Ieee 946 - 2020 [PDF]

Zitiervorschau

IEEE Power and Energy Society

STANDARDS

IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

Developed by the Energy Storage and Stationary Battery Committee

IEEE Std 946™-2020 (Revision of IEEE Std 946-2004)

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IEEE Std 946™-2020

(Revision of IEEE Std 946-2004)

IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications Developed by the

Energy Storage and Stationary Battery Committee of the

IEEE Power and Energy Society Approved 30 January 2020

IEEE-SA Standards Board

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Abstract: Recommended practices for the design of dc power systems for stationary applications are provided in this document. The components of the dc power system addressed by this document include lead-acid and nickel-cadmium storage batteries, static battery chargers, and distribution equipment. Guidance in selecting the quantity and types of equipment, the equipment ratings, interconnections, instrumentation and protection is also provided. This recommendation is applicable for power generation, substation, and telecommunication applications. Keywords: auxiliary, backup, battery, battery charger, charger sizing, control, cross-tie, dc, direct current, distribution, duty cycle, generating station, ground detection, IEEE 946™, instrumentation, nuclear, panels, protection coordination, rectifiers, reserve, selective protection, short-circuit, substation, telecommunication

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Participants At the time this recommended practice was completed, the DC Power Working Group had the following membership: Haissam Nasrat, Chair Richard Hutchins, Secretary Amber Aboulfaida Curtis Ashton Robert Beavers Robert Beck Christopher Belcher Steven Belisle Duane Brock James Buniak Thomas Carpenter Larry Carson Murad Daana Rajesh Dhiman Robert Feisley Kevin Fellhoelter

David Franklin Ali Heidary David Hood Wayne Johnson Roger Kang Thomas Keels Yves Lavoie Rufus Lawhorn Jose Marrero Tania Martinez- Navedo Stephen Mccluer Matthew McConnell Daniel McMenamin James Midolo

Sepehr Mogharei Thomas Mulcahy Bansi Patel Art Salander Surendra Salgia Christopher Searles Joseph Stevens Thomas Stomberski Kurt Uhlir Gustavo Varela Lesley Varga Stephen Vechy Jason Wallis Donald Wengerter

The following members of the individual balloting committee voted on this recommended practice. Balloters may have voted for approval, disapproval, or abstention. Amber Aboulfaida Ali Al Awazi Steven Alexanderson Curtis Ashton Gary Balash Radoslav Barac Thomas Barnes Robert Beavers Robert Beck Mark Bowman Jon Brasher Duane Brock Jeffrey Brogdon Chris Brooks Demetrio Bucaneg Jr. David Burns William Bush William Cantor Paul Cardinal Thomas Carpenter Suresh Channarasappa Michael Chirico Randy Clelland Bryan Cole Matthew Davis Ray Davis John Disosway Gary Donner Michael Dood Edgar Dullni Kevin Fellhoelter

Robert Fletcher Rostyslaw Fostiak Dale Fredrickson David Giegel Mietek Glinkowski Jalal Gohari Joseph Gravelle Randall Groves Ajit Gwal Ali Heidary Lee Herron Werner Hoelzl Robert Hoerauf Richard Jackson Anil James Geza Joos Innocent Kamwa Peter Kelly Yuri Khersonsky Hermann Koch Boris Kogan Jim Kulchisky Saumen Kundu Mikhail Lagoda Chung-Yiu Lam Daniel Lambert Rufus Lawhorn Timothy Lensmire Albert Livshitz Jon Loeliger Debra Longtin

Jose Marrero Hugo Marroquin Michael May Stephen McCluer William McCoy James McDowall Daniel McMenamin Steven Meiners Larry Meisner James Midolo Sepehr Mogharei Daleep Mohla Thomas Mulcahy Jerry Murphy Haissam Nasrat Dennis Neitzel Arthur Neubauer Michael Newman Nick S. A Nikjoo Joe Nims James O’Brien Lorraine Padden Bansi Patel Anthony Picagli John Polenz Thomas Proios Robert Rallo Jan Reber Timothy Robirds Charles Rogers Thomas Rozek

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Ryandi Ryandi Art Salander Surendra Salgia Steven Sano Bartien Sayogo Robert Schuerger Christopher Searles Robert Seitz Nikunj Shah

Devki Sharma David Smith Jeremy Smith Ralph Stell Gary Stoedter Thomas Stomberski K. Stump Sercan Teleke Michael Thompson Wayne Timm

James Van De Ligt Lesley Varga Gerald Vaughn Stephen Vechy John Vergis Donald Wengerter Kenneth White Hughes Wike Jian Yu

When the IEEE-SA Standards Board approved this recommended practice on 30 January 2020, it had the following membership: Gary Hoffman, Chair Vacant Position, Vice Chair Jean-Philippe Faure, Past Chair Konstantinos Karachalios, Secretary Ted Burse Doug Edwards J.Travis Griffith Grace Gu Guido R. Hiertz Joseph L. Koepfinger* John D. Kulick

David J. Law Howard Li Dong Liu Kevin Lu Paul Nikolich Damir Novosel Jon Walter Rosdahl

Dorothy Stanley Mehmet Ulema Lei Wang Sha Wei Philip B. Winston Daidi Zhong Jingyi Zhou

*Member Emeritus

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Introduction This introduction is not part of IEEE Std 946-2020, IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications.

DC power systems continue to play a vital role in generating station, substation, and telecom controls and providing backup for emergencies. This recommended practice fulfils a need within the industry to provide common or standard practices for the design of dc power systems. The design features are applicable to all installations and systems capacities. The original issue of IEEE Std 946 was published in 1985 with the title IEEE Recommended Practice for the Design of Safety-Related DC Power Systems for Nuclear Power Generating Stations. The 1992 revision changed the title to apply to all generating stations, while including specific guidance and a detailed bibliography of nuclear design reference standards. This revision makes a general update to reflect the most recent industry practices as well as substantial additions to annexes. In addition to power generation applications, this recommended practice covers dc power system design in substations and telecommunication applications. Some discussions and illustrative figures have been retained as they offer a constructive comparison to designs without having to resort to additional standards. This recommended practice was prepared by a Working Group that is part of the Energy Storage and Stationary Battery Committee and was sponsored by the Energy Development and Power Generation Committee of the IEEE Power and Energy Society. Note that IEEE Std 1818™ and IEEE Std 946 are complementary documents, developed by independent working groups.1

1

Information on references can be found in Clause 2.

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Contents 1. Overview ................................................................................................................................................... 12 1.1 Scope .................................................................................................................................................. 12 1.2 Purpose ............................................................................................................................................... 12 1.3 Exclusions .......................................................................................................................................... 13 2. Normative references ................................................................................................................................ 13 3. Definitions ................................................................................................................................................. 14 4. Organization of this recommended practice .............................................................................................. 15 5. Description and operation ......................................................................................................................... 15 5.1 General ............................................................................................................................................... 15 5.2 System design considerations ............................................................................................................. 16 6. Batteries .................................................................................................................................................... 18 6.1 Number of battery strings ................................................................................................................... 18 6.2 Determination of battery duty cycle and battery size (capacity) ......................................................... 19 6.3 Installation design............................................................................................................................... 21 6.4 Maintenance, testing, and replacement ............................................................................................... 22 6.5 Qualification, relevant codes, and standards ....................................................................................... 23 7. Battery chargers ........................................................................................................................................ 23 7.1 Number of chargers ............................................................................................................................ 23 7.2 Load sharing between paralleled chargers .......................................................................................... 24 7.3 Determination of rated output ............................................................................................................. 24 7.4 Installation design............................................................................................................................... 26 7.5 Output characteristics ......................................................................................................................... 26 7.6 Qualification ....................................................................................................................................... 29 8. Distribution system ................................................................................................................................... 30 8.1 System layout ..................................................................................................................................... 30 8.2 Distribution panels ............................................................................................................................. 35 8.3 Available short-circuit current ............................................................................................................ 35 8.4 Protective device description and rating ............................................................................................. 37 8.5 Voltage ratings for loads ..................................................................................................................... 38 8.6 Qualification ....................................................................................................................................... 39 9. DC power system instrumentation, controls, and alarms........................................................................... 40 9.1 General ............................................................................................................................................... 40 9.2 DC power system ground detection .................................................................................................... 43 9.3 DC bus undervoltage alarm ................................................................................................................ 45 9.4 Special loading considerations ........................................................................................................... 45 9.5 Design features to assist in battery testing .......................................................................................... 46 9.6 Cross-tie between buses ..................................................................................................................... 46 10. Protection against electrical noise, lightning, and switching surges ........................................................ 46 10.1 Electromagnetic interference (EMI), radio-frequency interference (RFI) ........................................ 47 10.2 Lightning and switching surges ........................................................................................................ 47 11. Spare equipment ...................................................................................................................................... 48 Annex A (informative) Bibliography.............................................................................................................. 49

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Annex B (informative) Battery charger sizing—Sample calculations������������������������������������������������������������ 51 Annex C (informative) Battery available short-circuit current—Sample calculations����������������������������������� 56 Annex D (informative) Battery charger and dc power system, short-circuit current contribution������������������ 59 Annex E (informative) Effect of unintentional grounds on the operation of dc power systems���������������������� 62 Annex F (informative) Telecommunication-specific considerations�������������������������������������������������������������� 65 Annex G (informative) Load sharing of chargers������������������������������������������������������������������������������������������� 70 Annex H (informative) Center tapped battery design considerations������������������������������������������������������������� 71 Annex I (informative) Additional batteries in nuclear power generation applications����������������������������������� 72

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IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications 1. Overview 1.1 Scope This recommended practice provides guidance for the design of stationary dc power systems. The components of the stationary dc power system addressed by this recommended practice include the following: — Storage batteries — Static battery chargers/rectifiers (including sizing) — Distribution equipment — Protection equipment — Control equipment — Interconnections — Instrumentation Guidance for selecting the quantity, types, and ratings of equipment is also provided. The considerations of each of these different components and the issue of load voltage and other load specifics are discussed in terms of their effect on the design of the whole system. Guidance on short-circuit calculation and contribution of different dc power system components is also offered to improve reliability, performance, and safety of the installation.

1.2 Purpose The purpose of this document is to provide the user with information and recommendations concerning sizing and designing dc power systems in stationary applications. While the recommended practices in this document apply to dc power systems in substations, additional guidance for substations is provided in IEEE Std 1818.2

2

Information on references can be found in Clause 2.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

1.3 Exclusions Electrically self-contained ac-ac equipment and the following components of the dc power system, with the exception of how they influence the dc power system design, are specifically excluded from the scope of this recommended practice: — The ac power supply to the battery chargers/rectifiers — Photovoltaic, wind, and other alternative dc power source system designs — Loads served by dedicated engine starting battery systems — Applications requiring dc voltage supply above 1000 V nominal — Motor generator sets — Battery technologies other than lead-acid (L-A) and nickel-cadmium (Ni-Cd) Separate systems are usually recommended for the following special service applications, and are not within the scope of this document: — Engine (cranking) starting — Emergency lighting — Fire detection and annunciation — Fire protection actuation

2. Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. IEEE Std 308™, IEEE Standard Criteria for Class 1E Power Systems for Nuclear Power Generating Stations.3,4 IEEE  Std  344™, IEEE Standard for Seismic Qualification of Equipment for Nuclear Power Generating Stations. IEEE Std 450™, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Vented LeadAcid Batteries for Stationary Applications. IEEE Std 484™, IEEE Recommended Practice for Installation Design and Installation of Vented Lead-Acid Storage Batteries for Stationary Applications. IEEE Std 485™, IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications. IEEE Std 649™, IEEE Standard for Qualifying Class 1E Motor Control Centers for Nuclear Power Generating Stations.

3 The IEEE standards or products referred to in Clause 2 are trademarks owned by The Institute of Electrical and Electronics Engineers, Incorporated. 4 IEEE publications are available from The Institute of Electrical and Electronics Engineers (https://​standards​.ieee​.org/​).

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

IEEE Std 650™, IEEE Qualification of Class 1E Static Battery Chargers, Inverters, and Uninterruptible Power Supply Systems for Nuclear Power Generating Stations. IEEE Std 666™, IEEE Design Guide for Electric Power Service Systems for Generating Stations. IEEE Std 693-2005™, IEEE Recommended Practice for Seismic Design of Substations. IEEE Std 979™, IEEE Guide for Substation Fire Protection. IEEE Std 1106™, IEEE Recommended Practice for Installation, Maintenance, Testing, and Replacement of Vented Nickel-Cadmium Batteries for Stationary Applications. IEEE Std 1115™, IEEE Recommended Practice for Sizing Nickel-Cadmium Batteries for Stationary Applications. IEEE Std 1187™, IEEE Recommended Practice for Installation Design and Installation of Valve-Regulated Lead-Acid Batteries for Stationary Applications. IEEE Std 1188™, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Valve Regulated Lead-Acid (VRLA) Batteries for Stationary Applications. IEEE Std 1189™, IEEE Guide for Selection of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications. IEEE Std 1375™, IEEE Guide for the Protection of Stationary Battery Systems. IEEE Std 1491™, IEEE Guide for Selection and Use of Battery Monitoring Equipment in Stationary Applications. IEEE Std 1578™, IEEE Recommended Practice for Stationary Battery Electrolyte Spill Containment and Management. IEEE Std 1635™, IEEE/ASHRAE Guide for the Ventilation and Thermal Management of Batteries for Stationary Applications. IEEE Std 1818™, IEEE Guide for the Design of Low Voltage Auxiliary Systems for Electric Power Substations. NEMA PE 5, Utility Type Battery Chargers.5 NFPA 70®, National Electrical Code® (NEC®).6 NFPA 70E®, Standard for Electrical Safety in the Workplace.

3.  Definitions For the purposes of this document, the following terms and definitions shall apply. For terms not defined in this clause, IEEE Std 1881 [B20], IEEE Standard Glossary of Stationary Battery Terminology [B20], and the IEEE Standards Dictionary Online shall be consulted.7 NEMA publications are available from the National Electrical Manufacturers Association (https://​www​.nema​.org/​). NFPA publications are published by the National Fire Protection Association (https://​www​.nfpa​.org/​). 7 IEEE Standards Dictionary Online is available at: http://​dictionary​.ieee​.org. An IEEE Account is required for access to the dictionary, and one can be created at no charge on the dictionary sign-in page. 5 6

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

battery capacity: The quantity of electrical energy, measured in ampere-hours (Ah) or watt-hours (Wh), produced by a battery during discharge. battery charger: A device to restore and maintain the charge of a secondary battery. For easier reading, “charger” is used throughout this recommended practice to refer to battery charger or a rectifier connected to a battery. battery state of charge: The stored or remaining capacity in a battery expressed as a percentage of its fully charged capacity. duty cycle: The sequence of loads a battery is expected to supply for specified time periods. nominal battery voltage: The value assigned to a battery of a given voltage class for the purpose of convenient designation. The operating voltage of the system may vary above or below this value. N+X: Parallel redundancy to ensure that the system is always available. N is the minimum required number of modules/systems. X is the variable referring to extra units needed for reliable operation.

4.  Organization of this recommended practice Stationary dc power systems appear in many applications and industries. All have certain commonalities, while some have some unique requirements. It should be noted that these commonalities or unique requirements are derived from variance in environmental conditions, reliability expectations, and importance of application. That translates to specific feature requirements or technology differences (thyristor versus high-frequency switched mode chargers, or lead-acid versus Ni-Cd batteries) that can provide an engineering approach to the selection/design of dc power systems. For example, a substation charger can be used in a telecom application and vice versa as long as it can meet the requirements. Describing every application is beyond the scope of this document, therefore the three dominant applications are generation, substations, and telecommunications. Large telecommunication carriers may have their own internal dc power system standards that examine dc power systems and their requirements in detail. This is also beyond the scope of this document, but is worthy of a mention. In this recommended practice, each section includes subparagraphs reserved for these three applications when there are unique requirements. For other industrial applications, one can use the recommendations—in part or as a whole—of one of the three dominant applications. For example, substation application recommendations may be used, where applicable, for the design of an industrial process control dc power system. It is not the intent of this recommended practice to exclude other industrial applications. Lead-acid and nickel-cadmium batteries are the types of batteries primarily used in these applications. Some other battery technologies may be used but are not fully addressed in this document.

5.  Description and operation DC power systems provide reliable power to critical loads. Examples of critical loads include auxiliary motors, circuit breakers and switchgear, relays, solenoids, SCADA, telecommunications equipment, inverters, emergency lighting equipment, fire suppression equipment, etc.

5.1 General A dc system normally consists of one or more battery strings, one or more battery chargers/rectifiers and one or more distribution panels. If a battery isolation/protective device is used, refer to the battery protection guidelines of IEEE Std 1375. Refer to simplified typical connection in Figure 1 for the line of demarcation that limits the application of IEEE 946 versus IEEE 1375.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

Figure 1—IEEE 946 versus IEEE 1375 line of demarcation In normal operation, the battery and battery charging system are both connected to the loads through a common bus or via a dc distribution panel. Therefore, they operate as parallel sources. The battery charging system applies voltage and supplies current to the battery in order to maintain a full state of charge in the battery. The charger also generally supplies the continuous load and/or other loads as specified. If the load exceeds the maximum current rating of the battery charging system, the battery charging system output voltage will drop, causing all current in excess of the battery charging system rating to be supplied by the battery. In the event of a failure of the ac power supply to the battery charger (in case the charging system consists of only one charger), a battery charger failure, or the battery charger being removed from service, the battery should supply all the power required by the load(s) for some specified periods of time. This is commonly referred to as the “duty cycle.” 5.1.1  Power generation Specific design guidance for dc power systems for nuclear generating plants are discussed fully in numerous design standards listed in Annex A. 5.1.2 Substation For additional guidance refer to IEEE Std 1818. 5.1.3 Telecommunications Telecommunication installations require dc power for almost all equipment, as only a small percentage of telecommunications equipment is ac-powered. Therefore, dc power systems are required for normal and backup powering of most telecommunications installations. Refer to Annex F for the specific considerations in telecommunication applications. Telecommunication loads in substations commonly used for telemetry and telecontrol generally operate at lower voltage (e.g., −48 V dc) than the main substation dc power system voltage (e.g., 125 V dc). A dedicated dc power system might be needed to feed such loads. In cases when the power consumption is relatively low, they can be fed by the substation dc power system through dc-dc converters (e.g., 125 V dc to 48 V dc converters).

5.2  System design considerations In addition to the load’s requirements, the design should accommodate any regulatory agency requirements, safety, or other requirements. The design should also address factors such as reliability, design philosophy, maintenance, testing, ventilation, floor and wall loading, and space limitations, etc.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

The equipment to be connected to the battery and battery charger will govern the minimum and maximum voltage operating voltage range of the dc power system. The designer should consider a single system or multiple systems based on the voltage, current, and redundancy requirements of the components. For example, if a communication system requires 48 V dc input and the dc bus is 125 V dc, then consider whether the communication equipment would be either supplied by its own battery and charger or by a dc-dc converter fed from the main 125 V dc system. A recommended practice is to create a diagram at the start of the design process showing the battery or batteries, charger(s), dc panel(s), and all connected loads. Consideration should also be given for future growth. Redundant dc power systems may also be considered. Redundancy may provide flexibility for maintenance, testing, or replacement in the event of equipment failure or the need to upgrade in the future. The ability to connect other dc power systems may also aid in maintenance activities. Although not recommended, center-tapped battery designs have been utilized in special applications. In most cases, a center-tapped battery design should be considered as the least advantageous design, as it may result in issues such as a more complicated ground fault detection, battery state of charge imbalance, cell voltage imbalance, load imbalance, improper autonomy, etc. Designing separate systems for different voltages is recommended in lieu of center tapped battery. See informative Annex H for more details on center tapped battery. When special loads such as inverters and dc-dc converters are connected to a dc power system, considerations should be made to understand their potential effect on the dc bus in terms of charger regulation, current and voltage ripple, transient behavior, or other disruptive interactions. 5.2.1  Power generation The most common nominal system voltages utilized in a power generation dc power system are 24 V dc, 48 V dc, 125 V dc, and 250 V dc. The following dc voltage values are commonly seen on equipment within a plant and are provided for illustrative reference only: — 250 V dc — Motors for emergency pumps — Large valve operators — Large inverters — 125 V dc — Motors and valve actuators — Control power for relay logic circuits — Opening and closing of switchgear circuit breakers — Smaller inverters — Field flashing — 48 V dc, 24 V dc, 12 V dc — Communication systems — Specialized instrumentation and controls Some battery technologies (e.g., lead-acid) may exhibit a momentary voltage dip phenomenon known as the “coup de fouet” when supplying high initial inrush currents. The coup de fouet effect value is provided by the manufacturer and should be considered when sizing battery capacity in order to reduce load disturbances or shutdowns. The coup de fouet becomes greater as a battery ages.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

Once the voltage limits and design parameters are established, the size of the battery (number of cells and capacity) can be established as outlined in applicable best practices or standards, e.g., IEEE Std 485 and IEEE Std 1115. It may be useful to size the battery such that the design duty cycle can be met with a reduced number of cells (N−1, N−2). When determining system voltage in facilities with long cable runs, voltage drop should be considered. In order to evaluate the voltage drop across a cable, Ohm’s law shall be applied using the maximum current times the cable resistance provided by the manufacturer. Legacy equipment or available equipment may constrain the choice of dc voltages. Considerations should be taken for special operation conditions. For example, the capabilities of a black start or system restoration plan may require more than one attempt to restore the station ac service. This can be the case when the main electrical grid power is unavailable, and electrical power is provided by a backup generator. 5.2.2 Substation Telecommunication loads in substations commonly used for telemetry and telecontrol generally operate at lower voltages (e.g., 24 V dc or −48 V dc) than the main substation dc system voltage (e.g., 125 V dc). A dedicated dc system might be needed to feed such loads. In cases where the power consumption is relatively low, it can be fed by the main substation’s dc system through dc-dc converters (e.g., 125 V dc to 24 V dc or −48 V converters). In such cases, considerations for additional needs, such as required incremental power, voltage regulation, and high-frequency noise should be made when designing the main substation’s dc system. For additional guidance refer to IEEE Std 1818. 5.2.3 Telecommunication Nominal system voltages for telecommunications dc power plants are most commonly 48 V dc, 24 V dc, and 12 V dc. Minimum and maximum telecommunication equipment operating voltage levels are defined in ANSI/ATIS-0600315.2013 [B2] and ANSI/ATIS 0600315.01.2015 [B3]. It is recommended that chargers be equipped with filtered outputs to the appropriate level as charted in NEMA PE 7 [B28].

6. Batteries Stationary batteries are used to supply dc power to specified dc loads when the source of ac power to the battery charger has been disrupted or is insufficient to support the loads. Refer to appropriate IEEE standards or the battery manufacturer’s guidance for the proper selection of battery type to be used in the application. Correct battery selection is essential for reliability, useful life, cost, and maintenance planning. Factors such as operating temperature, duty cycle, battery life, and deep cycling should also be considered.

6.1  Number of battery strings The number of battery strings in an independent dc power system should be considered at the design stage. More than one battery string for capacity or redundancy should be considered to help ensure compliance or reliability requirements. 6.1.1  Power generation Some examples of how to improve the reliability and protection of critical loads include the following: — When loads are divided into two or more independent systems, each independent system should be provided with its own dc power system.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

— When maximum dc power requirements exceed the capacity of one battery string, the system designer should consider either of the following: — The use of parallel battery strings. — The use of two independent dc power systems, each with its own battery. In this case, selectivity and coordination of dc buses and protective devices should be implemented. — When modifications to the facility cause the dc power demand to exceed the existing dc power system’s capacity, the installation of a new independent system should be considered. Other alternatives include the following: — Replacing the existing dc power system (battery, battery charger, and possibly an addition to the main distribution bus/board) with a larger capacity system. (Note that replacing the existing battery with a larger battery may require the replacement of the main distribution.) — Installation of a parallel battery. (Note that this may also require additional considerations such as matching existing battery capacity, recharging duration, replacement of the main distribution, etc.) — When dc power system independence is required and multiple systems are installed, each system should be powered from separate and/or independent sections of the ac power system if an alternate ac source is available (refer to 8.1.1.1). — When maintenance and/or emergency reasons require isolating one battery string from another, individual disconnects or a circuit breaker should be recommended in the design. — When maintenance and/or emergency reasons require isolating one battery string from another, the loading should be reviewed to ensure that the remaining one battery string has adequate capacity to operate the worst case loading scenario. Additional power generation applications cases include specific guidance for center tapped battery design considerations used in a power generating station (see Annex H) and for the number of batteries to be used in a nuclear generating station (see Annex I). 6.1.2 Substation For additional guidance refer to IEEE Std 1818. 6.1.3 Telecommunications Parallel strings are very common in telecommunications in order to meet the required backup times as needed for both the application and redundancy.

6.2  Determination of battery duty cycle and battery size (capacity) To size a battery correctly, it is important to know the following: — The number of loads (if more than one load) — The load size and duration — The loads sequence in case of multiple loads — The desired design minimum voltage for any dc load when the battery reaches its end of discharge point — The lowest anticipated temperature the battery electrolyte should encounter Refer to IEEE Std 485 for lead-acid and IEEE Std 1115 for nickel-cadmium. The overall duration (total battery discharge time) of the duty cycle cannot be less than the estimated time interval necessary to restore the charger output to the battery and connected dc loads. This estimated time 19

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

interval is determined by engineering judgment, which is greatly influenced by operating experience, and by the quantity, reliability, and flexibility of the specific off-site power sources and on-site power sources. The duty cycle may also be influenced by the accessibility of the site. In theory, a minimum sizing scenario would only require the battery to supply dc power to the load for approximately 1 min (the time needed between the loss of ac power and the loading of an operational standby power source), assuming that after such time, the charger output and dc loads would return to normal. In practice, however, the duration of the battery duty cycle is generally estimated to be anywhere from 5 min to 72 h. The selected time depends upon the overall design requirements. It may consider what to do if the standby power source fails to automatically restore power. 6.2.1  Power generation The types of dc loads encountered in a generating station can include, but are not limited to, any or all of the following: — Annunciator system — Inverter — Emergency lighting — Emergency lube oil and seal oil pumps — Engine starting and control — Generator field flashing — Fire detection and actuation — Main generator output breaker control — Offsite power recovery — Relays and solenoids — Standby or black-start power source starting and control — Switchgear breaker operation, including spring charging motors — Communication systems — Squib valves — Motorized valves 6.2.2 Substation Most substations have a duty cycle of 8 h or more. Substations located in areas that are remote and/or difficult to access by field service personnel may require more time to respond and therefore may require a longer duty cycle. Develop a load profile in accordance with appropriate IEEE sizing standards (i.e., IEEE Std 485 or IEEE Std 1115) to help ensure correct battery sizing for the application. A typical substation duty cycle needs to account for the following types of load: — Continuous loads, e.g., relays, communications, security, and monitoring — Non-continuous loads, e.g., emergency lights — Larger momentary loads, e.g., breaker coils and motor-operated switches For additional guidance refer to IEEE Std 1818.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

6.2.3 Telecommunication For traditional telecommunications sites, a minimum of 3 h to 4 h of battery backup is typically provided for sites with permanent on-site, auto-start, and auto-transfer standby engine-alternators. A minimum of 8 h is typically provided for sites not served by a standby engine-alternator. In order to maintain the battery backup for the most critical loads, non-critical loads are sometimes shed. The battery is generally sized based on a constant load current during a predefined period of time. However, if the load is constant power type in which the current rises as the battery voltage drops, then this increased current has to be considered in the battery sizing. Typical nominal dc bus voltages are +12 V, +24 V, −48 V, 380 V, and 575 V. In general, there may be regulatory rules or guidelines that determine the amount of backup time for traditional voice service. The most common of these rules requires a minimum of 8 h of battery backup for sites not backed up by an on-site permanent auto-start, auto-transfer engine-alternator. For sites with a permanent onsite auto-start, auto-transfer engine alternator, the rule is typically 3 h of battery backup plus travel time to the site, or simply a straight minimum of 4 h of battery backup. Non-voice loads (such as broadband data and video) may be shed in some sites after a certain period of time, or after a certain voltage is reached in the discharge in order to extend the operation of more essential loads. Typically, this time could range from 5 min to several hours and is company specific. When this is done, it is typically done off of a timer activated by a commercial ac failure or “battery on discharge” alarm, or the timer starts when a certain discharge voltage is reached. Most telecommunication dc power systems (commonly known in the telecom industry as a “dc plant”) designed for three or more hours of battery reserve use a battery designed for long-duration discharge. If the site is powered by renewable energy or has poor grid quality, a battery designed for cycling duty should be considered.

6.3  Installation design The design of each battery installation, signage, battery room design, and access (when applicable) should be in accordance with the appropriate IEEE standards and local codes. IEEE standards typically include but are not limited to the following: — IEEE Std 484 – Installation of vented lead-acid (VLA) batteries — IEEE Std 1187 – Installation of valve regulated lead-acid (VRLA) batteries — IEEE Std 1106 – Installation, maintenance and testing of nickel-cadmium batteries — IEEE Std 1491 – Battery monitoring — IEEE Std 1578 – Spill containment — IEEE Std 1635 – Ventilation and thermal management — IEEE Std 979 – Substation fire protection Codes commonly enforced in the US include the following: — National Electrical Safety Code® (NESC®) (Accredited Standards Committee C2) [B4] — National Electrical Code® (NEC®) (NFPA 70®) — NFPA 70E®, Standard for Electrical Safety in the Workplace

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

A controlled environment is recommended for all battery installations. Operating outside the manufacturer’s recommended temperature range will impact the performance and lifespan of the battery. Factors to consider in designing the optimum environment for a battery system may include the following: — Airborne contaminants, such as corrosive coastal environments (e.g., salt, humidity) — Particulates — Seismic requirements (refer to IEEE Std 693-2005) — Natural disaster exposure (e.g., floods, tsunamis, wildfires) — Adequate hydrogen ventilation — Spill containment — Access, egress, and safety of personnel — Occupancy — Security requirements — Fire detection and suppression — Heat ventilation — Space layout — Clearance requirements (NESC [B4], NEC) and additional space needs for large cell lifting devices Different battery types can vary substantially not only in physical dimensions, but also in the weight of the filled containers. Design should consider rack size and type (tiered, stepped, or step-tier) and limitations on number of tiers to limit the height. With very large batteries, lifting devices may be required to place containers on tiered racks. The design needs space to accommodate both installation and maintenance requirements of the battery. If racks with more than two tiers are selected, there may be a temperature gradient between top-tier and bottom-tier cells, which can affect loading of the cells and impact battery life. Applicable national/local codes and standards should be researched and analyzed to improve compliance as required. Spill containment systems may be required in certain jurisdictions. Check with the authority having jurisdiction (AHJ) responsible for the particular jurisdiction where the site is being designed for installation if in doubt. 6.3.1 Substation For additional guidance refer to IEEE Std 1818.

6.4  Maintenance, testing, and replacement Batteries should be maintained, tested, and replaced in accordance with IEEE Std 450 (VLA), IEEE Std 1188 (VRLA), IEEE Std 1106 (Ni-Cd), and/or IEEE Std 1184 [B18] for UPS battery replacement determination and local regulations. The dc supply may be equipped with switching devices to facilitate the removal of a battery string from service for the purposes of offline maintenance and testing. Taking a battery offline may require prior connection of a temporary battery in order to provide continuous backup power supply to the station. Refer to Clause 8 for more guidance on additional safety isolation devices needed for this operation.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

6.4.1 Substation For additional guidance refer to IEEE Std 1818.

6.5  Qualification, relevant codes, and standards Qualification of batteries may be required depending on the application specifics. Seismic and/or environmental qualification of battery installations is based on the appropriate local codes where the battery will be installed or on good engineering judgment based on the application and criticality of the installation. For seismically active areas, the battery cabinets or racks, and the batteries should be seismically qualified in accordance with the seismic requirements of the International Building Code (IBC). 6.5.1  Power generation Nuclear power plants have additional requirements based on their licensing commitments. Refer to IEEE Std 535 [B12]. 6.5.2 Substation Transmission entities might have additional requirements based on North American Electric Reliability Corporation (NERC). Other regions may have similar regulations. 6.5.3 Telecommunication Equipment used in telecommunications applications by many large North American telecommunication companies is required to be Network Equipment-Building System (NEBS) compliant. See Annex F for further details.

7.  Battery chargers The battery chargers are used to restore electrical energy in the stationary batteries and to supply power to dc loads during normal operation.

7.1  Number of chargers At a minimum, one battery charger should be provided for each battery. Additional battery chargers should be considered if increased operation flexibility, redundancy, or capacity is desired and/or required. For example, instead of using one very large charger, one can consider using two identical smaller size chargers in parallel. As long as the constant loads are fully supported by one charger, utilizing two chargers provides flexibility for routine maintenance and in case of a failure of one of the chargers. The continuous load would be maintained without affecting the battery, but the recharge time would be increased. Constant load would be maintained without affecting the battery. The use of modular chargers in an N+x configuration (i.e., one or more additional chargers than are necessary to support the critical load) may provide greater reliability than a single charger, but less than two fully redundant chargers. 7.1.1 Substation For additional guidance refer to IEEE Std 1818.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

7.1.2 Telecommunication Typically, N+1 or greater rectifier redundancy is used in telecommunication stations.

7.2  Load sharing between paralleled chargers When more than one charger is connected in parallel, load sharing between chargers should be considered. It is not recommended that chargers of different designs and/or technologies be operated in parallel. Refer to Annex G for more details about load sharing of chargers. 7.2.1 Telecommunication Parallel operation is the normal mode of operation for telecommunication chargers. For modern rectifiers, a microprocessor controller gives them all an exact voltage to synchronize, and because of the precision of their output regulation, they may all share the load equally. If the controller fails, the rectifiers should be equipped with circuitry that will put them at a “fallback” default output voltage (typically very near, if not the same as the float voltage at 25 °C). In most systems, this “fallback” voltage is set through the controller, which the miniature microprocessor in each rectifier remembers in case of controller failure. So, even when the controller fails, a modern telecommunication rectifier system may still share the load equally among the rectifiers because all rectifiers are programmed with the exact same “fallback” voltage.

7.3  Determination of rated output 7.3.1 General The battery charger output current should be sized to feed the connected load while recharging a fully discharged battery to reach a level greater than 90% of its capacity within a predetermined recharging time. This estimated time is a combination of engineering, judgment, and/or regulatory requirements, and can be influenced by operating experience as well as the quantity, reliability, and flexibility of other power sources, e.g., on-site or off-site generators, auxiliary network power sources, or redundant dc power systems (batteries and battery chargers). Equation (1) should be used to calculate the size of the charger as follows: ​Ic  =  K​(​(C × e)​/ t + ​IL​  ​)​​

(1)

where ​C​ ​e​ ​t​ ​IL​  ​​

​K​ ​Ic​ ​

is the ampere-hour removed from the battery as calculated from the battery sizing calculation based on the actual duty cycle. For other considerations (7.3.3) or if this value is not known, then use the published ampere-hour rating of the battery based on the expected discharge time duration is the recharging efficiency factor. Suggested values, which may be overridden by the battery manufacturer, are given in Table 1 is the recharge time in hours, usually between 6 h and 24 h. Recharge time may be impacted based on the minimum or maximum charging current. Consult the battery manufacturer refers to all constant loads and the duty cycle of non-continuous (not transient loads) currents expected to be supported by the battery charger while recharging or floating the battery (in amperes). If the load is a mixture of constant current and constant power type and the average voltage is not known, then it is recommended using the minimum voltage to determine the equivalent constant current load of the constant power load part refers to any factors such as but not exclusive to factors related to future load growth, temperature, altitude, etc., desired by the specified charger. If no other factor is needed, then k = 1 is the minimum charger output current in amperes

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

Table 1—Typical battery recharge factors Battery type

Suggested -e- value

VLA

1.1

VRLA

1.15

Vented Ni-Cd

1.3

Partially recombinant Ni-Cd

1.14

Special considerations include the following: — For some battery technologies, such as VRLA, excessive charging current might affect their expected life. Therefore, if the sum of all the loads is used to size the charger and those loads do not operate concurrently, then the charger will have too much current available for charging the battery. If that current is higher than the manufacturer recommendations, additional controls, such as charger current limiting, may be required to protect the battery. — Recharge time factor “t” in constant voltage charging: Due to the load voltage range limitation, chargers may operate in constant voltage charging rather than in constant current charging. In this case, the battery is recharging at a lower rate and will take much more time to reach its full capacity. Hence, it is recommended that “t” is fixed to a value (between 6 h and 24 h, e.g., 8 h) depending on the site criticality and conditions. Sample calculations of battery charger rating are given in Annex B. 7.3.2  Power generation In addition to the information in 7.3.1, in power generation applications, the charger shall have a minimum output current (I2) to be capable of simultaneously supplying the constant load (IL) and the non-continuous load (ILN). The following equations apply: ​I2​ ​  =  ​IL​  ​ + ​ILN  ​ ​​

(2)

​I3​ ​  =  Max. [​ ​Ic​ ​ , ​I2​ ​]​​

(3)

where ​I3​ ​ ​Ic​ ​I2​ ​​ ​ILN ​  ​​ ​I3​ ​​

is the higher value of I2 or Ic is the charging current calculated in the above paragraph is the charger minimum output current is the largest combination of non-continuous loads (e.g., as defined in 4.2.2 of IEEE Std 485-2020) that would likely be connected to the bus simultaneously during normal plant operation, including periodic testing of dc components such as emergency lighting and emergency oil pumps is the final charger output current value for this application

7.3.3 Substation In cases where substations are located in areas that may require more time to respond, it is recommended to consider using the battery nominal Ah capacity for charger sizing instead of the removed Ah. Refer to IEEE Std 1818 for more guidance.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

7.3.4 Telecommunication Modern telecommunication charging is typically done with constant voltage charging, where rectifiers are connected in parallel with the batteries and loads. Telecommunication rectifier sizes are typically given in nominal rated dc output Amperes (e.g., 50 A, 200 A, etc.) for typical nominal 48 V dc or 24 V dc rectifiers. While some of these rectifiers may put out slightly more current (if the current limit is set to allow it) than the nominal Ampere rating, the nominal rated output number is used for rectifier sizing calculations. Telecommunication rectifiers may also be rated in constant-power watts, which means that their output varies by voltage. For sizing purposes, this number is typically converted to Amperes at the expected battery float voltage. For example, a 3500 W rectifier floating 24 series-connected 1.215 specific gravity (s.g.) cells at an average of 2.20 V/cell would be an approximately 66.3 A rectifier (3500 W/52.8 V).

7.4  Installation design 7.4.1 General A controlled environment is recommended for all charger installations. Additionally, installation design should consider the following: — Corrosive atmospheric conditions — Nearby industrial processing sites — Coastal environments — Particulates from nearby industrial processing sites, dust, and dirt — Environmental conditions that would accelerate component degradation, e.g., sunlight exposure, high ambient temperature — Humidity — Seismic requirements required by authority having jurisdiction (refer to IEEE Std 693-2005) — Natural disaster exposure, e.g., floods, tsunamis, wildfires — Accessibility for maintenance of components, adequate space for egress and safety for personnel — Security requirements, e.g., physical access, cybersecurity — Requirements for replacement under live service 7.4.2 Substation In addition to the information in 7.4.1, working clearance per AHJ such as the NESC [B4] or NEC in the US should be considered.

7.5  Output characteristics All charging sources should meet the requirements of applicable standards such as NEMA PE 5, and the following performance characteristics. A connected battery may need additional requirements such as: — Battery maximum charging current and voltage — Maximum current and voltage ripple — Temperature voltage compensation — Other characteristics in order to maximize its service life under the operating conditions 26

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Connected dc loads may also have additional requirements such as: — Maximum and minimum voltage levels — Maximum noise and ripple levels — Inrush currents, etc. Certain site conditions may require the charger to perform with or without the battery connected with continuous and non-continuous loads, but not momentary loads. 7.5.1  Ripple and transients Output ripple, transients, electromagnetic interference (EMI), and radio frequency interference (RFI) should be limited to: — Help extending battery life — Avoid impact on sensitive loads — Comply with the applicable electromagnetic compatibility (EMC) regulations, such as the Federal Communications Commission (FCC), IEC 61000-4 series [B7], and IEEE Std 650 standards Ripple frequency and current magnitude are the components of concern to batteries. The lower the frequency and the higher the amplitude of the current ripple, the more damage is being done to the battery. Chargers compliant to international standards, with properly functioning filter circuits, produce ripple at such low magnitudes as to be inconsequential to battery life. Almost all damaging ripple current to batteries is due to connected ac inverters and other converters. Ripple values are usually measured at rated charger voltage and current conditions into a resistive load with a connected battery. The maximum voltage and current ripple should be specified based on the connected load(s) ripple limitations. NOTE—Although inverter reflected ripple effect on the dc bus is not covered by this document, special considerations may be needed to reduce the total ripple content to a level to help ensure correct load operation and to help extending battery life.

7.5.1.1  Power generation Chargers being equipped with filtered outputs to the appropriate level as charted in NEMA PE 5 is recommended. 7.5.1.2 Substation Same as the information provided in 7.5.1.1. 7.5.1.3 Telecommunication Historically, telecommunication dc systems provided power to equipment that served analog voice circuits. For this reason, it was particularly important to keep electrical transients and noise to a minimum in the voice band between 20 Hz and 10 kHz. Because dc ripple in older silicon controlled rectifier (SCR) and ferroresonant chargers with 50 Hz ac or 60 Hz ac input was primarily of the primary frequency and lower order harmonic multiples, this unfiltered ripple fell right into the voiceband. For this reason, telecommunication rectifiers have historically been well filtered, producing minimal ripple to the battery and load (some older rectifiers were not as well-filtered as others, and thus needed the battery to provide additional filtering to negate the lowfrequency ripple for the “talk battery.” However, all modern telecommunication rectifiers are well-filtered).

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

It is recommended that chargers be equipped with filtered outputs to the appropriate level as charted in NEMA PE 7 [B28]. 7.5.2  Operation without a connected battery For some designs or operation needs, disconnecting the battery from the dc power system for maintenance may be appropriate. Chargers should be capable of supplying the small incremental and continuous loads within the charger’s rated capacity. NOTE—Due to the dynamic response time of chargers, they are not usually capable of supplying the large inrush currents required by some loads. Therefore, it is not recommended to use chargers without a connected battery. Other operation techniques, such as using a temporary battery or transferring the load to other dc power systems, should be considered.

Also, a deterioration of the voltage regulation and output ripple may be experienced when the battery is disconnected. If the increase in voltage regulation or ripple cannot be tolerated, the maximum allowable values should be specified. A charger with improved filter circuit may further reduce the voltage and current ripple magnitude when battery is disconnected. 7.5.2.1  Power generation Operation without a connected battery is not recommended. Caution is to be taken if the dc power system is a source for the protective relays and circuit breaker tripping. Disconnecting the battery may affect the protection system. 7.5.2.2 Substation Operation without a connected battery is not recommended. Breakers may operate at any time, particularly during a fault. A charger may not have the instantaneous response time needed to respond to a sudden large current demand, resulting in possible breaker misoperation. 7.5.2.3 Telecommunication Typically, telecommunication loads do not include excessive transient loads; hence modern telecommunication rectifiers can operate without a connected battery due to their fast response time and inherent high output filter. 7.5.3  Remote voltage sensing and battery temperature compensation When chargers and batteries are located at a distance from each other, a significant voltage drop may occur in cables. Remote sensing leads should be considered to help ensure proper voltage regulation to the battery terminals. The loss of remote sensing shall not cause the charger voltage to increase. When sensors at the battery indicate that operating temperature has deviated outside the acceptable temperature range, temperature compensation can adjust voltage in proportion to the temperature shift. Temperature compensation can extend the life of a battery and when the battery is overheating, can mitigate and possibly prevent thermal runaway and fire. Temperature compensation might be an option on some chargers and is recommended for use with lead-acid and nickel-cadmium batteries. A temperature compensation sensor installed on the negative terminal of the pilot cell or block is recommended. Minimum and maximum voltage compensation levels should be considered to help ensure proper load operation. In installations with parallel chargers, any remote voltage and temperature sensing should be located at the same point.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

7.5.3.1 Telecommunication The rectifier controller can set the output voltage of the rectifiers based on internal sense, or external sense. Internal sense is when the output voltage setting is matched to the common “hot” dc output bus voltage in that bay or shelf relative to the grounded return bus in that bay or shelf. This is commonly used in small dc plants where the batteries are very near the rectifiers. In larger dc plants where the batteries are further away, there is usually more voltage drop between the rectifiers and batteries, and charging the batteries based on the voltage at the batteries is more important. In those cases, external sense is used. A pair of external sense wires are run to the positive and negative battery termination buses above the battery stand(s). The rectifier output voltage(s) are adjusted to provide the float voltage set in the controller so that voltage is produced at those “remote” battery term buses. Some older rectifiers, while connected to a controller, cannot have their output voltage finely adjusted by the controller; thus, each rectifier has its own potentiometer (pot) for setting its output voltage. In such plants, where they still exist, the rectifiers have to have their “pots” adjusted periodically in order to share the load fairly equally. Since each rectifier regulates itself rather than receives a voltage regulation signal from the controller, the voltage of each rectifier may drift ever so slightly over time in relation to the other paralleled rectifiers. A difference of even a few millivolts can cause huge variations in current sharing between rectifiers; thus the “pots” have to be periodically adjusted (typically this is done every 6 to 12 months). 7.5.4  Output current limit Stationary battery chargers are typically constant voltage, designed to limit their output current. When the charger operates in current limit mode, it is operating as a controlled constant current source to restore the bulk Ah of the battery and feed the dc load. The current should taper as the battery approaches full charge. The charger can also enter current limit mode if it is overloaded or a fault appears on the dc bus. The current limit should protect the charger components from premature failure due to overload. For some battery technologies such as VRLA, where charging capacity exceeds the battery’s allowable charging limits, dedicated battery charge current limiting shall be employed.

7.6  Qualification The objective of equipment qualification testing is to demonstrate that the equipment functions within its specified range during normal and abnormal operating conditions. Equipment qualification can normally be divided into the following two major subsets: — Performance qualification — Environmental qualification The purpose of environmental qualification is to demonstrate that the charger will operate on demand to meet system performance requirements within specified environmental parameters. Environmental qualification covers radiation, electrical environment, temperature, humidity, seismic, and EMI/RFI conditions among other parameters as defined by each component’s application and setting. All chargers/rectifiers are required to be qualified per performance and environmental standards. Specific performance and environmental qualification requirements for chargers/rectifiers applicable to all industries covered in this recommended practice are described below. 7.6.1  Power generation Performance and functional qualification for chargers is defined in NEMA PE 5.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

For Class 1E, environmental qualification for chargers/rectifiers is defined in several standards. Radiation and temperature qualification are dictated by IEEE Std 650. Seismic qualification is defined in IEEE Std 344. Installation, inspection, and testing requirements for power, instrumentation, and control equipment at nuclear facilities are defined in IEEE Std 336 [B8]. For non-Class 1E, seismic qualification for chargers/rectifiers are defined by IEEE Std 693-2005 and the International Building Code (IBC), Chapter 5. EMI/RFI qualification guidance is defined by EPRI TR-102323 [B6] among other documents. 7.6.2 Substation Performance/functional qualification for battery chargers is defined in NEMA PE 5. Seismic qualification for chargers/rectifiers is defined by IEEE Std 693-2005. 7.6.3 Telecommunication Rectifiers used in telecommunications applications by many large North American telecommunication companies may be required to be Network Equipment Building System standards (NEBS) compliant. The documents applicable to rectifiers and their racks/shelves/cabinets are GR-63, GR-1089, and SR-3580. In addition to the flammability and seismic requirements of GR-63 mentioned for batteries, GR-1089 testing covers electromagnetic emissions and susceptibility, short-circuit testing, power input fault immunity, and safety. In addition to the basic NEBS specifications produced by Telcordia, the telecommunications company may require the manufacturer to have their rectifiers and rectifier controllers meet GR-151 (central office rectifier requirements), GR-221 (rectifier controller microprocessor requirements), GR-947 (switchmode rectifiers), GR-1515 (VRLA thermal runaway detection and control, which is called for by the Fire Codes), GR-3108 (temperature-hardening for outdoor application environments), TR-NWT-000154 (central office power plant control and distribution equipment), and TA-NWT-000406 (very small dc power systems). Telecommunication rectifiers used in buildings not owned by the telecommunications company are typically safety-tested and listed to at least UL 60950-1. Telecommunication operators wanting to specify higher-efficiency rectifiers in order to save money on the electric bill and reduce carbon emissions may require compliance with ANSI/ATIS-0600015.04.2010 and/ or the US EPA Energy Star Uninterruptible Power Supplies specification (while specifically using UPS in the title, the specification also covers dc power system rectifiers). In addition to the traditional ISO 9001 quality manufacturing standards required by most users, many telecommunications companies also require manufacturing compliance to the additional telecommunications specific quality requirements of TL9000 [B31].

8.  Distribution system 8.1  System layout 8.1.1 Electrical The dc power system layout and connection depend on the site design criteria. The chargers, batteries, and loads can be connected at various points. Fcodes cFor example: — In substations, chargers can be directly connected to battery terminals, to the load side of battery disconnect switch (if one exists), or to a dc panel branch circuit. Batteries can also be directly connected to the dc panel(s) main lugs, dc panel branch circuit, etc., with or without a disconnect switch. — In power generation applications, all dc components are typically connected to a common dc distribution panel. 30

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

The layout would also affect the reliability or service of the dc power system. For example, if the charger is directly connected to the battery side of the disconnect switch, it could be considered a reliable method of charging the battery, since there are minimal points of failure in between the charger and battery. However, since the charger also serves to supply power to continuous loads under normal operation, a fault on the battery or removal of the battery for replacement (by opening the battery disconnect switch) may disconnect the charger from the loads. 8.1.1.1  Power generation The optimum dc supply selection and distribution system design for any given plant is based on the design criteria established for that plant. Figure 2 is the key diagram for a typical 125 V dc power system.

NOTE 1—All breakers are normally closed except those marked “N.O.” NOTE 2—Optional or alternate features indicated by --------. NOTE 3—Fuses may be substituted for breakers. NOTE 4—Diagram is not meant to depict redundancy.

Figure 2—125 V dc power system key diagram If replacement of equipment is required in the future without performing a major shutdown of loads, two distribution panels or accommodation for connecting alternate connection point to the dc bus would be required. This also applies to systems that require spare, redundant, or temporary system to be connected. For critical facilities, a dual-dc power system design may be considered to help ensure compliance or reliability requirements. Under normal operation, redundant dc power systems are meant to be operated in isolation from one another. In the case that one dc power system fails, the other shall remain available in order to maintain continuity of power supply to critical loads in spite of a contingency scenario.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

Figure 3 is the typical diagram for a class 1E dc power system.

NOTE 1—All breakers are normally open except those marked “N.O.” NOTE 2—Fuses may be substituted for breakers. NOTE 3—Optional or alternate indicated by --------. NOTE 4—Class 1E refers to nuclear safety related dc power systems. NOTE 5—Div. 1 and div. 2 refer to redundant loads required in nuclear systems.

Figure 3—125 V Class 1E dc power system key diagram 8.1.1.2 Substation It is preferable that all dc components would be connected to a dc distribution panel. In redundant dc power systems, this distribution panel can be fed from another dc source through a tie overcurrent protective device (OCPD). For additional guidance, refer to IEEE Std 1818. 8.1.1.3 Telecommunication DC plants for central offices typically are equipped with two or more rectifiers based on the load, capacity to recharge the battery, and N+1 redundancy as a maintenance spare. The battery often consists of two or more parallel battery strings sized to carry the load for a specified reserve time within the operating voltage window of the telecommunications systems. The overcurrent protection (OCP) system is made up of primary bays that in turn feed secondary distribution bays [generally called battery distributing fuse bays (BDFB), breaker bays (BDCBB), power distributing (PD) bays, or similar terms] (see Figure 4). These secondary distribution bays feed the telecommunications bays, cabinets, or relay racks by way of a tertiary level of overcurrent protection (fuse panels) within bays. A microprocessor-based controller governs dc plant operation and provides monitoring and alarm reporting. The battery disconnects shown in Figure 5 are optional based on system design. The majority of grounded systems will not break the battery to ground connection. Breaking this connection is typical in non-grounded dc power systems. Figure 4 shows a typical central office dc power plant, and Figure 5 shows a typical redundant dc design.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

Figure 4—Typical central office dc power plant

Figure 5—Redundant rectifier design DC plants for small facilities, such as cell or microwave sites, typically are equipped with two or more rectifiers based on the load, capacity to recharge the battery, and n+1 redundancy as a maintenance spare (see Figure 5). The battery consists of one or more parallel battery strings sized to carry the load for a specified reserve time within the operating voltage window of the telecommunications systems. DC power distribution consists of primary panels of fuses, circuit breakers, or a combination of devices that feed the telecommunications bays, cabinets, or relay racks, usually by way of a second level of overcurrent protection (fuse panels) within bays. A microprocessor-based controller governs dc plant operation and provides monitoring and alarm reporting.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

While a larger telecommunications site may have multiple dc plants, almost all dc plants only have one set of power distribution panels or bays connected to the common dc bus to which the batteries and rectifiers are connected. From this single point, typically dual (A and B) feeds are derived to power secondary and tertiary fuse or breaker panels. Most loads are dual fed so that loss of a single feed does not disrupt service. 8.1.2 Physical Consideration needs to be given to where the dc distribution equipment is to be located. It is usually advisable to place the equipment (battery, charger, and main distribution panel) as close to the electrical load as possible, thus reducing voltage drop issues and accommodating maintenance and testing activities. DC control and instrumentation cables should be routed in separate raceways from power cables in order to reduce the impact of surges or transients. These surges and transients can be present in ac and dc power systems and/or grounding cables as a result of switching activities, lightning strikes, or undetected ground faults. 8.1.2.1  Power generation If the ac auxiliary power system is separated into two or more independent divisions with a dc power system for each ac division, then the equipment and cables for each dc power system division should be separated from the equipment and cables at the other division to the same extent as employed for the ac system. Sizing and routing of cables should take into consideration inrush currents from motor loads that occur during breaker or switch mechanism operation, and their impact on voltage drop throughout the dc power system. In any arrangement, it is recommended to run the positive and negative main cables in separate conduits so that any fault on these cables will first be polarity-to-ground before a polarity-to-polarity fault can develop. The use of nonmagnetic conduit should be considered so as to reduce the inductance of this circuit. A circuit with lower inductance will reduce the magnitude of voltage spikes generated and reflected into the dc power system when high-current load circuits (such as motors, inverters, and faults) are interrupted. In addition, a highly inductive circuit may adversely affect the performance of current-limiting fuses if utilized. A throw-over or transfer switch should be considered to allow removal of one of the batteries (and chargers) from service for maintenance purposes. Depending on the application, other standards related to cable specifics (e.g., installation, splicing, flame resistance) such as IEEE Std 383 [B9] and IEEE Std 1202 [B19] may also be required. 8.1.2.2 Substation For additional guidance refer to IEEE Std 1818. 8.1.2.3 Telecommunication In typical telecommunication sites with only a single dc plant using lead-acid batteries, the plant is typically located on the ground floor or basement because of the additional floor loading capacity provided by a slabon-grade floor to support the heavy weight requirements of lead-acid batteries. When multiple plants exist in a site, they may be located much closer to the loads they serve in order to reduce voltage drop, power losses, copper cable size, and cost. The batteries, rectifiers, and primary distribution bays are typically located in a compartmentalized “power room” for medium and larger plants, but there are now smaller distributed dc plants collocated with the loads they serve. Battery positive and negative leads should be run adjacent to each other on the same cable trays, conduits, or raceways throughout the dc distribution system to provide electromagnetic noise field cancellation.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

In typical customer premises applications (commercial customer-owned building where the telecommunications company provides the equipment), the dc plant (including its batteries) takes up a portion of a bay in the communications room of the customer, or in small applications may even be wall-mounted batteries, rectifiers, and miniature distribution. In modern remote terminal (RT ) outdoor cabinets, the dc power plant may be in the same chamber as the electronic equipment, or in an end chamber of the cabinet. The batteries are usually located in a completely separate (and separately-ventilated) compartment, generally in the lowest level of the cabinet. “Unfused” dc cabling (the cabling that connects the rectifiers, batteries, and distribution systems in parallel) within the dc power plant is typically run on separate overhead open frame cable racks in telecommunications company facilities. The primary distribution cable to the BDFB or larger runs to equipment bays (these feeds are typically fused at 100 A or greater) are typically run on a separate overhead “power” cable rack from the rest of the communications type cables in larger facilities. Secondary power distribution from the BDFB to the equipment bays or smaller runs directly from the PBDs to equipment bays (these feeds are typically fused at ground smaller than 100 A) are typically run on cable hangers or horns attached to the outside of the overhead cable rack used for the copper communications cables. DC grounding system cables are also typically run in this way (on hangers or horns attached to overhead cable rack). In huts, customer premises, and similar facilities, the dc power cable is typically run on the same overhead rack with the rest of the communications cables, but is usually segregated on the rack from the rest of the communications cables.

8.2  Distribution panels The number of distribution panels is determined by the anticipated number of loads served from the dc power system and system design requirements. Consideration should be given to probable growth requirements over the expected useful life of the site as well. It may be desirable in certain cases to employ one distribution panel for each independent dc power system. A minimum of one distribution panel should be provided for each dc power system. It should be located as close as possible to the battery. The buses in the main distribution panel should be protected for personnel safety and to reduce the probability of bus faults. However, these buses must be designed to handle mechanical and electrical stresses caused by transient faults or during normal operation. Proper spacing or barrier rated for the maximum system voltage should be provided between the positive and negative line leads at the main distribution panel. The design should follow applicable safety standards and meet or exceed expected short-circuit currents. 8.2.1 Substation For additional guidance refer to IEEE Std 1818.

8.3  Available short-circuit current The maximum available short-circuit current for the dc power system is the sum of that delivered by the battery, charging system (one or more chargers in parallel), and inductive load, such as motors (as applicable). This available short-circuit current must not exceed the interrupting capacity of feeder breakers and fuses, as well as the withstand capability of the distribution buses and disconnecting devices. When a more accurate value of maximum available short-circuit current is required, the analysis should account for interconnecting cable impedance. When a fault occurs on a dc power system and depending on the impedance of the fault circuit, a substantial voltage drop can occur that could affect or disrupt operating connected loads. As a result, the overcurrent protective device (OCPD) should be selected and coordinated to minimize the dc power system low voltage and the duration caused by a fault. DC busses and panels should also meet or exceed expected short-circuit currents. Only the OCPD feeding the fault and the closest to the fault should trip, leaving the rest

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

of the system intact to continue supplying power to unaffected areas. Refer to Annex C and Annex D for details and examples of battery and charger short-circuit current contributions. 8.3.1 Batteries The short-circuit current across a battery terminal is a function of its actual voltage, internal impedance, intercell connectors impedance, and other series connected components. Note that internal impedance is significantly affected by the chemistry and internal design for all battery types. Also, the rise time when a short-circuit occurs is relatively fast. If connecting multiple battery strings in parallel, the fault current contribution from each string needs to be considered. It is recommended to contact the battery manufacturer for the maximum short-circuit current value. Refer to Annex C for calculations. As an example, the fault current from a large vented lead-acid storage battery resulting from a bolted short at the battery terminals will typically exhibit a rate-of-rise that delivers the peak current within 17 ms. The fault current for a short at the dc distribution switchgear or panelboard peaks later (typically within 34 ms to 50 ms) depending on the cable size and layout, due to the inductance of the dc power system in series with the fault. The magnitude of the fault current for a short at the distribution bus will also be lower than the value at the battery due to the resistance of the cables between the battery terminals and the bus. 8.3.2 Chargers The available short-circuit current at the charger output depends on the charger design topology. Two predominant designs are used: — Low-frequency rectifying chargers such as SCR, ferro-resonant, and magnetic amplifier — High-frequency rectifying chargers commonly called switched mode chargers Tests on current-limited low-frequency battery chargers have shown that the initial short-circuit current contributed from the battery charger can exceed the current-limited value. A large transient current spike may occur during a certain time before the charger current limit mode activates or a protective device opens. Whether the battery charger is isolated from the battery or is operating in parallel with the battery can change the response and contribution to the fault current. The following considerations should be taken in account: a)

The stored energy in filter circuits (capacitors): This instantaneous peak short-circuit current may approach a value 200 times the charger rated current. However, the time duration of the initial transient current is short (in the order of 2 ms due to the inter connections impedance) and generally does not affect the ratings of equipment and protective devices.

b)

The rectifier and the upstream circuit impedance: After the stored energy in the filter circuits is dissipated, the magnitude of the transient short-circuit current is dependent on the L/R ratio of the ac supply as well as the inductance and resistance of the transformer-rectifier-fault circuit (rectifiers, filter inductors, shunt, and other series components).

c)

Response time to current limit: This time is typically 300 ms. for a controlled SCR type charger, and the transient current amplitude is typically 10 to 12 times the rating of the charger. The time to current limit for a controlled ferro resonant charger is less due to its inherent design. In all cases, consult the battery charger’s manufacturer for short-circuit capabilities, response to a short-circuit, and time to current limit under short-circuit conditions. The battery charger’s time to current limit under a shortcircuit condition should be considered along with the impedance and L/R ratio of the fault circuit when selecting and coordinating OCPDs.

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

High-frequency battery chargers behave in a different manner. Because of the high-frequency controllers, current limiting and voltage regulation are relatively faster, so their contribution in the fault is usually limited to no more than 150% of the rated charger output current. Generally, at the distribution panel charger connection, external cable gauge and length may reduce the fault contribution to levels where charger internal protection may not operate. In all cases, consult the charger’s manufacturer for the charger time constant and its short-circuit capabilities at its output terminals. It may be appropriate to select a battery charger technology with higher short-circuit current capabilities if needed to more efficiently trip the protection device (OCPD) closest to the fault. 8.3.3  Other equipment Operating inductive loads, such as dc motors, contribute to the total fault current. The maximum current that a dc motor delivers to a short-circuit at its terminals is limited by the effective transient armature resistance (Rd') of the motor. For dc motors of the type, speed, voltage, and size is typically used in generating stations. Rd' is in the range of 0.1 to 0.15 per unit. Thus, the maximum fault current for a short at the motor terminals typically ranges from seven to ten times the motor’s rated armature current. Therefore, it is conservative to estimate the maximum current that a motor contributes to a fault as ten times the motor’s rated full load current. When a more accurate value is required, the short-circuit contribution should be calculated using specific Rd' data for the specific motor, or actual test data should be obtained from the motor manufacturer. For additional accuracy, the calculation should account for the resistance of the cables between the motor and the fault.

8.4  Protective device description and rating Selective protection coordination of the OCPDs in dc power system components is of the utmost importance. In a fault condition, the closest OCPD to the fault should trip first to isolate the fault so that less equipment is affected. An OCPD may have different ratings for ac and dc applications. In dc application, it is highly recommended to only use dc rated OCPD. Although empirical methods can be found to convert OCPD ac characteristics to dc equivalent values, it is recommended that the OCPDs and associated hardware such as fuse holders, are certified to withstand the application maximum operating voltage (not nominal voltage) and current. An OCPD, such as a circuit breaker or a fuse, is recommended between the dc sources [e.g., battery, charger(s), etc.] and the rest of the connected load equipment. Associated OCPD status monitoring (e.g., breaker open relay or blown fuse indication) may be needed to provide annunciation of alarm conditions. In addition, if a highly inductive load or circuit is connected, it may adversely affect the performance of OCPDs. OCPD dc ratings are usually based on a fixed maximum time constant (L/R). All protective and disconnecting devices should be properly rated for short-circuit current and maximum operating voltage of the dc application. (e.g., float, equalize). Also, a manual isolation device is recommended between the battery terminals and the main distribution buses and panels. If more than one battery string is used, a manual isolation device is recommended between each string and the main distribution buses and panels. When using an OCPD on each string, OCPD size and characteristics should to be selected based on the worst-case scenario operation, such as if one string is disconnected and the balance of the battery is subject to carry a heavy load, or in case of a fault on the dc bus. Refer to IEEE Std 1375 for the guide of battery protection. The distribution bus and battery OCPD should have interrupting capacity or short-circuit current withstanding capability that exceeds the maximum short-circuit current available for the system voltage and ambient temperature. 37

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

The continuous current rating of the battery OCPD should be selected to accommodate the maximum sustained current in the battery duty cycle, and should have instantaneous current rating and delayed current rating (I2t) to: — Help prevent the undesired operation of the battery OCPD during the highest duty-cycle current magnitude and duration — Facilitate proper coordination with downstream OCPDs — Protect the main distribution bus and cabling When cross-tie OCPD and/or battery test-OCPDs are used, then protection and coordination for transfer inrush currents from all energy sources and short-circuit currents shall be considered. It is important to understand that there is a difference between protection and coordination (see IEEE Standards Dictionary Online). 8.4.1  Power generation For the battery to meet the demands of momentary or random loads (see definition in IEEE standards such as IEEE Std 485, IEEE Std 1115, etc.), the protective device’s interrupting rating should be sized adequately. Consult the battery manufacturer for ratings for discharge duration less than 1 min. The main protective device should coordinate with all downstream protective devices. The distribution bus and any manual disconnecting device should have a short-circuit current withstand capability (i.e., bracing) that exceeds the maximum short-circuit current available. NOTE—For grounded systems, protective devices should be coordinated. For ungrounded systems, these devices may not be fully coordinated; however, the devices should be selected to protect the associated infrastructure, including cables.

8.4.2 Substation Same as 8.4.1. 8.4.3 Telecommunication Same as 8.4.1.

8.5  Voltage ratings for loads A confirmation that each component powered by dc power systems can operate without damage over the system voltage window (e.g., from equalize to the final end-of-discharge voltage) at the location of its input terminals should be done. Voltage drop due to large loads, such as motor starting and charging capacitor inrush, may result in a momentary voltage dip between the terminals of the battery and the terminals of the dc loads, and should be addressed. Ensure that cables are sized to reduce excessive voltage drop during transitory load conditions. External transient events such as lightning and transmission faults affecting the ground grid should be considered as applicable; control cable shielding, surge protective devices and filters may need to be employed. Standards such as IEEE Std C37.90.1 [B23] should be consulted for more information about external transients. 8.5.1  Power generation Equipment specifications for components powered by dc power systems should require the equipment to operate as designed and without damage over the input terminal voltage window corresponding to the variation in system voltage. For designs in which the battery is equalized while connected to the load, this range should

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IEEE Std 946-2020 IEEE Recommended Practice for the Design of DC Power Systems for Stationary Applications

cover the variation from equalization to the final end-of-discharge voltage (e.g., from 140 V dc to 105 V dc for a nominal 125 V dc power system, or from 280 V to 210 V dc for a nominal 250 V dc power system). This is representative of the system operating voltage window. Table 2 provides the recommended voltage range of some (typical) dc powered components for those designs in which the battery is equalized while connected to the load. Note that the dc voltage ratings of components may not have a plus or minus 10% tolerance that is typical of ac rated components. Table 2—Recommended voltage range of 125 V dc and 250 V dc (nominally rated components for designs in which the battery is equalized while connected to the load; voltages do not include transient events) Voltage range

Component

125 V dc (nominal)

250 V dc (nominal)

Circuit-breaker close coil

90–140

180–280

Circuit-breaker trip coil

70–140

140–280

Motor-starter coil

90–140

180–280

Solenoid valve

90–140

180–280

Valve-operator motor

90–140

180–280

Auxiliary motor

100–140

200–280

Electromechanical relay coil

100–140

200–280

Solid-state relay

100–140

200–280

Instrumentation including protection relays

100–140

200–280

Indication lamp

100–140

200–280

For ungrounded dc power systems, external transients, such as lightning and transmission faults affecting the ground grid, may result in significant voltage increases, line to chassis. In addition to ampacity considerations, supply cables for dc-powered components should be sized to provide adequate voltage for proper operation during the individual component’s worst-case operating condition. The worst-case condition for a constant power load, such as an inverter, may occur at a reduced battery-terminal voltage in which case the load current increases. A dc valve-operator motor may have a locked-rotor (starting) current of four to ten times the rated full-load current; therefore, voltage drop during the starting of the valve operator motor is typically the worst-case condition. In addition, the voltage drops in four (rather than two) feeder conductors (from starter to motor) should be included in the total voltage drop, due to the necessity of switching the series field when reversing the valve motor. For small horsepower motors, the voltage drop across the thermal-overload relay element may be significant and should be considered in the cable sizing. 8.5.2 Substation Same as power generation. Additional guidance on cables and dc power system can be found in IEEE Std 525 and IEEE Std 1818. 8.5.3 Telecommunication Equipment specifications for components powered by dc power systems should require the equipment to operate, as designed and without damage, over the input terminal voltage range corresponding to ANSI/ATIS 0600315.2013 [B2] and ANSI/ATIS 0600315.01.2015 [B3].

8.6  Qualification Distribution equipment should be qualified for the application.

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8.6.1  Power generation For Class 1E nuclear applications, distribution equipment shall be qualified in accordance with IEEE Std 649 (as applicable). Cable, field splices, and connections shall be qualified in accordance with IEEE Std 383 [B9]. 8.6.2 Telecom Telecom distribution systems shall be designed to meet NEBS compliance.

9.  DC power system instrumentation, controls, and alarms 9.1 General Control devices, instrumentation, and alarms should be provided in order to both enable monitoring and control of the dc power system (e.g., voltages, currents, OCPD operation, temperatures), and to annunciate during abnormal conditions. Figure 6 is a one-line diagram showing the recommended instrumentation and alarms for the 125 V dc power system shown in Figure 2 and Figure 3. The recommended instruments, controls, alarms, and their locations are described in Table 3.

Figure 6—125 V dc power system instrumentation and alarms

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Table 3—Instruments, controls, and alarms Location Instrument/alarm/control

Main control room (MCR)

Local

Battery current (ammeter, charge/discharge)

 

Xa

Battery charger output current (ammeter)

 

X

DC bus voltage (voltmeter)

X

X

Battery charger output voltage (voltmeter)

 

X

Ground detector

 

X

DC bus undervoltage alarm

Xb

 

DC power system ground alarm

c

X

 

Battery breaker/switch open alarm

Xb

 

Battery-charger output-breaker open alarm

X

b

 

Battery-charger dc output failure alarm

X

b

 

Cross-tie breaker closed alarm

Xc

 

Battery-charger ac power failure alarm

X

b

 

Charger low dc voltage alarm

Xb

 

Charger high dc voltage shutdown relay (opens main ac supply breaker to the charger)

 

X

Battery test breaker closed alarm

Xc

 

a Hall-effect instrument, or a jack (connected to the battery ammeter shunt) for use with a portable test instrument (microvoltmeter) may be provided to read battery charging current, and thus determine the state of charge as described in IEEE Std 450. Other methods for measuring float current may be used. b These alarms should not be combined with others. c These alarms may be tied to a common alarm. Common alarm may not reflect a less important event and has to be resolved.

To confirm proper battery voltage, it is important to monitor the voltage specifically at the battery terminals, rather than at the output terminals of the charger or throughout the dc distribution system. Additional instrumentation may be considered to determine the battery capability for trending purposes. Refer to applicable IEEE standards (e.g., IEEE Std 450, IEEE Std 1106, IEEE Std 1188) to identify which parameters to monitor (e.g., float current, temperature). For details on how to monitor these parameters, refer to IEEE Std 1491. 9.1.1  Power generation Cybersecurity concerns and access control must be considered when determining whether or not to employ remote operation of dc power system components. Figure 6 shows a typical 125 V dc power system instrumentation and alarms. The controls for the battery should be principally located in the battery area. All switching associated with the battery system should be performed at the local equipment. Careful considerations should be made when using remote controls. It is recommended that in new dc power systems all alarms should be individually reported. Additional instrumentation (e.g., battery monitoring systems, float current monitoring, AH metering) may be incorporated into the dc power system design as needed to improve its reliability.

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9.1.2 Substation Cybersecurity concerns and access control must be considered when determining whether or not to employ remote operation of dc power system components. As a minimum, the following alarms and readings should be remotely monitored: dc bus voltage, dc bus low-voltage alarm, battery current (float, charge/discharge), breaker closed alarm for test/service battery (if applicable), and ground detection. The following alarms and readings may be remotely monitored: charger output current, dc bus voltage, charger output voltage, battery OCPD open alarm, charger OCPDs open alarm, charger dc output failure alarm, crosstie breaker closed alarm (if applicable), charger ac power failure alarm, charger low dc voltage alarm, charger high dc voltage shutdown relay (opens main ac supply breaker to the charger), and ambient temperature. Transmission entities might have additional requirements imposed by transmission control agencies or other governmental entities, such as NERC in the US. 9.1.3 Telecommunication The typical controller used in telecommunications dc plants is a microprocessor device that governs the power plant and provides alarm, monitor, and control functionality. Some controllers also provide features such as thermal management to protect VRLA battery cells from thermal runaway. The controller uses Form C contacts for local audio/visual annunciators as well as points for local site monitoring equipment. Many controllers also provide graphical user interface (GUI) and SNMP access for remote monitoring through a network connection. Figure 7 shows a typical dc plant controller, while Table 4 shows typical telecom instruments and alarms.

Figure 7—DC plant controller (typical) Table 4 lists typical instruments and alarms.

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Table 4—Instruments and alarms Location

Instrument/alarm/control

Controller local

Remote

Load discharge current (ammeter, discharge)

X

X

DC bus voltage (voltmeter)

X

X

Individual rectifier output current

X

X

Thermal management (VRLA)

X

 

Load sharing/energy management

 

 

Feeder drain monitoring (overcurrent protection > 100 A [typical])

X

X

Form C Contacts

X

Critical alarm/major alarm

X

X

Minor alarm

X

X

AC fail/multiple ac fail alarm

X

X

Battery on discharge alarm (BOD)

X

X

Low-voltage alarm (may be one or two voltage levels)

X

X

Distribution alarm

X

X

Discharge fuse alarm (DFA)

X

X

High-voltage alarm (HV)

X

X

High-voltage shutdown alarm (HVSD)

X

X

Rectifier fail alarm (RFA)/multiple rectifier alarm (MRFA)

X

X

More than one rectifier fail (critical)

X

X

Alarm battery supply fuse fail (ABS) (if applicable)

X

X

Alarms

Most alarms in a telecommunications dc plant are organized into two master alarms: MAJOR and MINOR, which have their own Form C contacts. This does not preclude any alarm from having its own Form C contact.

9.2  DC power system ground detection Depending on the application design and requirements, dc power systems can be designed to operate as grounded or ungrounded (floating) systems. Low-voltage communications and back-up generator circuits usually have one pole grounded and do not require ground detection. Refer to Annex E for discussion about the effect of unintentional grounds on the operation of dc power system. 9.2.1  Power generation In typical power generation, a dc power system is designed as an ungrounded system instead of a grounded system so that a low-resistance ground fault on one of its two polarities cannot affect the operation of the system, thus increasing system reliability and continuity of service. Negatively grounded systems are used in older generation wireless communication. Positive and negative grounded systems do not require ground detection circuits. A grounded system design may be used if there is a desire to isolate low-resistance ground faults by means of protective devices. While protected for single line-to-ground faults, ungrounded systems are sensitive to relative changes in the grounding plane as a result of storm activity or transmission line faults. Also, when a ground occurs, the voltages to ground in the system adjust and the capacitive charges redistribute. Sensitive relays have been

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known to energize momentarily, while the cable and capacitive charge to ground shifts. Electronic loads, such as inverters and protective relays with instantaneous undervoltage trips, have operated erroneously. Sufficient redundancy and confirmation signals should be used in any logic which is dependent on the voltage of the dc power system. Many relays with dc coils have low dropout voltages and are not affected by the momentary line voltage dips. Ground detection should be provided for an ungrounded dc power system. It is recommended that ground fault resistance magnitude also be monitored so as to lessen the likelihood of a lowresistance (polarity-to-polarity) fault caused by multiple grounds occurring and affecting the operation of the dc load(s). A symmetrical deterioration occurs when the insulation resistance of all conductors in the affected system decreases at a similar rate. An asymmetrical deterioration occurs when the insulation resistance of one conductor decreases substantially more than that of another conductor. Insulation deterioration may lead to a leakage current to ground. Symmetrical and asymmetrical deterioration of insulation shall be checked. Multiple high resistive grounds are frequently present on a distribution system. A ground detection system that actively and continuously evaluates both the positive and the negative leg of the dc power system is preferred. Figure 8 shows a balanced ground detection design for an ungrounded system. A galvanometer or milliammeter provides indication and recording capability. The ground detector should provide a high polarity-toground resistance so that a single ground occurring on the system can not affect the operation of that system. Consideration should be given to the individual load (device) characteristics to determine the magnitude of ground resistance that could initiate operation of normally de-energized loads or inhibit drop-out of normally energized loads. A conservative approach to determine the ground detector alarm setpoint for a dc power system is to measure the normal ground leakage current of the system and set the ground detector to alarm at that value plus a margin to be determined by engineering judgment. This approach should result in a very sensitive ground detection system that alarms on a high resistance ground. A suitable, less sensitive ground detector alarm setpoint may be determined by the method provided in Annex E.

NOTE—Resistance value may change with different application.

Figure 8—Typical ground detection for an ungrounded dc power system

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NOTE—High resistance to ground used in some ground detector designs implies minimal and balanced current leakage to ground, so the floating dc power system is not 100% floating. Strong consideration might be needed when multiple detectors are used (e.g., in a redundant configuration, one in each parallel charger and one on the common dc bus) to reduce nuisance alarms or sporadic system tripping since all resistor networks will be connected in parallel resulting in an increased leakage current to ground.

9.2.2 Substation Same as power generation. 9.2.3 Telecom Traditional telecommunication systems are positive grounded, e.g., –48 V dc. Negative grounded systems are sometimes used in wireless telecommunication systems, e.g., +24 V dc. Neither positive nor negative grounded systems require ground detection circuits.

9.3  DC bus undervoltage alarm The function of the dc bus undervoltage alarm is to alert the operator that the battery is being discharged. The dc bus undervoltage relay should be adjustable and set to alarm at a voltage slightly less than the open circuit voltage of the battery (e.g., approximately 119 V for a 58-cell, 125 V battery or 49 V for 24 cell, 48 V battery) rather than at the minimum allowable system voltage (typically 105 V for a 125 V system). This undervoltage value setting alerts the operator whenever the battery is supplying energy to the dc bus load (e.g., more load than the charger can handle), sufficiently early to take appropriate corrective action. It is also recommended that this alarm is equipped with a time delay (e.g., 1 s) to reduce nuisance alarms caused by a sudden voltage drop following an inrush on the dc bus.

9.4  Special loading considerations The following equipment characteristics and system design features should be given consideration when sizing and selecting equipment. 9.4.1  Load transfers If the dc power system design is such that a load group can be automatically or manually transferred to another dc source during equalizing, testing, etc., that source (battery, charger, and distribution equipment) should be sized to supply both (original plus transferred) load groups. This would be typical of a system with a cross-tie feature. When load transferring from one system to another, the voltage window of both must be compatible. 9.4.2  Constant-power dc loads Many dc loads are constant power, especially loads that power electronics such as computer servers, inverters, and dc-dc converters. Constant power loads draw more current as the battery voltage decreases. For systems where most of the load is constant power, as many telecommunications loads are, constant power rating tables should be used when sizing the battery. If the dc load is a mixture of constant power and current loads, using constant power rating tables may result in a significant oversizing of the battery. In these instances, it may be more realistic to convert the constant power loads into constant current values by using a voltage value that approximates the average voltage during the duty cycle. If the average voltage is unknown, then we recommend using open circuit voltage of the fully charged battery.

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9.5  Design features to assist in battery testing The design features of each dc power system should provide an effective and safe means to periodically perform a capacity discharge test on each battery. Considerations should also be made to feed the dc load by another dc source, such as a parallel battery permanently connected to the dc bus or a dispatch service battery. In the latter case, a service-battery switching device, such as a normally open circuit breaker or a disconnectfused switch may be required. Figure 3 provides one configuration that allows the battery to be isolated from the dc power system for testing via a dedicated test circuit breaker in the associated distribution panel. This test breaker should be maintained in the open position during all modes of system operation, except during the battery test, with an alarm in the main control room when this breaker is closed. Cables from this test circuit breaker should be routed to a convenient location and terminated so that connection to a discharge load bank is convenient. This method (or similar means) for a safe connection of temporary discharge test cables should be provided. The design can also provide means for partial battery discharge test, where the rectifier output voltage is lowered below open circuit, but the rectifiers are not shut off to reduce the possibility that a battery failure would cause a catastrophic load failure.

9.6  Cross-tie between buses Cross-ties between dc distribution buses may be utilized to supply critical loads when a battery or charger is taken out of service for maintenance or testing. Cross-ties can also provide beneficial switching flexibility during such situations and can aid in accomplishing orderly plant shutdowns. A cross-tie to any independent dc power system is acceptable provided that the independent dc power system meets the sizing requirements of 6.2. One acceptable design provides a manually operated circuit breaker at each end of the cross-tie. The cross-tie circuit breakers should be normally open and should activate an alarm in the main control room if either is closed. Operating procedures should clearly define the operation of the cross-tie breakers. If crosstie operation results in paralleling two batteries, the duration of parallel operation should be limited to the time required for switching so as to reduce the impact that circulating currents may have on battery capacity. Increased available short-circuit current resulting from the parallel sources should also be considered.

WARNING OCPDs including circuit breakers and fuses, dc panels, and related equipment must be sized to withstand the momentary surge current flowing when two dc power systems are paralleled.

9.6.1  Power generation For Class 1E nuclear applications, a cross-tie to any independent dc power system, other than the dc power system in the redundant safety division, is acceptable only during cold shutdown or refueling modes, and only if it can be shown that the cross-tie does not impair the ability of the Class 1E dc power system to perform its safety function. In multi-unit nuclear stations, Class 1E dc power systems shall not be shared between units unless it can be shown that such sharing does not impair their ability to perform their safety functions.

10.  Protection against electrical noise, lightning, and switching surges Electrical noises, switching surges, and lightning effects should be adequately addressed for a reliable dc power system and its connected load operation. These disturbances are generally caused by lightning, inverters, dcdc converters, inductive loads switching, arcing, radio equipment, grid transients, voltage sags from large connected loads, equipment failure, relay actuation, cable crosstalk, etc. If not suppressed or filtered, they may impress voltage spikes on the dc power system of magnitudes that are dangerous for connected equipment, and in addition to damages, they can also cause reset and disturb sensitive loads.

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Precautionary techniques should be used to eliminate, reduce, and to damp out the noise and surge amplitude with surge suppressors and/or low and band pass filter circuits. Segregation of power and signal cables can also be deployed to help assuring better immunity. In ungrounded dc power systems, suppressing and filtering circuits should not establish additional conductive paths, in the power frequency range, to ground.

10.1  Electromagnetic interference (EMI), radio-frequency interference (RFI) EMI and RFI are propagated by conducted and radiated effects. Adequate equipment filtering should be considered to help assuring necessary immunity to disturbances such as: — Radiated noise, radio-frequency, electromagnetic field — Conducted disturbances induced by radio-frequency fields — Conducted and common mode disturbances For guidance refer to EPRI TR-102323 [B6], IEC 61000-4 series [B7], FCC, and ATIS 0600013 [B5].

10.2  Lightning and switching surges For applications where the power and control cables connected to the dc system are exposed to transient voltages, considerations should be made to protect electrical equipment against their damaging effects. Examples of typical disturbances are as follows: — Lightning — Voltage surges, high-frequency disturbances — Oscillatory and fast transients, and bursts generated by inductive loads and power equipment switching — Electrostatic discharges — Damped oscillatory magnetic fields — Voltage dips, short interruptions, and voltage variations Adequate surge protective devices (SPD) networks [e.g., metal oxide varistors (MOVs), avalanche diodes, spark gap protection devices, etc.] should be installed at the distribution panel level and at each connected equipment level. In addition to SPDs, twisted and shielded cables may also be needed for signaling and control circuits. For protected indoor installations, equipment should typically be able to withstand 2 kV to ground momentarily without damage, while for outdoor equipment, the level is typically 4 kV. Depending on the site conditions and needed protection, refer to standards such as IEEE C62 family on surge protection (e.g., IEEE Std C62.41.1 [B24], IEEE Std C62.41.2 [B25]), IEEE Std C37.90.1 [B23], IEEE Std 525, IEC 61000-4 series [B7], etc.

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11.  Spare equipment The need for spare equipment/components depends on system design features, as well as the criticality of the system. Other factors, such as operating experience, the availability of replacement equipment/parts, manufacturing lead time, the capabilities of performing in-house repairs, and component failure rates should be given consideration in determining the specific components that should be maintained as spare parts. For example, if the system design provides a readily available backup battery charger, the need for maintaining spare charger components could be reduced and possibly eliminated. Other considerations, such as shelf life, may make it undesirable to obtain spares at an early stage in plant life. For example, spare battery containers can typically be maintained for only one year if they are of the dry type (or three to six months if wet type) and thereafter should be charged and maintained the same as a battery in service. With an adequate design margin, the battery may be able to fulfill the service requirements with one or more cells electrically removed. The quantity of batteries provided and the capability to utilize backup battery capacity through the use of cross-tie circuits for example, may also be factors in determining the need and urgency for obtaining spare cells. In any case, an evaluation should be performed to determine the need for spare equipment based on the combination of system design features, operating requirements, etc.

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Annex A (informative)

Bibliography Bibliographical references are resources that provide additional or helpful material but do not need to be understood or used to implement this standard. Reference to these resources is made for informational use only. [B1] ANSI/ATIS 0600329.2008: Network Equipment – Earthquake Resistance.8 [B2] ANSI/ATIS 0600315.2013: Voltage Levels for DC-Powered Equipment Used in the Telecommunications Environment. [B3] ANSI/ATIS 0600315.01.2015: Voltage Levels for 380V DC-Powered Equipment Used in the Telecommunications Environment. [B4] Accredited Standards Committee C2-2017, National Electrical Safety Code® (NESC®).9 [B5] ATIS 0600013: Electromagnetic Compatibility (EMC) And Electrical Protection used for telecommunication equipment. [B6] EPRI TR-102323, Guidelines for Electromagnetic Interference Testing of Power Plant Equipment.10 [B7] IEC 61000-4 (all parts) Electromagnetic Compatibility Testing and Measurement series of publications.11 [B8] IEEE Std 336™, IEEE Recommended Practice for Installation, Inspection, and Testing for Class 1E Power, Instrumentation, and Control Equipment at Nuclear Facilities.12,13 [B9] IEEE Std 383™, IEEE Standard for Qualifying Electric Cables and Splices for Nuclear Facilities. [B10] IEEE Std 384™, IEEE Standard Criteria for Independence of Class 1E Equipment and Circuits. [B11] IEEE Std 446™, IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications (The Orange Book™). [B12] IEEE Std 535™, IEEE Standard for Qualification of Class 1E Vented Lead Acid Storage Batteries for Nuclear Power Generating Stations. [B13] IEEE Std 603™, IEEE Standard Criteria for Safety Systems for Nuclear Power Generating Stations. [B14] IEEE Std 627™, IEEE Standard for Qualification of Equipment Used in Nuclear Facilities. [B15] IEEE Std 666™, IEEE Design Guide for Electric Power Service Systems for Generating Stations.

ATIS publications are available from the Alliance for Telecommunications Industry Solutions (https://​www​.atis​.org/​). The NESC is available from the Institute of Electrical and Electronics Engineers (https://​standards​.ieee​.org/​). 10 EPRI publications are available from the Electric Power Research Institute (https://​www​.epri​.com). 11 IEC publications are available from the International Electrotechnical Commission (https://​www​.iec​.ch) and the American National Standards Institute (https://​www​.ansi​.org/​). 12 The IEEE standards or products referred to in this annex are trademarks owned by The Institute of Electrical and Electronics Engineers, Incorporated. 13 IEEE publications are available from The Institute of Electrical and Electronics Engineers (https://​standards​.ieee​.org/​). 8 9

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[B16] IEEE Std 690™, IEEE Standard for Design and Installation of Cable Systems for Class 1E Circuits in Nuclear Power Generating Stations. [B17] IEEE Std 741™, IEEE Standard for Criteria for the Protection of Class 1E Power Systems and Equipment in Nuclear Power Generating Stations. [B18] IEEE Std 1184™, IEEE Guide for Batteries for Uninterruptible Power Supply Systems. [B19] IEEE Std 1202™, IEEE Standard for Flame-Propagation Testing of Wire and Cable. [B20] IEEE Std 1881™, IEEE Standard Glossary of Stationary Battery Terminology. [B21] IEEE Std C37.16™, IEEE Standard for Preferred Ratings, Related Requirements, and Application Recommendations for Low-Voltage AC (635 V and below) and DC (3200 V and below) Power Circuit Breakers. [B22] IEEE Std C37.90™, IEEE Standard for Relays and Relay Systems Associated with Electric Power Apparatus. [B23] IEEE Std C37.90.1™, IEEE Standard for Surge Withstand Capability (SWC) Tests for Relays and Relay Systems Associated with Electric Power Apparatus. [B24] IEEE Std C62.41.1™, IEEE Guide on the Surge Environment in Low-Voltage (1000 V and less) AC Power Circuits. [B25] IEEE Std C62.41.2™, IEEE Recommended Practice on Characterization of Surges in Low-Voltage (1000 V and less) AC Power Circuits. [B26] NEMA MG 1, Motors and Generators. [B27] NEMA PE 1, Uninterruptible Power Systems. [B28] NEMA PE 7, Communication Type Battery Chargers. [B29] Stationary Battery Short-Circuit Test Report, AEI Test No. 0591-1, May 1991, Alber Engineering, Inc. [B30] Stationary Battery Short-Circuit Test Report, ATI Test No. 0792-1, July 1992, Alber Technologies, Inc. [B31] TL9000: Quality Management System (QMS). [B32] US NRC NUREG/CR-7229: Testing to Evaluate Battery and Battery Charger Short-Circuit Current Contributions to a Fault on the DC Distribution System.

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Annex B (informative)

Battery charger sizing—Sample calculations B.1 General This annex outlines the method, including sample calculations, for determining the required rating of battery chargers.

B.2 Equation The facility battery charging equipment should be sized in accordance with the following equation: ​Ic  =  K​(​(C × e)​/ t + ​IL​  ​)​​

(B.1)

Refer to 7.3 for the equation and variable definition.

B.3  Example 1: Power generation Determine the rating of the charger required for a battery where the continuous dc load is 100 A, the largest combination of non-continuous loads is 80 A; ampere-hours discharged from the VLA battery is 400 Ah with 12 h to recharge the battery (no abnormal service conditions) at an altitude of 1500 m (5000 ft) above sea level, and 10% future load growth:

C × e ​Ic​ ​  =  K​(_ ​  t    ​  + ​IL ​ ​)​​

(B.2)

Altitude de-rating factor value at 1500 m provided by the manufacturer is 1.07. ​K  =  altitude factor × future factor : K  =  1 . 07 × 1 . 10  =  1 . 18​

(B.3)

400 × 1 . 1 ​      ​   + 100)​  =  161 . 3 A​ ​Ic​ ​  =  1 . 18​(_ 12

(B.4)

To account for the largest combination of non-continuous loads, the following applies: ​I2​ ​  =  1 . 18 × ​(100 + 80)​  =  212 . 4 A​

(B.5)

Recommended charger rated output I3: ​I2 ​​  =  max​[​Ic​ ​ , ​I2​ ​]​  =  212 . 4 A​

(B.6)

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B.4  Example 2: Substation Simple calculation for an existing 150 Ah VLA battery with 12 h recharge and 25 A of constant load including the future growth. The charger is installed at 100 m (333 ft) above sea level. The load profile is not known.

150 × 1 . 1 ​Ic​ ​  =  1​(_ ​      ​   + 25)​  =  38 . 8 A :  use 40 A output charger​ 12

(B.7)

Caution Refer to 7.3 for special considerations.

B.5  Example 3: Telecommunication Some telecommunication rectifier sizes are given in nominal rated dc output amperes (e.g., 50 A, 200 A) for typical nominal 48 V dc or 24 V dc rectifiers. While some of these rectifiers may put out slightly more current (if the current limit is set to allow it) than the nominal ampere rating, the nominal rated output number is used for rectifier sizing calculations. More and more telecommunication rectifiers are now rated in constant-power watts, which means that their output varies by voltage. For sizing purposes, this number may be converted to amperes at the expected battery float voltage. For example, a 3500 W rectifier floating 24 series-connected 1.215 s.g. cells at an average of 2.20 V/cell, would be an approximately 66.3 A rectifier (3500 W/52.8 V). Or, the calculations may be done wholly in watts (see examples below). While individual telecommunication equipment shelf loads may vary more widely, overall load on a modern telecommunication dc power plant is relatively constant throughout the day and year, unless end-use equipment is added or removed. This load may vary slightly, but at nominal float voltage, the average plant current during the busiest hour of the year is typically described as an average List 1 load (Telcordia GR-513 gives more exact definitions of List 1 drains for overall dc plants and individual pieces of telecommunication equipment). List 1 drain (which can be simply described as a peak average well below the worst-case peak) is used to size rectifiers and batteries in telecommunications dc plants. Because most of the loads are constant power, and battery voltage drops during a full discharge by up to about 20%, GR-513 recommends that the total rectifier capacity be at least 20% greater than the List 1 load (most telecommunication network operators specify a minimum rectifier sizing guideline of 20% to 40% excess capacity). In addition to helping to ensure that the rectifiers can handle recharge when the batteries have been drained to the minimum operating voltage, this excess capacity sometimes helps to verify that the GR-513 requirement to recharge the batteries to 80% of capacity within 24 h is met (returning the first 80% of capacity is a near 100% efficient operation—returning the last 20% becomes less and less efficient). However, calculations must be done to prove this, and if not, then additional rectifiers may be needed. As noted in above in this document, and per Telcordia GR-513 and FCC Best Practices, multiple rectifiers in parallel are commonly used in telecommunications, designed with n+1 redundancy so that if one rectifier fails, the load is not partially on the battery. Some telecommunications companies even require n+2 redundancy in certain types of sites (such as the most critical ones, or the ones that are hardest to reach in certain climatic conditions or furthest away), or in all sites. Note that the “redundant” rectifier(s) is really whichever rectifier fails first, as all installed rectifiers are typically set up to share the load fairly equally. If one rectifier is lost, the output current of all of the remaining rectifiers that are turned on and properly set will increase to cover the capacity that was being carried by the rectifier that failed.

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Depending on the recharge time desired, typical rectifier sizing ranges from 120% to 140% minimum of the typical relatively constant current load. Both rules (n+1, and recharge) must be satisfied. The paralleled chargers in a modern telecommunication system are connected to a common microprocessor controller that has the ability to shut down and restart the rectifiers. The controller should shut down the rectifier(s) in case of a high voltage (known as high-voltage shutdown [HVSD]). Modern rectifiers used in charger systems also have their own HVSD protection scheme. Some have a maximum value set by hardware in the unit as well as a software threshold that is able to be configured by the charger system controller. The controller may turn off and restart individual rectifiers in order to help ensure that the working rectifiers are operating near the highest point on their energy efficiency curve (this is typically called energy management). The controller also has the ability to control the rectifier output voltages (usually for temperature-compensated charging of VRLA batteries based on input from one or more battery temperature sensors, or to turn down the voltage to a level that allows a partial discharge test of the batteries). Illustrating telecommunication rectifier sizing is probably best done with an example. A typical telecommunication central office –48 V dc power plant might be designed with the following inputs/standards: — 10 500 W nominal –48 V rectifiers — Plant float voltage of –52.80 V — Float busy-day peak average (List 1 drain) load of 2400 A (this includes future growth over the relevant planning horizon) — Minimum recharge sizing rule of 20% extra capacity above and beyond List 1 for this particular telecommunication company — N+1 rectifier sizing rule for this particular telecommunication company — 80% recharge within 24 h — 12 parallel strings of flooded 1680 Ah cells designed for an approximate backup time of 4 h — The 4 h rate of these particular cells to the designed minimum voltage per cell (mVpc) is 295 A, making their capacity at the 4 h rate 1180 Ah Given these inputs, the minimum number of rectifiers can now be designed. First, the dc current per rectifier for sizing purposes must be determined:

10 500 W _ ​ 

   ​  ≈  199 A​

(B.8)

52 . 8 V

To barely meet the future planning horizon List 1 load, 13 rectifiers are required (decimals must be rounded up when sizing rectifiers):

2 400 A _ ​ 

  ​   =  12 . 06​

(B.9)

199 A

Due to the N+1 reliability rule, this means that 14 rectifiers are required. This means that there is 2786 A of total rectifier capacity: ​14 × 199 A  =  2 786 A​

(B.10)

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This total rectifier capacity must now be compared against the 20% excess recharge capacity requirement and the 80% recharge within 24 h rules. In order to meet the recharge capacity rules, the projected List 1 drain is multiplied by the excess capacity factor: ​2 400 A × 1 . 2  =  2 880 A​

(B.11)

As can be seen, the minimum recharge capacity rule is going to require 15 rectifiers:

2 880 A _ ​ 

  ​   =  14 . 47​

(B.12)

199 A

This means that the total rectifier capacity is now: ​15 × 199 A  =  2 985 A​

(B.13)

The spare capacity available for recharge is: ​2 985 A − 2 400 A  =  585 A​

(B.14)

This excess capacity needs to return 80% of the capacity of a fully discharged battery plant within 24 h per the rules of this particular telecommunication company. The capacity of the battery plant is: ​12 strings × 1  180 Ah  =  14  160 Ah​

(B.15)

80% of this capacity is: ​14 160 Ah × 80 %   =  11 328 Ah​

(B.16)

The number of hours required to return 80% of the capacity of a fully discharged battery is:

11 328 Ah _ ​ 

     ​  ≈  19 h and 20 min​

(B.17)

585 A

This is less than the maximum 24 h requirement, so no additional rectifiers are needed. Because the modern telecommunications loads are primarily constant power (as has been described previously in this document) and the rectifiers themselves are rated in watts, all of the calculations from the preceding example can be done in watts. The first step is to convert the List 1 drain to watts. While technically, List 1 drains are calculated by equipment manufacturers at 52.0 V, it is more accurate with actual loads to run the calculations at the actual float voltage. Assuming constant power loads, the List 1 load in watts is: ​2 400 A × 52 . 80 V  =  126 700 W​

(B.18)

To barely meet the load, 13 rectifiers are required.

126 700 W _ ​ 

 ​    =  12 . 07 rectifiers​

(B.19)

10 500 W

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Due to the n+1 rectifier redundancy/reliability requirement, this means that 14 rectifiers are required. Total rectifier capacity is thus: ​14 × 10 500 W  =  147 000 W​

(B.20)

This total rectifier capacity must then be compared against the 20% recharge capacity rule, and the 80% recharge in 24 h rules. In order to meet the recharge capacity rules, the List 1 drain is multiplied by the excess capacity factor: ​126 700 W × 1 . 2  =  152 000 W​

(B.21)

To meet this rule, 15 rectifiers are required:

152 000 W _ ​ 

10 500 W

 ​    =  14 . 48​

(B.22)

With 15 rectifiers, the total rectifier capacity is now: ​15 × 10 500 W  =  157 500 W​

(B.23)

The spare capacity available for recharge is then: ​157 500 W − 126 700 W  =  30 800 W​

(B.24)

To determine if 80% of battery capacity can be returned within 24 h (the first 80% of lead-acid battery capacity returned is almost 100% coulombically efficient). The full capacity of all the batteries is: ​12 × 1 180 Ah × 52 . 80 V  =  747 600 Wh​

(B.25)

80% of that capacity is: ​747 600 Wh × 80 %   =  598 100 Wh​

(B.26)

The number of hours required to return 80% of the capacity, using the spare rectification is:

598 100 Wh ___________ ​ 

   ​   =  19 . 4 h​

(B.27)

30 800 W

This is less than the 24 h required by the specification, and thus no more rectification is needed to meet this final rule.

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Annex C (informative)

Battery available short-circuit current—Sample calculations C.1 Introduction The current that a battery delivers on short-circuit depends on the total resistance of the short-circuit path. The battery nominal voltage should be used when calculating the maximum short-circuit current. Tests have shown that an increase in electrolyte temperature (above 25 °C) or elevated battery terminal voltage (above nominal voltage) has no appreciable effect on the magnitude of short-circuit current delivered by a battery (see Stationary Battery Short-Circuit Test Report 0591-1 [B29] and Stationary Battery Short-Circuit Test Report 0792-1 [B30]). For Ni-Cd cells the short-circuit capability can range between 7 and 50 times the rated Ah capacity. Refer to the manufacturer for the Ni-Cd battery short-circuit capability. Although an increase in temperature results in an increase in the chemical activity of the battery, it also increases the resistance of the metallic components of the battery, thereby offsetting any appreciable change in the magnitude of short-circuit current the battery can deliver. However, the elevated temperature results in the battery’s capability to deliver the short-circuit current for a longer duration. Elevated battery terminal voltage (above nominal voltage) during float and equalize charge does not increase the chemical energy available from the battery during short-circuit. For lead-acid batteries, the effective voltage driving the short-circuit current is dependent on the acid concentration in direct contact with the active material in the plates. Therefore, the battery nominal voltage (2.00 V per cell for lead-acid and 1.2 V for NiCd) should be used when calculating the maximum short-circuit current. The fault current from a large lead-acid battery resulting from a bolted short at the battery terminals typically exhibits a rate-of-rise that delivers the peak current within typically 10 ms to 50 ms. For the actual time, consult the battery manufacturer. Refer to references [B27], [B29], and [B30].

C.2 Discussion The total resistance is made up of two major parts as follows: — The apparent internal resistance of the battery — The external circuit resistance The total internal resistance of the battery is equal to the sum of the internal resistance of the cells plus the resistances of the intercell connections. The value of internal cell resistance is a variable quantity that is significantly influenced by many factors, e.g., the temperature, the age, and the state of charge of the cell. The total external circuit resistance is the sum of the resistances of the various components, e.g., the connecting cables and the fault resistance. The following sample calculations illustrate one method of calculating the internal resistance of any cell (utilizing manufacturer’s published discharge characteristic curves for that cell) and then calculating the current that cell can deliver: — Through a bolted short-circuit at the cell terminals (i.e., zero external resistance) — Through a short-circuit at the main distribution bus with a specific (0.01) external resistance and with no charger or motor contribution 56

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C.3  Sample calculations for a lead-acid battery Sample calculations for a lead-acid battery include the following: a)

Internal resistance of a cell can be calculated from the slope of the initial volts line. See Figure C.1, which shows a discharge characteristic fan curve for a typical 7 through 15 plate (total) cell. (When fan curves are not available or other battery technologies are employed, contact the manufacturer for Rt values for specific cell types and recommended voltages to be used for short-circuit current calculations.)

Rp ​Rt  =  _ ​   ​​ Np

(C.1)

where ​ t​ R ​Rp​ ​Np​

is the total internal resistance of cell (ohms) is the resistance per positive plate (ohms) is the quantity of positive plates

​V1​ ​ − ​V2​ ​  Ω / positive plate​ ​Rp  =  ​ _ ​  ​I2​ ​ − ​I1 ​​

(C.2)

Rp is the resistance per positive plate for any two voltage and current points along the line. If we pick 1.90 V as V1, and 1.50 V as V2, I1 is found to be 60 A/positive plate and I2 is 370 A/positive plate. The calculation is given in Equation (C.3):

1 . 9 − 1 . 5 0 . 4 ​Rp  =  _ ​   ​    =  ​ _  ​  =  0 . 00129 Ω / positive plate​ 270 − 60 210

(C.3)

Assume that the cell being investigated has 15 (total) plates. Since the cell has seven positive plates (connected in parallel), the total internal resistance is:

0 . 00129     ​  =  0 . 00018 Ω​ ​Rt  =  _ ​  7 b)

(C.4)

The short-circuit current available at the cell terminals is found from Ohm’s law as follows:

Ec 2    ​   =  11 111 A​ ​  ​Ic  =  ​ _ ​   =  _ Rt 0 . 00018

(C.5)

where

c)

I​ c​ is the available short-circuit current (in amperes) ​Ec​ is the nominal cell voltage (2.00 V) ​Rt​ is the total internal resistance of cell (ohms) The short-circuit current available at the main distribution bus (the load terminals of the main/battery circuit breaker) from a battery made up of 58 cells with an average internal resistance of 0.00018 Ω as calculated in Equation (B.5), with 0.010 Ω total external resistance, and with no charger or motor contribution, is found from Ohm’s law as follows: ​Eb  =  58 cells  =  2 Vpc  =  116 V​ EB is the nominal battery voltage RB is the total internal resistance of battery: ​Rb  =  Rt per cell * number of cells  =  0 . 00018 * 58  =  0 . 01044 Ω​

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Rx is the total external circuit resistance (including resistance of main lead cables; intercell [if not included in RB], inter-tier, and inter-rack connections; main circuit breaker or fuse; and the fault) ​Rx  =  0 . 0100 Ω​ RT is the total circuit resistance ​RT  =  RB + Rx  =  0 . 01044 + 0 . 0100  =  0 . 02044 Ω​

Eb 116 ​IB  =  _ ​   ​   =  _    ​   =  5675 A​ ​  Rt 0 . 02044

(C.6)

where ​IB​

is the available short-circuit current (amperes) at load terminals of main/battery circuit breaker

The short-circuit currents calculated in Equation (B.6) may, when combined with the charger and motor contribution, be used to determine required interrupting capacity of the circuit breakers or fuses. NOTE—For comparison, 10 times the 1 min ampere rating (to 1.75 V per cell at 25 °C) of the battery is 10 × 1139 = 11 390 A.

Figure C.1—Typical lead-acid battery fan curve

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Annex D (informative)

Battery charger and dc power system, short-circuit current contribution D.1 Introduction This annex provides a rationale for the selection of the maximum value of battery charger short-circuit current that occurs coincident with the maximum battery short-circuit current. The reason for determining the maximum combined short-circuit current is to specify equipment that is suitable for the expected fault current. It is necessary to include the contributions from connected motors, battery chargers, and batteries when calculating the total short-circuit current for a fault in a dc power system.

D.2  Combined effect of currents from battery, charger, and connected inductive load The fault current for a short at the dc distribution switchgear or panel board peaks later (typically within 34 ms to 50 ms) due to the inductance of the dc power system in series with the fault. The magnitude of the fault current for a short at the distribution bus will also be lower than the value at the component due to the impedance of the cables between this component’s terminals and the bus. Due to the battery time constant, the maximum coincident short-circuit current can be conservatively calculated as the sum of the peak short-circuit current from the battery and the peak short-circuit current value from the charger (Figure D.1). Inductive loads such as dc motors, if operating, will contribute to the total fault current. The maximum current that a dc motor delivers at its terminals is limited by the effective transient armature resistance Rd of the range of 0.1 to 0.15 per unit. Thus, the maximum fault current for a fault at the motor terminals typically ranges from seven to ten times the motor’s rated armature current. Therefore, it is conservative to estimate the maximum current that a motor contributes is ten times the motor’s rated full-load current. When a more accurate value is required, the short-circuit current should be calculated using specific rd data or from test data obtained from the motor manufacturer. (IEEE Std 446 [B11]: Emergency and standby systems for industrial and commercial applications and IEEE Std 666.) Each installation should be evaluated by the design engineer to determine the magnitude of the short-circuit currents from the battery, charger, motors, etc. Any non-typical installation should be evaluated by the design engineer to verify that the peak values of the battery and charger short-circuit currents are not coincident.

D.3  Sample evaluation The following example illustrates one method for determining the relative fault-current contributions of a battery and a current-limited charger to a fault at the distribution panel bus. (Note that the following values may not be typical for any given charger type, design or installation.) Charger and feeder cables Charger current rating: I = 300 A Charger transient current: ​Ic  =  10 × 300  =  3000 A​

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Cable and breaker resistance: Rc = 0.0228 Battery and feeder cables Battery: 60 cells, rated 1950 Ah at 8 h rate. Battery resistance under fault: ​Rb  =  0 . 0001131 × 60 cells  =   0 . 006786 Ω​ Time to peak short-circuit current @ five-time constants = 11 ms Battery cables: 2-1/C 350 kcmil cables/leg, each 12.3 m (40.5 ft) Total calculated loop resistance: R2 = 0.0017 Total calculated inductance: L = 36.2 μH The battery time constant and apparent inductance under short-circuit conditions are calculated as follows:

11 ms Battery time constant: ​ T  =  ​ _     ​  =  2 . 2 ms​ 5 Battery inductance: ​Lb  =  T × Rb  =  2 . 2E − 3 × 0 . 006786  =  14 . 9 μH​ The time constant for the combination of the battery and cables is calculated as follows:

14 . 9E − 6 + 36 . 2E − 6 Lb + L ___________________    ​T1  =  ​ _   ​   =  ​      ​  =  6 ms​ 0 . 006786 + 0 . 0017 Rb + R2

(D.1)

Fault at the distribution panel bus Battery fault current:

2 Vpc * 60 120 V       ​   =  __________________ ​    ​  =  14 141 A​ ​Ib  =  _ ​  ( Rb + R2 ​ 0 . 006786 + 0 . 0017)​ Battery fault current peaks at (5) (T1) = (5) (6 ms) = 30 ms The charger short-circuit contribution: Ic = 3000 A Conclusion As illustrated in Figure D.1, the maximum total combined short-circuit current is: ​I  =  14 141 + 3 000  =  17 141 A​ 17 141 A is a maximum current when the battery current peaks at 30 ms after the fault.

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Figure D.1—Typical short-circuit current evolution at the dc bus connected to a battery in parallel with current-limited charger

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Annex E (informative)

Effect of unintentional grounds on the operation of dc power systems E.1 Introduction This annex provides guidance in determining: a) the threshold value of ground-fault resistance that may affect equipment operation if a second ground occurs in an ungrounded dc power system and b) a suitable, less sensitive, ground-detector alarm setpoint. Note that unintentional grounds in grounded systems (positively or negatively grounded) make themselves obvious by creating a quite large short-circuit event.

E.2 Discussion A single low-resistance ground on one of the two polarities of an ungrounded dc power system should not affect the operation of the system. However, a ground of sufficiently low resistance on one polarity followed by a second ground can produce ground currents of sufficient magnitude to initiate operation of de-energized dc loads (devices) or inhibit dropout of energized dc loads (devices). The grounding configuration shown in part A of Figure E.1 illustrates how multiple grounds could initiate operation of a de-energized load. This condition could result in false operation of a normally de-energized load (device).

Figure E.1—Ground faults may energize a normally de-energized device or prevent de-energizing a normally energized device The grounding configuration shown in part B of Figure E.1 could exist in a normally energized logic circuit, such as an engineered safety feature or a reactor protection system, and could inhibit the drop-out of energized loads (devices). Figure E.2 shows three ground combinations that could short out an actuating coil and/or the dc source. For low-resistance grounds, a fuse or circuit breaker would trip a circuit designed with overload and fault protection, clearing the faulted circuit and dropping out all energized loads (part B of Figure E.2), or preventing operation of de-energized equipment (part A and part C of Figure E.2) on that circuit. A fault configured as shown in part A or part C of Figure E.2 could inhibit the trip of a breaker for switchgear, generator, or a large motor. The ground configuration shown in part A or part B of Figure E.3 could cause the relay contacts associated with one piece of equipment in one circuit to affect the operation of a similarly grounded device in another circuit.

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Figure E.2—Ground faults may short out an actuating coil and/or dc source

Figure E.3—Ground faults may cause contacts in one circuit to actuate devices in a different circuit In order to determine the threshold resistance of a ground fault that, if followed by a solid ground, could initiate operation of a normally de-energized load or could inhibit dropout of a normally energized load; the most sensitive dc loads (devices) should be identified and their minimum pickup current and maximum dropout current should be evaluated.

E.3  Sample evaluations E.3.1  Example 1 (low-resistance device) The control devices for a Type XYZ switchgear breaker trip device is rated at 125 V dc. The closing coil and trip coil are as given below. Assuming a dc power system operating (float) voltage of 130 V, the current through the closing coil or trip coil having 20.83 Ω, in series with a 20 kΩ ground leakage in parallel with the control contacts (so when the control contact is the open position, the coil will be fed through the 20 kΩ ground leakage resistance) so the current as per part A of Figure E.1 would be:

130 V _______________

​       ​  =  6 . 5 mA​ 20 kΩ + 20 . 83 Ω

(E.1)

Minimum pickup closing current:

90 V _

  ​   =  4 . 32 A​ ​  20 . 83 Ω

(E.2)

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Minimum pickup tripping current:

70 V _

​    ​   =  3 . 36 A​ 20 . 83 Ω

(E.3)

(In this example we assumed that the values of 90 V dc minimum pickup closing voltage and 70 V dc minimum pickup tripping are provided by the coil data sheets.) Since the minimum pickup current for the coils is 4.32 A (closing) and 3.36 A (trip), these values are well above the value of current (6.5 mA) that would be experienced with a 20 kΩ total ground-fault resistance. Therefore, a 20 kΩ ground, followed by a solid ground, would not cause spurious operation of the switchgear breaker. The threshold value of ground-fault resistance is:

130 V _

   ​   − 20 . 83 Ω  =  17 . 86 Ω​ ​  3 . 36 A

(E.4)

neglecting the effect of the ground detector resistance. Assuming that this circuit breaker trip coil has been identified as the most sensitive (lowest pickup or dropout current) device connected to the dc power system, the ground detector alarm can be set at any value above 17.86 Ω; the appropriate margin (above the threshold value) being based on engineering judgment. A setpoint of 20 Ω will alarm at a ground fault current (3.18 A) that is 5.4% below the minimum pickup current (3.36 A). This example is for illustrative purposes only since most ground detection systems detect grounds several orders of magnitude greater than 20 Ω.

E.3.2  Example 2 (high-resistance device) The operating characteristics of a normally energized Type XYZ 125 V dc relay are as given in the following paragraph. Assuming a dc power system operating (float) voltage of 130 V, the current passing through the 2 kΩ relay coil and the 20 kΩ ground leakage resistance (in parallel with the control contacts) (part B of Figure E.1), after they open, would be:

130 V _____________ ​       ​  =  5 . 91 mA​ (​ 20 kΩ + 2 kΩ)​

(E.5)

Since the maximum dropout current for the relay is 6.25 mA, a 20 kΩ ground followed by a solid ground would be of high enough total resistance to produce a low enough ground current that would not prevent the relay from dropping out when the control contacts open. The threshold value of ground-fault resistance is (130 V/0.00625 A) – 2 000 Ω = 18 800 Ω, neglecting the effect of the ground detector resistance. Assuming that this relay has been identified as the most sensitive (lowest pickup or dropout current) device connected to the dc power system, the ground detector alarm can be set at any value above 18 800 Ω; the appropriate margin (above the threshold value) being based on engineering judgment. A setpoint of 20 kΩ will alarm at a ground fault current (5.91 mA) that is 5.4% below the maximum dropout current (6.25 mA).

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Annex F (informative)

Telecommunication-specific considerations F.1  Telecommunication qualifications Network Equipment-Building System (NEBS) is a family of Ericsson/Telcordia (Telcordia is a vestige of the Bell Labs research division of the old Bell system) documents that helps to ensure safety (similar to UL), as well as reliability at the highest levels. The documents applicable to batteries and their racks/trays/cabinets are GR-63 and SR-3580, where various flammability (the NEBS flammability requirements are based in part on UL 94, ANSI/ATIS-0600307.2007, and ANSI/ATIS-0600319.2008). Transportation, vibration, seismic (Telcordia earthquake testing requirements are more similar to the old Uniform Building Code (UBC) requirements than the International Building Code (IBC) requirements and are based in part on ANSI/ATIS0600329.2008 [B1]), and other requirements applicable to batteries are covered in GR-63, and the various levels (Level 1 being basic safety and Level 3 being the highest level of reliability) are specified in SR-3580. Most telecommunication companies require the NEBS testing (and determination of the applicability of the various NEBS requirements) are to be done by a third-party NRTL (OSHA Nationally Recognized Testing Laboratory), or at least have the tests witnessed by a third-party NRTL. In addition to the basic NEBS specifications produced by Telcordia, the telecommunications company may require the manufacturer to have their batteries tested to at least portions of GR-232 for vented lead-acid batteries, GR-4228 (which also references GR-1200 for accelerated life testing) for VRLA batteries, GR3150 for Li-based batteries, GR-3168 for NiMH batteries, and GR-3020 for Ni-Cd batteries used in outdoor cabinets. Finally, most major telecommunication companies require battery (and other telecommunication equipment) manufacturers to be ISO 9001:2008 or TL9000 [B31] (a telecommunication-specific set of manufacturing quality standards produced by the Quest Forum) certified by a registrar.

F.2  Telecommunication dc power system distribution design While a larger telecommunications site may have multiple dc plants, almost all dc plants only have one set of power distribution panels or bays connected to the common dc bus to which the batteries and rectifiers are connected. From this single point, typically dual (A and B) feeds are derived to power secondary and tertiary fuse or breaker panels. Most loads are dual fed so that loss of a single feed does not disrupt service. Figure F.1 shows a typical dc power system layout in telecommunication plant and in the information and communication technologies (ICT ) equipment room including: — Rectifiers — Batteries formed of multiple strings — Primary, secondary, and tertiary distribution panels — Interconnection cable runs between different parts of the dc power system In huts, customer premises (Customer Prems) and similar facilities, the dc power cable is typically run on the same overhead rack with the rest of the communication cables, but is usually segregated on the rack from the rest of the communications cables.

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Figure F.1—Typical telecom dc power system and distribution equipment The dc distribution panels/bays are similar to a power distribution PDU or wall-mount cabinet in a traditional ac distribution system, except they are typically full bays, although there are mini- and micro-battery distribution fuse bays (BDFB) that are smaller and mounted in a cabinet or relay rack. The miscellaneous fuse or breaker panels typically sit at the top of each bay/relay rack to feed the individual equipment shelves. In smaller sites, the BDFBs may not exist. In addition, some shelves are fed directly from the power boards (PBDs) of the main dc plant, or directly from a BDFB, bypassing the miscellaneous fuse/breaker panel.

F.3  Telecommunication power plant location In typical telecommunication sites with only a single dc plant using lead-acid batteries, the plant is typically located on the ground floor or basement because of the additional floor loading capacity provided by a slab-ongrade floor to support the heavy weight requirements of lead-acid batteries. When multiple plants exist in a site, they may be located much closer to the loads they serve to reduce voltage drop, power losses, and minimize copper cable size and cost. The batteries, rectifiers, and primary distribution bays are typically located in a compartmentalized “power room” for medium and larger plants, but there are now smaller distributed dc plants collocated with the loads they serve. In typical customer premises applications (commercial customer-owned building where the telecommunications company provides the equipment), the dc plant (including its batteries) takes up a portion of a bay in the communications room of the customer, or in small applications may even be wall-mounted batteries, rectifiers, and miniature distribution. In modern remote terminal (RT ) outdoor cabinets, the dc power plant may be in the same chamber as the electronic equipment, or in an end chamber of the cabinet. The batteries are usually located in a completely separate (and separately-ventilated) compartment, generally in the lowest level of the cabinet.

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F.4  Telecommunication battery sizing considerations Most telecommunication dc plants designed for three or more hours of battery reserve use a battery designed for long-duration discharge. If the site is powered by renewable energy, or has poor grid quality, the cycling duty of the battery should be considered for battery type selection. Battery sizing in telecommunication typically de-rates from the constant-current manufacturer battery capacity tables due to generally decreasing capacity as the battery ages. Since the generally accepted knee of the life curve of a lead-acid battery is 80% capacity, many users simply de-rate the battery capacity from manufacturer tables by 20%. In a larger site, there may be many paralleled battery strings in a dc plant of differing ages, so the deration might be averaged to be less then 20%. Typical nominal dc system bus voltages are +24 V, –24 V, and –48 V. There are a few other rarer types, such as +48 V, –130 V, + 130 V, +140 V, 220 V, 380 V, and 575 V (the latter three are relatively new dc architectures for data centers, with limited deployments so far). Batteries are placed in series in a string to achieve the desired float voltage (for example, –52.80 V is typical for 24 cell strings of 1.215 s.g. lead-calcium vented lead-acid cells, and –54.48 V might be more typical for 38 cell strings of Ni-Cd batteries). For more advanced batteries (such as larger format Li-ion batteries), the entire battery module typically operates near the nominal plant voltage (while a typical Li-ion cell has an operating voltage of about 4 V, most Li-ion battery blocks sold into telecommunication markets come pre-packaged in a box with multiple internally series-paralleled cells and a battery management system that accepts a telecommunication charger float voltage of –52.08 V, –52.21 V, –52.80 V, –53.52 V, –54.00 V, or –54.50 V, for example). The maximum allowable float voltage of a plant is typically determined by the lowest maximum operating voltage of all the connected loads, minus a volt or so to help ensure that any fluctuations do not cause problems (guidelines for telecommunication equipment voltage operating windows are found in ANSI/ATIS-0600315 [B2]). Battery strings (or the more advanced modules) are typically paralleled to achieve the desired hour reserve based on the de-rated constant current capacity of the string from the manufacturers’ tables. For reliability reasons (especially with VRLA batteries), in general, even if a single string would meet the capacity requirements, the telecommunication power engineer may put in more than one string. For example, if the design load is 200 A and the desired reserve time is 8 h, two strings with de-rated 8 h rates (to the desired minimum volts per cell or mVpc, or minimum plant voltage) of 100 A apiece could be used. This approach means that loss of a string due to an open cell or connector significantly reduces the available reserve time (to less than half of the design), but there may still be plenty of reserve time. The mVpc for multi-cell strings in telecommunication applications is determined by taking the highest minimum operating voltage of the many pieces of load equipment, adding the designed voltage drop of the cabling between the batteries and the loads (which is typically no more than a volt or two for the low-voltage nominal 24 V and 48 V plants), then dividing by the number of cells in a series string. The design load is determined by taking the float load and increasing it by either the maximum current at the end voltage of the plant (the aforementioned highest connected load minimum operating voltage), or increasing it to a current that would be found in the middle of the discharge window. For example, if the designed reserve were 4 h, the current of a constant power load at a plant voltage 2 h into the discharge curve might be used. Illustrating telecommunication battery sizing is probably best done with an example. A typical telecommunication central office –48 V dc power plant might be designed with the following inputs/standards: — Lead-calcium long-duration flooded 1680 Ah cells with 1.215 s.g. from manufacturer XYZ — Plant float voltage of –52.80 V (average of 2.20 V/cell for a 24-cell string) — Equipment voltage operating window of –42.75 V to –56.00 V — 4 h minimum of designed battery reserve — Float busy-day peak average (known as List 1 drain in telecommunication power lingo, per Telcordia GR-513) load of 2400 A (this includes future growth over the relevant planning horizon)

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— Designed voltage drop from the battery terminals to the load during discharge of 2.0 V loop (combined voltage drop of the battery and return cables) — Battery capacity de-rating for end-of-life (80% of manufacturer’s listed capacity) — 100% constant-power loads assumed — Average discharge voltage of 47.50 V (2 h into the rated 4 h discharge) Given these inputs, the battery reserve can now be designed. First, the mVpc must be determined. Adding the designed voltage drop to the minimum equipment operating voltage, and dividing by 24 yields: ​42 . 75 + 2 . 0  V  =  44 . 75  V​

(F.1)

44 . 50 V _

​   ​    =  1 . 865 Vpc​ 24 cells

(F.2)

Because battery manufacturers do not have a table describing battery capacity at 1.865 V/cell, rounding to the nearest table value of 1.86 V/cell is the prudent choice. An end-of-life de-rating (multiply by 80%) must then be applied to the 4 h rate for the 1.86 mVpc for 1680 Ah 1.215 s.g. lead-calcium long-duration cells from manufacturer XYZ. Manufacturer XYZ lists a 100% current capacity figure of 295 A at the 4 h rate for their 1680 Ah 1.215 s.g. lead-calcium long duration battery for an mVpc of 1.86 V. De-rating this yields: ​295 A × 80 %   =  236 A / string​

(F.3)

The next step is to determine the average discharge load. Because the loads are constant power, and we know the float voltage load, we can calculate the average discharge load, given the average discharge voltage of 47.50 V. ​2 400 A × 52 . 80 V  =  126 720 W​

(F.4)

126 720 W _ ​ 

   ​   =  2 668 A​

(F.5)

47 . 50 V

Knowing the de-rated string capacity, and the future protected constant power average load, the number of strings of 1680 Ah 1.215 specific gravity flooded lead-calcium batteries can now be computed:

2 668 A ____________

  ​   =  11 . 3 strings​ ​  236 A / string

(F.6)

Rounding up to help ensure adequate capacity for the future load yields a result of 12 parallel strings of 1680 Ah flooded lead-calcium long-duration 1.215 s.g. battery strings.

F.5  Filter of charging/rectification equipment The good output filtering circuitry found on telecommunication rectifiers was previously referred to as “battery eliminator” circuitry, since the rectifiers could be feed telecommunication voice circuit equipment with no connected battery. Many rectifiers even have high ripple pulldown (HRPD) circuitry that senses when the filters are failing and shut themselves down to keep from introducing ripple to the loads. While technology has changed (analog plain old telephone service or POTS is slowly dying), thus lessening the need

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to filter the low-frequency ripple as strictly in telecommunication plants, the legacy has carried through, and telecommunication rectifiers remain well filtered, producing very little ultimate dc bus output ripple in either the lower frequency or higher frequency bands. While this may no longer be necessary for many of the loads, it is definitely beneficial to the batteries since they are not used as primary capacitive filters.

F.6  Telecommunication rectifier/charger controls and signals The paralleled chargers in a modern telecommunication system are connected to a common microprocessor controller that has the ability to shut down and restart the rectifiers (typically only in case of a runaway high voltage in an individual rectifier, or for energy management purposes in order to make the rectifiers left on run at their most economical loading percentage). This type of installation also has the ability to control the rectifier output voltages (usually for temperature compensated charging of VRLA batteries based on input from one or more battery temperature sensors, or to turn down the voltage to a level that allows a partial discharge test of the batteries). Many of the charger system controllers have external voltage sense that monitors the dc battery bus. This provides the ability to control the rectifier output voltages for voltage regulation as well as for temperature compensated charging of VRLA or other battery types based on input from one or more battery temperature sensors. Features such as lowering the dc voltage to a level that allows a partial discharge test of the batteries can be achieved. The rectifier controller can set the output voltage of the rectifiers based on internal sense, or external sense. Internal sense is when the output voltage setting is matched to the common “hot” dc output bus voltage in that bay or shelf relative to the grounded return bus in that bay or shelf. This is commonly used in small dc plants where the batteries are very near the rectifiers. In larger dc plants where the batteries are a little further away, there is usually more voltage drop between the rectifiers and batteries, and it is more important to charge the batteries based on the voltage at the batteries. In those cases, external sense is used. A pair of external sense wires are run to the positive and negative battery termination buses above the battery stand(s), and the rectifier output voltage(s) are adjusted to provide the float voltage set in the controller so that voltage is produced at those “remote” battery term buses. As partially described above, telecommunication rectifiers need to generally be capable of having their output voltage controlled (typically in the range of 100% to 117% of the nominal plant voltage) by the dc plant controller in order to do the following: — Have the proper float voltage setting for the various types of batteries and number of series-connected cells that could be used. — Temperature-compensated charging of VRLA batteries. — To adjust their voltage in order to meet the required voltage at the remote sense points. This adjustment is load dependent, as the changing current changes the voltage drop between the rectifiers and the remote voltage sense points nearer the batteries. — Potential partial discharge tests (where the rectifier output voltage is lowered below open circuit, but the rectifiers are not shut off to reduce the possibility that a battery failure would cause a catastrophic load failure).

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Annex G (informative)

Load sharing of chargers Operating considerations such as increased power, reliability, and serviceability may dictate that more than one charger is connected in parallel on the same dc bus. Active and passive load sharing can be available: — Active load sharing mode: In this mode, each charger controller actively regulates its output voltage based on its output current in order that the load current is equally shared between parallel connected chargers. Load sharing between two parallel chargers can be achieved as follows: — Load sharing control through a common connection control cable. — Independent load sharing control with no control wire connection between the two parallel chargers. — Passive load sharing mode: In this mode, the load-current is passively split between chargers based on each charger output voltage static settings. Active load sharing is normally only done with chargers of the same brand, model, and output rating. In telecommunications systems where, controlled energy management is used, some rectifiers may be shut down (or placed in “hot” standby) to make the rest of the rectifiers load-share at a level that achieves the highest energy conversion efficiency.

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Annex H (informative)

Center tapped battery design considerations For ungrounded 250 V systems that also supply 125 V dc components in a split battery arrangement. Some installations may utilize a center-tapped design where the battery is separated into two sections. The battery can be used as a single higher voltage dc power system for larger voltage loads, (e.g., a 250 V dc configuration), or separated into two lower voltage dc power systems for a control system or other loads (e.g., 125 V dc configurations). When using a center-tapped design, each half of the battery shall have its own charger(s). Care must be taken to help ensure that the battery is properly sized to carry any and all loads for the anticipated cycle of all elements. Since any bus could become grounded, the 125 V components, including any surge protection and filters, should be capable of withstanding the full system voltage at equalize. Refer to Figure H.1.

Figure H.1—Typical center-tap battery, chargers, and load connections

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Annex I (informative)

Additional batteries in nuclear power generation applications For Class 1E nuclear applications, as a minimum, a separate battery shall be provided for each Engineered Safety Feature (ESF) Division in each unit in order to provide the required independence between redundant Class 1E power systems. For increased operating flexibility in designs where the reactor protection system channels are dependent on dc, the number of safety-related batteries provided on each unit should equal the number of independent and redundant reactor protection system (instrumentation and control) channels. For example, in a unit with four reactor protection channels, four batteries should be provided. The rated capacity of each battery should be determined as described above in this guide.

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