CT Dimensioning in Low-Impedance Differential Protection PDF [PDF]

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 ABB Oy, Medium Voltage Products

1MRS758333 EN

Technical Note Issued: June 2015 Revision: B / 26 Jan 2016

CT dimensioning in low-impedance differential protection Application guide for transformer, motor and generator protection

Contents: 1

Scope ..................................................................................................................... 2

2

Introduction ......................................................................................................... 3

3

CT requirements for differential protection ..................................................... 4 3.1 CT requirements for low-set stage differential protection............................ 4 3.2 CT requirements for high-set stage differential protection .......................... 8 3.2.1 Operation of high-set stage ............................................................... 9 3.3 CT requirements for short-circuit protection .............................................. 11 3.4 Notes for generator differential protection ................................................. 12 3.5 Notes for power transformer differential protection................................... 14

4

List of symbols ................................................................................................... 16

5

Reference list ...................................................................................................... 17

Appendix 1: Calculation of the CT actual accuracy limit factor ................................. 18 Document history, Disclaimer and Copyrights, Trademarks, Contact information .... 20

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1MRS758333 EN

Scope

This document describes current transformer (CT) dimensioning and effects of CT saturation in stabilized low-impedance differential protection of power transformers, motors and generators. First, for introduction, the concept of CT time-to-saturate is introduced. Then, the CT dimensioning for low-set and high-set stage differential protection is presented with an illustrative example study. CT dimensioning for non-directional instantaneous overcurrent protection is discussed in briefly. Finally, some special features in the generator and power transformer differential protection is discussed with example studies. The rules given in this document are applicable for Relion® 615, 620 and 630 series protection relays with TR2PTDF and MPDIF low-impedance differential protection functions for transformers or machines, respectively. KEYWORDS: differential protection, current transformer, accuracy limit factor, current transformer dimensioning, saturation.

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CT dimensioning in low-impedance differential protection Application guide for transformer, motor and generator protection

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1MRS758333 EN

Introduction

For reliable and correct operation of the differential protection the current transformer (CT) has to be chosen correctly. The distortion of the secondary current of a saturated CT may endanger the stability and/or operation of the protection. A correctly selected CT, on the other hand, enables fast and reliable protection. The selection of a CT parameters depends on fault current magnitudes, DC component time-constant and actual CT burden. In low-impedance differential protection, traditionally a common practice has been to use similar CTs at both ends of the protected object and to equalize the CT burdens. The goal was to match CT performance so that in case of saturation, CTs at both ends will saturate as equally as possible. Earlier this was a requirement because it was impractical to size CTs for fully avoiding saturation, especially with high system X/R ratios. Nowadays, modern numerical protection relays of today are designed to tolerate some amount of saturation. Proper operation of a protection system requires that a sufficient part of the fault current can be sensed by the protection relays, despite of CT saturation. This ability can be specified with the parameter tal, time-to-saturate, which is the time during which the secondary current is a faithful replica of the primary current (fig 2.-1).

Fig. 2.-1. Illustration of time-to-saturate (tal). In case of external, i.e. through-fault situation, sufficient time-to-saturate ensures enough time for protection algorithm to detect situation and block low-set differential stage. Certain amount of uniformity on both sides CT performance is still required for the high-set protection stage. The requirement on the time-to-saturate depends on several factors, such as the required operation speed of the protection, the applied measuring method and the way of implementation of the protection function. Therefore time-to-saturate varies between protection functions and also between relay manufacturers.  Copyright 2016 ABB Oy, Medium Voltage Products, Vaasa, FINLAND

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CT dimensioning in low-impedance differential protection Application guide for transformer, motor and generator protection

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1MRS758333 EN

CT requirements for differential protection

The following CT requirements are valid for the Relion® 615, 620 and 630 series relays lowimpedance (stabilized) differential protection functions: -

TR2PTDF MPDIF

Transformer differential protection for two winding transformers. Stabilized differential protection for machines.

In these functions, allowed time-to-saturate is half of the cycle time (10ms in 50Hz network). 3.1

CT requirements for low-set stage differential protection

Current transformers for mentioned differential protection functions should work 1. at maximum short-circuit current 2. without saturation required time-to-saturate assuming maximum DC-component 3. and assuming 40% remanence, if CT remanence is considered. In 615, 620 and 630 series protection relay technical manual, equations for CT actual accuracy limit factor is given. Here we make things easier and present the same requirement in form of 𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙 ≥

𝐼𝑡𝑓_𝑚𝑎𝑥 × 𝐾𝑡𝑑 × 𝐾𝑟𝑒𝑚 𝐼𝑝𝑟

Where ALFactual is the CT actual accuracy limit factor (or CT total overdimensioning factor) Itf_max

is the maximum through fault current in external, out-of-zone fault

Ipr

is the CT rated primary current

Ktd

is the transient dimensioning factor considering effect of the asymmetrical fault current, i.e. DC component

Krem

is the remanence dimensioning factor considering effect of the CT remanent flux

The transient dimensioning factor depends on the DC time-constant (DC = X/R / ) and can be read either from the figure 3.1.-1 or 3.1.-2. The remanence dimensioning factor can be read from the figure 3.1.-3. Closed iron core CT can have, in worst case conditions, 80…90% remanence. However, tests and studies on CTs shows that typical remanence is about 40…50 % at maximum, giving Krem = 1.67… 2.0. When remanence is not considred, Krem = 1 (fig 3.1.-3).

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CT dimensioning in low-impedance differential protection Application guide for transformer, motor and generator protection

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Fig. 3.1.-1. Transient dimensioning factor as a function of DC time-constant.

Fig. 3.1.-2. Transient dimensioning factor as a function of power system X/R ratio.

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Fig. 3.1.-3. Transient remanence factor.

Example: maximum through fault current is 12kA, CT is 1000/1A, system X/R ratio is 50, ignoring CT remanence Krem = 1.0, and from the figure 3.1.-2, Ktd  4.1. 𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙 ≥

𝐼𝑡𝑓_𝑚𝑎𝑥 12000𝐴 × 𝐾𝑡𝑑 × 𝐾𝑟𝑒𝑚 = × 4.1 × 1.0 = 49.2 𝐼𝑝𝑟 1000𝐴

Note that by oversizing CT rated primary current, the CT requirement becomes smaller. This is one possibility which can be used for fulfilling the CT requirements. For example if 1600/1A CT would be used 𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙 ≥

12000𝐴 × 4.1 × 1.0 = 30.8 1600𝐴

With Relion® 615 and 620 and 630 series protection relays, the CT rated primary current can be at maximum four times higher than the protected object rated current. However, too high oversizing means somewhat worsened CT and relay accuracy at rated load current.

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Class X defines CT knee-point voltage as sinusoidal voltage applied to the CT secondary (primary open), which when increased by 10 % causes the CT magnetizing current to increase 50 %. The relationship between actual accuracy limit factor and CT knee-point voltage (Ukn), assuming resistive burden, is 𝑈𝑘𝑛 ≈ 0.9 × 𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙 × 𝐼𝑠𝑟 × (𝑅𝐶𝑇 + 𝑅𝑤𝑖𝑟𝑒𝑠 + 𝑅𝑟𝑒𝑙𝑎𝑦 ) 𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙 ≈

𝑈𝑘𝑛 0.9 × 𝐼𝑠𝑟 × (𝑅𝐶𝑇 + 𝑅𝑤𝑖𝑟𝑒𝑠 + 𝑅𝑟𝑒𝑙𝑎𝑦 )

Where Isr = CT rated secondary current, Rct = CT secondary winding resistance, Rwires = resistance of the connection wires, Rrelay = impedance of the relay current input. The factor 0.9 is used because knee-point voltage and accuracy limit factor are not exactly in the same point in the CT magnetizing curve. Actual value depends on CT, and is typically between 0.8 … 1.0. In the IEC 61869-2:2012 value 1.0 is used.

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CT dimensioning in low-impedance differential protection Application guide for transformer, motor and generator protection

3.2

1MRS758333 EN

CT requirements for high-set stage differential protection

Dissimilarity of the CT performance, i.e. unequal time-to-saturate, must be considered together with the sensitivity of the high-set stage. First, the ratio of CT dissimilarity can be calculated as follows 𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙_1 × 𝐼𝑝𝑟_1 𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙_2 × 𝐼𝑝𝑟_2 ∶ 𝐼𝑡𝑓_𝑚𝑎𝑥1 𝐼𝑡𝑓_𝑚𝑎𝑥2 In case the ratio is -

0.83…1.20: setting can be 60% of through fault current, or higher 0.67…1.50: setting can be 80% of through fault current, or higher other values: setting should be higher than through fault current

The reason of this recommendation is discussed in the next chapter.

Example 1: 25MVA power transformer maximum through-fault current is 1312A and 6873A on HV and LV-sides, respectively. Actual accuracy limit factors are 35 and 40 on HV and LV side CTs, respectively. CT rated currents are 300 and 1000, respectively. Fig. 3.2.-1.

Fig. 3.2.-1. Example case. Ratios are 35 × 300𝐴 40 × 1000𝐴 ∶ ⇒ 8.00 ∶ 5.92 = 1.35 1312𝐴 6873𝐴 Result: setting can be 80% of through fault current, or higher

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Example 2: 33kV/6.6kV, 25MVA, zk=8.4%. The Power transformer maximum through-fault current is 5.21kA and 26.03kA on HV and LV-sides, respectively. Actual accuracy limit factors are 45 and 57 on HV and LV side CTs, respectively. CT rated currents are 500 and 2500, respectively. Ratios are 45 × 500𝐴 57 × 2500𝐴 ∶ ⇒ 4.32 ∶ 5.47 = 0.79 5.21𝑘𝐴 26.03𝑘𝐴 Result: setting can be 80% of through fault current, or higher

3.2.1 Operation of high-set stage For ensuring fast and reliable operation in case of inside fault, the high-set differential protection stages in the TR2PTDF and MPDIF protection functions will operate (trip) under the following principles: -

The fundamental frequency component (DFT) of the differential current exceeds the setting, or The instantaneous peak value of the differential current exceeds 2.5 times the setting. In addition, the TR2PTDF also monitors ratio of bias and differential current and current phase angles, then halving the high set stage setting in case of inside fault.

In low-set differential protection stage the dissimilarity of the CT performance, i.e. unequal time-to-saturate, is handled by internal blocking from through-fault detection. As the high-set stage has no blocking, the CT dissimilarity must be considered together with the sensitivity of the stage. Figure 3.2.1.-1 illustrates CT performance dissimilarity in case of external, i.e. through-fault situation. DC time-constant 110 msec, no CT remanence. -

-

The upper part of the figure shows current waveforms assuming full DC-component, no CT remanence, with 1.25, 1.5, 1.75, and 2.0 times higher saturation point (actual accuracy limit factor in comparison to the protected object rated current) to the reference (1.0) CT. The middle part of the figure shows DFT component of the differential current. The lower part of the figure shows peak value of the differential current divided with factor 2.5.

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CT dimensioning in low-impedance differential protection Application guide for transformer, motor and generator protection

1MRS758333 EN

Fig. 3.2.1-1. Effects of CT performance dissimilarity. From the figure we can conclude that -

Ratio should not exceed 1.5 when high-set stage setting is about 80% of the throughfault current or higher (otherwise protection might trip from peak value) Ratios over 1.5 can be used when high-set stage setting is over the through-fault current.

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CT dimensioning in low-impedance differential protection Application guide for transformer, motor and generator protection

3.3

1MRS758333 EN

CT requirements for short-circuit protection

Differential protection is typically backed-up with an instantaneous short-circuit protection. In the Relion® 615, 620 and 630 series relays this is done with PHIPTOC function, which in case of transformer protection is on the HV-side. Therefore, the CTs should fulfil the CT requirements both for -

differential protection, based on the maximum through-fault current, and non-directional overcurrent protection, based on the highest possible fault current: the requirements are presented in relay technical manual and in reference /1/.

However, when the CTs are selected according the rules given in this document, the PHIPTOC will operate reliable up to about 20 times the though-fault current. Example: transformer is 22MVA, 33kV/6.6kV, 10%, the 33kV side CT is 400/1A. -

transformer through fault current is 3.85kA at 33kV side required actual accuracy limit factor for diff.protection is  3.85kA/400A×4 = 38.5 if selected CT has actual accuracy limit factor 38.5, the PHIPTOC is capable to operate reliable up to about 38.5 x 400A x 5 = 77kA.

Figure 3.3-1. illustrates the example case. Dotted red and black curves shows current seen by the relay using peak-to-peak and DFT measurement mode, respectively.

Fig. 3.3-1. Example case CT simulated with three different fault current levels.  Copyright 2016 ABB Oy, Medium Voltage Products, Vaasa, FINLAND

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3.4

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Notes for generator differential protection

The MPDIF differential protection function for machines have a special algorithm for 1. Blocking low-set stage in case of CT saturation. Basically, if algorithm first finds high bias current and low differential current, then followed by high differential current, this indicates that CTs have saturated and blocking will be activated until CTs recovers from the saturation. However, at the same time the angle between neutral and line side phase currents is monitored, if angle is not as expected for external fault, the blocking is overruled/removed. This ensures reliable tripping in case of inside fault and CT saturation after time-to-saturate. 2. DC component detection, and de-sensitizing the low-set stage accordingly. This (DC restrain enable -setting) is recommended to be used in case of network long DC timeconstants can be expected. In this algorithm, the highest DC component found in any of the three differential current is used to rise the relay operation curve. Real case example of de-sensitized settings When a big power transformer was energized in a bus fed by a single generator, the transformer inrush-current caused generator neutral side IL3 phase CT to partially saturate after about 2 cycles (fig 3.4.-1) which created differential current causing protection to operate.

Fig. 3.4-1. Generator currents during transformer inrush, upper graph has IL3 current waveforms, middle has differential current waveform and lower shows protection trip.  Copyright 2016 ABB Oy, Medium Voltage Products, Vaasa, FINLAND

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Differential protection operation curve settings: - low operate value (base value) = 15% of generator rated (pu) - slope (starting ratio) = 20% - turning points in the curve: 50% and 150%. Figure 3.4.-2, upper graph shows bias current (green curve) rise when inrush begins. After about 2 cycles, the CT saturation causes differential current (red curve) to rise. The blue curve shows how the protection low-set setting is changed (de-sensitized) based on the DC component measurement. The lower graph shows the same in the protection relay operation curve characteristics (bias-diff characteristics): Relay operation curve rises due to desensitization.

Fig. 3.4.-2. Example case bias and differential currents, normal and de-sensitized settings. Notes: -

The temporarily de-sensitized setting is brought down to normal exponentially with time-constant of one second. The DC component is calculated from the differential current (not from the phase currents). This can be seen in the figure 3.4.-2: de-sensitization occurs only after CT saturates.

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3.5

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Notes for power transformer differential protection

In case of an inside fault, where protection is required to trip, the CT saturation can cause activation of the 2nd harmonic blocking. As an example, figure 3.5.-1 shows result of a CT simulation, where fault current was 5 x CT rated, but high DC component causes saturation at about 0.04 sec. -

Upper plot shows CT secondary current waveform scaled to primary. Dashed line shows the unsaturated primary current. Lower plot shows the amplitudes of fundamental (1st harmonic) and 2nd harmonic components

Fig. 3.5.-1. Harmonic components at inside fault and CT saturation.

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From the figure 3.5.-1, we can see that when CT saturates the fundamental component drops and 2nd harmonic content rises, which can cause activation of the blocking. However, please note that: 1. Because the differential protection operates instantaneously, the question is not “will there be 2nd harmonic blocking”. We are interested on minimum time to saturate for reliable trip operation. Saturation after trip command is insignificant. 2. The instantaneous high-set differential protection stage is not biased and not blocked by harmonics. The higher fault current, the faster CT saturation, but when fault current exceeds the high-set stage the harmonics comes again insignificant. 3. In addition, the relay manufactures have own algorithms to handle this. Relion® series transformer protection relays function TR2PTDF have following features in the differential protection function TR2PTDF -

-

-

If the biasing current is small compared to the differential current or if both side currents are flowing inside the transformer, fault is most certainly inside. Then the instantaneous high-set stage operation point is automatically halved and low-set stage harmonic blocking is prevented. The 2nd harmonic blocking is based on ratio of 2nd harmonic and fundamental frequency (Id2f /Id1f) instead of absolute value of 2nd harmonic. If the peak value of the differential current is very high, the 2nd harmonic blocking threshold is desensitized (in the phase in question) by increasing it proportionally to the peak value of the differential current. The connection of the power transformer against a fault inside the protected area does not delay the tripping, because in such a situation the 2nd harmonic blocking is prevented by a separate algorithm based on a different waveform and a different rate of change of the normal inrush current and the inrush current containing the fault current. The algorithm does not eliminate the blocking at inrush currents, unless there is a fault in the protected area.

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List of symbols

ALF

CT rated accuracy limit factor: ratio of the rated accuracy limit primary current (value of primary current up to which the CT will comply with the requirements for composite error) to the CT rated primary current

ALFactual

CT actual accuracy limit factor

Ipr

CT rated primary current

Isr

CT rated secondary current

Itf_max

Maximum through fault current of the protected object, rms value

Ktd

CT transient dimensioning factor considering effect of the asymmetrical fault current

Kr

CT remanence, per cent of the maximum flux

Krem

CT remanence dimensioning factor considering effect of the CT remanent flux

Ra

CT actual burden, Rwires+Rrelay

Rct

actual CT secondary winding DC resistance in corrected to 75°C

Rrelay

impedance of the relay CT input

Rwires

resistance of the connection wires between CT and relay in corrected to 75°C

Sa

CT actual burden, Ist2 × (Rwires+Rrelay)

Sin

internal burden of the CT, Ist2 × Rct

Sr

CT rated burden in VA

tal

time-to-saturate, the time during which the CT secondary current is a faithful replica of the primary current

DC

time-constant of the fault circuit

Ukn

CT knee-point voltage: rms value of the sinusoidal voltage applied to the CT secondary terminals, other terminals being open, which, when increased by 10%, causes the 50% increase in the exciting current



angular velocity, 2f

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Reference list

/1/

1MRS758334. Effects of CT saturation to overcurrent protection. Technical note.

/2/

1MRS756966. General CT dimension Guide for MV-applications. Technical note.

/3/

1MRS756887. 615 series Technical Manual.

/4/

1MRS756508. 630 series Technical Manual.

/5/

IEC61869-2:2012. Standard for Instrument Transformers

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1MRS758333 EN

CT dimensioning in low-impedance differential protection Application guide for transformer, motor and generator protection

Appendix 1: Calculation of the CT actual accuracy limit factor The rated accuracy limit factor (ALF) is the ratio of the rated accuracy limit primary current to the rated primary current. A protective current transformer type 5P10 has, for example, the accuracy class 5P and the rated accuracy limit factor 10. For protective current transformers, the accuracy class is determined by the highest permissible percentage composite error at the rated accuracy limit primary current specified for the accuracy class concerned, followed by the letter “P” (referring to protection). Table A3-1. Error limits for protective CT class P according IEC61869-2 Accuracy class

Composite error at Ratio error at rated Phase displacement at rated accuracy limit primary current rated primary current primary current

5P

1%

60 minutes (=1deg) 5%

10P

3%

not defined

10%

The CT accuracy primary limit current defines the highest fault current magnitude at which the CT will meet the specified accuracy. Beyond this level, the secondary current of the CT will be distorted, and this may effect on the performance of the protection relay. In practise, the actual accuracy limit factor (ALFactual) differs from the rated accuracy limit factor (ALF) and is calculated using the equation 𝑆𝑖𝑛 + 𝑆𝑟 𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙 ≈ 𝐴𝐿𝐹 × | | 𝑆𝑖𝑛 + 𝑆𝑎 where ALF Sin Sr Sa

is the CT rated accuracy limit factor is the internal burden of CT secondary winding (Isr2 × Rct) is the CT rated burden is the actual burden of CT (Isr2 × Ra)

The CT internal burden (Sin) is often assumed to be pure resistive (cos=1). Then the equation can be also expressed as

𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙

𝑆𝑟 2 𝐼𝑠𝑟 ≈ 𝐴𝐿𝐹 × 𝑅𝑐𝑡 + 𝑅𝑎 𝑅𝑐𝑡 +

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Example: CT is rated 300/5A, 5P20, 10VA, Rct = 0.07 ohm and the secondary burden (including wires and relay) is 0.117 ohm. 10𝑉𝐴 (5𝐴)2 ≈ 20 × ≈ 50.26 0.07Ω + 0.117Ω 0.07Ω +

𝐴𝐿𝐹𝑎𝑐𝑡𝑢𝑎𝑙

The CT actual burden (Sa, Ra) includes resistance of connection wires and impedances of all devices in the secondary circuit. In ABB Relion® 611, 615, 620 and 630 series protection relays the input impedance of the relay 1/5A current input is less than 0.020 ohm. If CTs are connected to the relay using 6-wire connection, the resistance of the connection wires must be calculated based on 2 x distance between CT and the relay. In case of common return wire is used, i.e. 4-wire connection, factor 1 can be used for 2 and 3-phase faults. This is because there is no current in the return wire in case of 2 or 3-phase faults.

Fig. A3-1 CT connections, 6-wire and 4-wire (common return conductor) Sometimes the connection is mixed so that 6-wire connection is used at begin of the distance, and 4-wire for the rest of the distance. For example, if 6-wire connection is used for 20%, and 4-wire for 80% of the total distance between CT and relay, factor 1.2 should be used. Example: 4-wire connection is used, distance is 15 meter and resistivity of the wires (at 75 deg) is 8.65ohm/km. -

in case of 2 and 3-phase faults: resistance is 15 meter x 0.00865ohm/m = 0.130 ohm in case of 1-phase faults: resistance is 2 x 15 meter x 0.00865ohm/m = 0.260 ohm

Table A3-2 Resistance per cable length (+75 C) for copper. Material 2.5 mm2 4 mm2 Copper 0.00865  m 0.00541  m

6 mm2 0.00360



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m

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Document revision history Document revision/date A / 26 June 2015 B / 26 January 2016

History First revision Chapter 3.2 changed, 3.3 added, 3.4 example changed to a real-life case. List of symbols and appendix1 added

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