RET670 - Matrix Calculation With The Measured Current [PDF]

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DIFFERENTIAL CURRENT CALCULATION IN THE RET670 RELAY WITH THE AVAILABLE DATA ON THE SECONDARY VALUES While doing the 3ф short circuit test on transformer, the following data’s are observed in the RET670 relay,

IRHV = 34.6 A (0°)

IrLV = 98 A (180°)

IYHV = 34.6 A (240°)

IyLV = 98 A (60°)

IBHV = 34.6 A (120°)

IbLV = 98 A (360°)

The differential currents are as follows

IDiff_Rph = 45.0 A IDiff_Yph = 22.9 A IDiff_Bph = 23.1 A The details of the transformer, Power = 20 MVA HV Rated Voltage = 33 kV LV Rated Voltage = 11.5 kV Short circuit current in the secondary when the primary get injected with low voltage(415V) and the secondary three phase got shorted. Short Circuit Current in HV side (If) = Applied Voltage X Rated Current on the primary % of Impedance X Rated Voltage on Primary Short Circuit Current in HV side (If) =

415 X 350 0.12 X 33000

Short Circuit Current in HV side (If) = 36.67A Short Circuit Current in LV side (If) = 105.22A

As per the RET670, the equation for the differential is as follows,

A&B

are the matrices with numerical coefficients.

So while taking the above equation in to consideration, then the W1 and W2 has to be taken as follows, W1 = The reference Winding, W2 = The secondary Winding, So while taking the reference always, the Star ‘Y’ has to be taken as reference. if it is not available then we can go for the primary Delta ‘∆’ as reference.

A = is the matrix for the reference winding B = is the matrix for the secondary winding.

So here for Dyn11, the reference winding will be ‘yn’ and for the secondary winding will be ‘D’. According to this if re-arranging and adopting the values, A = matrix of the reference winding. B = matrix of the 30° leading.

IDA

2

-1

-1

-98

IDB

= 1

-1

2

-1

98

IDC

3

-1

-1

2

98

IDA IDB IDC

+ 33.0 X 1 11.5

√3

-130.6 =

65.3 65.3

1

0

-1

34.6

-1

1

0

34.6

0

-1

1

34.6

0 +

0 0

These currents are referred to the LV side “yn”. So it has to converted to HV side since the relay parameters are shown in HV rating of settings. So it can be re-written as,(by dividing the KN)

So the Current measured in the relay will be as follows,

IDA IDB IDC

43.5 =

22.7 22.7

0 +

0 0

So finally we got the result as in the relay By Gopinath.S Power System protection engineer.