Transformer (∆→Y) protection device verification

Test object: NR RCS978 transformer protection device

Protection setting:In a 220kV substation, the rated capacity of the high, medium and low voltage side is 240MVA, the voltage level is 220kV/110kV/35kV, the CT transformation ratio is 600/1, 1200/1, 3000/1, and the main transformer wiring mode is Y/Y0-△11. The differential threshold value is 0.5Ie, and the differential quick-break value is 4Ie.

Protection straps setting: In the "setting value", the total protection control word "main protection input" in the system parameters is set to 1, the main protection trip control word is set, and the control words "differential quick-break input" and "ratio differential input" are set according to the test requirements. All other protections are withdrawn. On the protection screen, only the "differential protection" hard straps is cast.

Test method: VAHME702 only provides 3-phase current, so only single-phase or two-phase current can be added to each side for testing. In the high-voltage side to the medium-voltage side (Y/Y) inspection, the current I* is injected into phase A on either side, according to the phase adjustment method of the protection device:

 

 

and

 

   

 

so

 

 

 

That is, both phases B and C will be affected. In order to avoid this effect and make the inspection easier, the wiring method used on the high and medium voltage side is: the high voltage side current enters from the A-phase polar terminal, flows out into the B-phase non-polar terminal, and flows back from the B-phase polar terminal. The test device, the same wiring method on the medium voltage side, namely:

 

 

SO：

 

​

​The phase angle of the current added to the high and medium voltage side is 180°, and the size is I* (here I* is not the actual size of the high and medium voltage side current added but a standard unit value). According to the above example, the three-phase current tester IA The phase size is 1.05A and the phase is 0°, and the IB phase size is 1.05A and the phase is 180°. The device should have no differential current. It is only necessary to change the magnitude of the current amplitude on either side to generate a differential current. When the differential current value reaches the differential action threshold, the differential will act.
In the high-to-low-voltage side Y/∆ test, the wiring method should be adopted: the high-voltage side current enters from the A-phase polar end, flows out into the B-phase non-polar end, and flows back to the test instrument from the B-phase polar end. The side current enters from the A-phase polar end and flows back to the test instrument from the A-phase non-polar end.

Differential protection D-Y two sides wiring diagram (three-phase current)

 

HV side:

LV side

It can be obtained from the above that the phase angle of the current added on the high and low voltage side is 180°, the size of the I side is I*, and the size of the III side is, and the device should have no differential current. That is, the IA phase size of the three-phase current tester is 1.05A, the phase is 0°, the IC phase size is 1.732*1.32=2.28A, the phase is 180°, and the device should have no differential current. It is only necessary to change the magnitude of the current amplitude on either side to generate a differential current. When the differential current value reaches the differential action threshold, the differential will act.
     When verifying the differential slope of the ratio, the method used is the same as the Y→∆ three-phase current method in 2-2, that is, gradually reduce the output current value of the high (low) voltage side until the differential protection is activated. The current value on both sides, and then simultaneously zoom in (reduced) N times, first from balance to differential and then action to record the current value on both sides of the action at this time, from these two sets of data to calculate the slope of the ratio boundary, can be controlled The dynamic current value range is used to determine whether the selected two points are within the same slope. The same method can be used to calculate the slopes of the other two ends. This method is more complicated and requires a lot of calculations, but it can clearly understand the differential principle, and beginners must master it.

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