Intercore Coupling

[marks1080]

I'm testing some bushing CT's on a new 500kV SF6 ABB breaker. Recently intercore coupling has become a required test for us complete. I notice there is an IEEE on this. Our internal test procedure (which is only a brief paragraph) says that when measuring voltage on the companion CT - across the full winding - you should not measure more that 10V for the pass to be accepted.

The CTs have 5 taps. When energizing one CT at X1 to X2 to saturation the measured voltage on the companion CT is upwards of 100V. When energizing the CT across X1 to X4 (the inservice tap) the measured voltage on the companion CT is much less, abour 30V, still failing our standards.

I would like to understand why the voltage measurment is so different when saturating through X1-X2 vs. X1-X4, ie: why is the voltage on the companion CT higher when saturating through a smaller section of the CT winding.

Does anyone have any other insight on this that may think valuable?

Thanks,
Mark

 

[davidbeach]

The less of the CT you apply your voltage to, the higher the voltage across the whole CT.

 

[lindsayspeters]

Hi Mark,
Your CT secondary winding is not evenly distributed. If so,
there are high leakage flux outside of the core. The leakage flux induces voltage on companion CT. If the winding is evenly distributed, the leakage flux should be very low.
Thanks

Lindsays

 

[lindsayspeters]

Hi Mark,
This problem may be due to the core problem because of the air-gap. There is some leakage at the gap. The leakage flux will induce a voltage around windings. If the gap is the cause, the induced voltage can be eliminated by rotating the CT.

Thanks


Lindsays