AC current in the protective earth of power transformers 2

[dm300mw]

We measured an AC current - in the order of tenth Amps - flowing in the protective earth of large scale step up power transformers.What might be the source of this current?
Thanks everybody

 

[waross]

Capacitive coupling between the windings and the core.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

waross,let me disagree.This is much more complicated.This current-tank earth pad to ground, is from the voltage difference between tank and station earth pad.This varies with load ( leakage flux from windings generates a voltage on tank surface) and position of earthing etc,etc. Let me give actual values for a 1000 MVA 420 KV step up transformer bank.Actual tank earth currents at full load is 10-22 A for two single phase units and 0.6-13.8 A for the third phase.The capacitance current ( LV to core + outer winding to tank) will be only of the order of 0.1 A as estimated.

Some years back, I came across a pretty large rectifier transformer in an aluminum plant where the tank current to earth was the order of 800 A !One out of two earth links to ground was removed to bring down this current to the order of 300 A.

 

[waross]

Thank you for the information, prc. I don't doubt your information but I have a couple of questions to help me better understand this effect:
I have seen a lot of transformers where there did not seem to be excess current to ground. Is this an issue with three legged cores and unbalanced voltages?
I am under the impression that the open circuit voltage caused by capacitive coupling is quite high, but that the current is limited by the reletively high source impedance of the capacitive effect.
From your numbers I suspect that the open circuit voltage of flux induced currents is quite low but the source impedance is quite low.
From the OP, I took the current to be less than an Amp (tenths, not tens) which is consistent with capacitive leakage currents.
Comments?

Bill
--------------------
"Why not the best?"
Jimmy Carter