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Inadvertant Ground on Power Transformer

Inadvertant Ground on Power Transformer

Inadvertant Ground on Power Transformer

(OP)
thread238-296370: Inadvertent core ground on transformer
About a year ago the question was raised on how to deal with an inadvertant ground on a large power transformer (50MVA, 138kV/22.9kV), I was curious if the quick fixes were still a solution or if the Transformers condition continued to degrade or if DGA inicated problem solved and transformer is still healthy?

The several methods of dealing with inadvertant ground seem to be:
A. Untank and repair
B. Use arc welder to burn clear. (often only temporary fix)
C. Use resistor on intentional ground to limit current flow.
D. Isolate intentional ground, depending on inadvertant ground for core.

Assumptions (my own)
1. Intentional core ground is not required to carry load (under normal operation), it is used to keep core and tank at same potential.
2. Intentional core ground is used as path for fault current in event of winding shorted to core.
3. Ungrounded cores are more suseptible from damage from solar activity due to geomagnetically induced currents.

RE: Inadvertant Ground on Power Transformer

The problem of inadvertant core ground is actually inadvertant double ground problem, as core is always kept under one point earthing.This is required to avoid core picking up potential rise which may cause partial discharges.Of course it will act as a path for fault current when there is a ground fault to core from inner winding.Geo-magnetically induced currents are not coming on core.

In addition to core, core clamping structure is also kept under one point earthing.So in large transformers, insulation of core to ground is checked, core clamp to ground is checked and also between core to core clamp.Usually in field it checked at 500V DC and a value of 500 kilo-ohms is expected in new transformers.Factory insulation withstand test is done at 3.5 kV Dc or 2.5 KV AC ( In CEGB and some other utilities ask for 10KV AC )in air.The three terminals( Core-core clamp-ground) are brought up on a terminal board at tank top and earthed to tank.

Due to shifting during transport or ageing of insulation with time,some times this insulation value may come down or get shorted.Then double earthing is the result.Then a circulating current can happen from the induced voltages from leakage flux from windings.This can over heat core clamp,some times even melting metal.A,C&D are usually followed.Sometimes the double earth is kept as such as all double earths are dangerous.

RE: Inadvertant Ground on Power Transformer

Unfortunately the OP of that thread has not signed in since April 2011. I think it will remain a mystery.

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