Inadvertent core ground on transformer

[trobgherian]

I have an inadvertent core ground on a power transformer. I want to add a resistor to the designed core ground and rely on the inadvertent core ground. My goal is to minimize the circulating current as a result of the inadvertent core ground. What size resistor should I use? I am thinking 10 Ohm, 300 Watt. Any thoughts?

 

[waross]

Fire extinguisher? Call the vet after the horse has died! Replace the transformer before you experience an inadvertent total failure. An inadvertent ground means an insulation failure. quite possibly you have now or will have soon shorted turns. Shorted turns are the start of an avalanche type failure. As long as the transformer is energized, the shorted turns will be generating excess heat that will lead to further damage and failure. A resistor won't help shorted turns.

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

 

[trobgherian]

This is a ground on the core steel of the transformer, not the windings. I know there are many ways of removing inadvertent core grounds. For example, an engineer from GE in Pittsfield, MA told me I could burn an inadvertent core ground in the clear with an arc welder and he has done it before. I don't want to do that since it involves more work and risk. Adding a resistor has been done in the past, but I don't know what watt or resistance is reasonable. Thanks.

 

[stevenal]

We have a 12/16/20 MVA 115/12.47 kV unit with an inadvertent core ground. The manufacturer supplied a 1000 ohm 1500 W resistor along with a box and bushings. Been sampling GIO every two weeks, but have not yet seen a clear trend either direction. A Megger works fine to burn it clear, temporarily. Always came back.

 

[prc]

In large transformers,core and core clamping metal structures are individually insulated and isolated from ground and then shorted together to tank which, in turn, is earthed to ground.When core or coreclamp gets another inside earthing to metal tank,due to damage in the insulation structures( usually press board or FRP) there is a chance of circulating current through the double earthed part.The problem is more in case of core clamp due to voltage difference developed from leakage flux at full load.This current can some times cause arcing in current path and even melting of metal.This can be detected from DGA ( dissolved gas analysis of oil)results.Many times, such problem may not be there,say when second earthing is at bottom of core to tank.( a location where there will be no leakage flux )To prevent circulating currents, one way is to add a resistor in series to the double earthed component path so that circulating current is reduced.Some times the designed earth at tank top is separated out as the failed path inside will act as earthing.

My experience is that inadvertent earth in core laminations is not a issue and nothing need to be done.

 

[waross]

I apologize. I mistakenly assumed that you were an electronic type with a power transformer in an electronic chassis. Yes, I know, not the proper forum but it happens.
Sorry

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