Transformer potential rise of the core versus the ground under transient conditions

[qman5]

Hi All,

I have a customer specification that calls for a test, or in lieu of a test, a calculation to demonstrate the maximum potential between the transformer core and ground during a transient condition based off characteristics of its surge arresters on both the primary and secondary. The transient (unknown value) voltage would be applied to any terminal. The HV side of the transformer is in Y, with the secondary being Y but with a reactance inserted in series between the neutral and the ground.

Just a few questions/pointer requests from my side:

1. Do you have any good reference for the transient model of a transformer so I can model or at least do a rough hand calculation of this?
2. I don't have the electrical model of the transformer, so in parallel with 1, would this be something that I need to ask the manufacturer of the transformer for, or could I make assumptions?
3. Any idea on the purpose of this test? My understanding would be that the core is coupled to the ground permanently via a jumper, so in a practical situation (so long as the jumper/series resistor is not fried) there should be no significant effects.

Thanks!

 

[waross]

The tank should be grounded by a substantial jumper. Do not insert a resistor, reactor or any impedance in the tank grounding circuit.
The arrestors may be mounted on the tank but they should be grounded directly through substantial jumpers. The jumpers should be sized to withstand the currents associated with transients while the arrestor is conducting with no dangerous voltage drop or touch voltages on the tank. The tank should not be depended upon to carry arrestor currents. The arrestors should be grounded independently of the tank.

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

 

[prc]

Even when the core is solidly grounded, there will be voltage build up in core when a surge impinge the winding terminal. This is the reason why the core losses in large EHV transformer goes up by 1-5 % after the impulse testing. The voltage build up in core damages the insulation of lamination and eddy losses goes up. But this seems to be a reversible process as after some weeks,core losses will go down.
For simulation, use the inner winding to core capacitance to find out the transferred surge, assuming core in not grounded.

 

[krisys]

Even if there is a voltage build up during the transformer switching, the is no danger to the core. Each core is stacked, their potential rise will be identical.

As there is oil insulation around the core, there is no hazard to the personnel. Any power frequency voltage or static charges will be absorbed by the core earthing jumper.

So I did not understand, why your client is asking for this test. What inference he can make out of it? As far as I remember the standards (IEC) are not calling for such tests. If such tests are not in the standards, insisting this test to the supplier could be difficult. The warranty issues also to be looked into.

At least you can ask this your client and update us.