transfomer primary connections

[electricuwe]

Does anybody know a standard which specifies short-circuit withstand capabilty for primary connections inside of a large power transformer (cast resin) in case of a fault in the primary winding? Does anybody have experience with such a fault where this fault caused more severe problems due to rupture of primary connections?

 

[electricpete]

There is an IEEE standard for transformer THROUGH-fault withstand capability. It makes sense that you want your transformer to withstand an external fault for the time it takes the protection to clear it.

I have never heard of anyone specifying requirements for limiting collateral damage in the event of INTERNAL transformer fault. The need for such a specification is not as obvious to me since transformer internal fault is far less common and when it does occur draining/inspection of transformer will be required anyway (and possibly rewind). Furthermore in my limited experience of the 5-6 transformer failure investigations I have been involved with including some on hi-side, I have never seen evidence of any movement due to fault currents. Apparently these internal faults have all been relatively high impedance.

Obviously if you wanted to impose such a specification you would have to tell the manufacturer the fault current available at the high side since manufacturer cannot rely on transformer impedance to limit current as is done during though-fault test.

 

[prc]

Power transformer standards specify short circuit with stand capability with respect to a L-L or L-G fault on secondary of transformer ie for the fault current through the trf as limited by trf impedance+ primary system impedance.System impedance will be very ,very low compared to trf impedance esp in well interconnected high voltage system.If a trf fail to with stand such a fault current,windings mechanically fail followed by electrical failure.
But if a mechanical(followed by electrical) failure occurs in the primary connections to winding( getting shorted to earth)or in the early part of the primary winding ,there will be no trf impedance to limit the fault current ,and the current will be several times more than the current due to an external fault .Naturally the failure consequences will be severe and devastating.
I have seen some customers insist for higher with stand capability for primary bushing and inside primary connections(same fault levels as of primary system).Personally I believe there is not much sense in it.

 

[jbartos]

Suggestion: Since any transformer parameters or ratings are not posted, consider for example:
ANSI/IEEE C57.12.00a and b-1978 "American National Standard Thermal and Short-Circuit Requirements Supplements to ANSI C57.12.00-1973, General Requirements for Distribution, Power, and Regulating Transformers"
Table 21 "Distribution Transformer Short-Circuit Withstand Capability"
1phase KVA 3phase KVA Withstand Capability
Per Unit of Base Current
(Symmetrical)
5-25 15-75 40
37.5-100 112.5-300 35
167-500 500 25