Higher size transformer retrofit

[Sahib]

Existing transformer size in a telephone exchange is 500 KVA. The breaking capacity of the secondary side breaker fed by bus bar from the transformer is 55kA. The transformer suffered internal fault and has to be replaced. The spare transformer available at another distant exchange is 750KVA, 415V, 4% impedance. The question is if the existing breaker need to be replaced for the 750 KVA replacement transformer. Thanks.

 

[kingpb]

Nope, you can always limit the transformer with a smaller breaker. The max let through fault current on the 415V, 3 ph side is around 26kA (with the 750KVA @ 4%) so if your breaker is rated 55kA, shouldn't be an issue, assuming the LV bus bar is rated over 26kA as well.

 

[iceworm]

Existing transformer size in a telephone exchange is 500 KVA.
...The spare transformer available at another distant exchange is 750KVA, 415V, 4% impedance.

As king said, the secondary cb is okay.

Recommend checking the primary CB. Depending on how close it was sized, the 750kva inrush could be a problem. You may need to change setting.

ice

 

[Sahib]

Thanks.
The subject 11kV/415V, 750kva transformer is indoor type but kept outside for a year or two. I told to my supervisor that to energise the transformer only insulation test with a suitable meggar is necessary: if the insualtion value is above a minimum, the tranformer is fit for energisation as visible damages such as oil leakage is not present. But he is insisting to lift the transformer core, varnishing the windings etc., before energising the transformer. Who is right?

 

[kingpb]

I would say both, only because if the intended use is for a mission critical application, you probably don't want it being put in service to only fail and muck things up later, thus the bosses concern.

But, if its just a general application, and your trying to get service back up and running, then do the minimum. Honestly, if it ain't leaking it's probably ok.