Transformer Protection 1

[hugop973]

I have a Secondary Unit Substation, the transformer is rated 2500 KVA, 13.2KV Primary - 480V Secondary, 3 phase. I was providing overcurrent protection and differential protection to the transformer but other engineer eliminated the differential protection because he said is too expensive and not needed for this type of transformer.
My question is, transformer differential protection is not neccessary or recommended in this case? If not when is needed?. Thanks

 

[rbulsara]

There are no hard and fast rules but economics play a role. Differential protection is expensive (because of need for breakers, relays, CTs vs. say just switch and fuse) and becomes a question of accessing risk vs. probable costs, if at all there is a failure.

Typically transformer diff protection is found in transformers size 10 MVA or greater. The transformers are among the most reliable electrical equipments, so it helps taking more risk, especially for smaller size units.

Factors in decision making may include:
Probability of an internal failure.
Possibility of localized repairs to the winding.
Cost of repairs vs. replacement.
Criticality of the application.





Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

Hard to justify differential on a small transformer, particularly if not oil filled. Differential only operates for a transformer internal fault with the intent of catching the fault early. Once a dry type transformer has had a fault it is pretty much toast anyway and you don't have the risk of oil ignition from a fault that doesn't clear quickly. For bigger transformers, faster clearing may make something rewindable rather than scrap, but would you ever try to have your 2.5MVA transformer rewound if damaged or would you just get a new one to replace it?

 

[cranky108]

Just to add another aspect, not that it applies here, but you can use a differential for faster clearing if there is a chance of damaging other equipment. Or in cases where parallel transformers are used for high reliability.

Again it comes down to cost, or equipment protected, or cost of down time.