Two parallele Xfo

[Eclair]

Hi everyone like me,

A setup of two exactly similar transformer of about 22MVA connected togother via a tie breaker has a downstream earth fault.

I figure out that the ground resistors (400A) of each transfomer will contribute of a ground/earth fault for 800A.

How can I solve the problem: where is the fault? And how can I shutdown one the good problematic transformer?

I guess it is with a differential relay that this problem could be solve. But how and where in my circuit?

Thank you

 

[alehman]

I'm confused. The earth fault is downstream of the tie breaker, but you want to trip just one transformer?

Differential protection for the transformers is usually good, but will do nothing for a downstream earth fault (hopefully).

 

[jghrist]

Are there no breakers or relays on the outgoing feeder?

You should be able to parallel the CTs on one side of the tie breaker with CTs from the transformer on the other side of the tie breaker to sense a fault on that side of the bus.

 

[pwrtran]

Eclair

Is the fault downstream of the xfmrs on the secondary bus, or downstream of the secondary bus on any one of the outgoing feeders?

If the earth fault is on the secondary bus then a bus differantial will detect the fault. Say T1 and T2 connect to B bus and Y bus repectively with the tie breaker normally closed. A fault one B bus will cause diff to trip T1 bank breaker and the tie breaker. Some bus protection may also ask all feeder breakers associated with the faulty bus to trip. If the fault is downstream on a feeder then bus diff won't work and you need feeder protection (inst., overcurrent, distance).

Regards

 

[slavag]

Hi Eclair.
Could you please attached electrical scheme with CT's, CB's
and used protective relays.
I sure, Forum can help you.
Best Regards.
Slava