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For differential protection on a delta-star connected power T/F, CTs on delta side are connected in star & CTs on star side are connected in delta to compensate for 30 degree phase shift because of star-delta connection. What will be this arrangement for Star-Star connected T/F?

Responding to Sunny’s 17-Aug-08 (09:55) query...

Star-Star is preferred for two reasons:

a) CT ratio's are easily determined.

b) Earthing of the CT secondary wiring is simplified.

Regards, Phil Corso (cepsicon@aol.com)

Mr. Phil,

I didn't get it. I am asking that for a star-star connected power T/F, what would or should be CT's connection arrangement on both sides (primary/secondary) for differential protection?
Please explain!

Sunny, both sets of CTs can be star-star connected, just like the T/F primary and secondary winding's are!

Phil Corso (cepicon@aol.com)

In diff protection we have to create phase diff,
so for star star transformer cts should connected in delta delta on both sides.

The answer is using an example of 10 kVA transformer 13.8 kV primer & 4.16 kV secondair. In CT selection 1.5 factor is implemented.
CT ratio at 13.8 kV is 75/1 and 200/1 at 4.16 kV.
CT connection is Star-Star.
CT out put for the relay is 0.836A from 13.8 kV system and 1.04 A from 4.16 kV.

This mismatch can be adjusted if we use a new relay type but for the old type we havew to select tap. Remember connect phase A to phase A, B to B, and C to C for relay relay. Ground the neutral CT at one location, recommended at relay cubicle panel.