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Differential Protection: CTs arrangement for star-star connected power transformes?
What will be the arrangement for Star-Star connected T/F?

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?

1 out of 1 members thought this post was helpful...

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 (

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!

By Phil Corso, PE on 20 August, 2008 - 12:39 am
1 out of 1 members thought this post was helpful...

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

Phil Corso (

1 out of 1 members thought this post was helpful...

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

Yes, I agree with the CT connection suggested.

I would like to know that if I put Star connection of CT's on either side of STAR-STAR transformer then Is there any provision in relay so that we can compensate phase difference??

I am facing same differential problem in STAR STAR.

By Rasam Syamsudin on 31 August, 2008 - 1:28 pm

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.