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 (email@example.com)
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?
Sunny, both sets of CTs can be star-star connected, just like the T/F primary and secondary winding's are!
Phil Corso (firstname.lastname@example.org)
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.
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.