Power system protection

[bethy]

Master DK,

I need your help about this please! Here is the problem

A three phase, delta-wye connected, 15KVA, 33/11KV transformer is protected by CTs. Determine the CT ratios for differential protection such that the circulating current (through the transformer delta) does not exceed 5A.

Solution:

The line currents are: I(delta) = 262.44 A & I (wye) = 787.3 A (by using kVA/(sq^3 *V)

If the CTs on the high voltage side are connectied in wye, then the CT ratio on the high voltge side is 787.3/5 = 157.46.

Similarly, the CT ratio on the low voltage side is 262.44 *(5/sq^3) = 757.6

My question is if the CT on the high voltage side, it should be use I (delta) since delta is the primary, right?? Am I missing something here???

Thanks,

 

[DK PE]

Master DK,I need your help about this please! Here is the problem

A three phase, delta-wye connected, 15KVA, 33/11KV transformer is protected by CTs. Determine the CT ratios for differential protection such that the circulating current (through the transformer delta) does not exceed 5A.

Solution:

The line currents are: I(delta) = 262.44 A & I (wye) = 787.3 A (by using kVA/(sq^3 *V)

If the CTs on the high voltage side are connectied in wye, then the CT ratio on the high voltge side is 787.3/5 = 157.46.

Similarly, the CT ratio on the low voltage side is 262.44 *(5/sq^3) = 757.6

My question is if the CT on the high voltage side, it should be use I (delta) since delta is the primary, right?? Am I missing something here???

Thanks,

Look here: http://engineerboards.com/index.php?showtopic=13493