Connection of CTs to Differential Relay

[appunni]

Sir,
We have a 110/11kv star-star transformer (solidly earthed on both sides) with 100/1A CTs on 110kv side and 1000/1A CTs on 11kv side with PS class core. Do we have to connect CTs on both sides in delta before connecting the CTs to differential relay? What is the problem if the CTs are connected in star on both sides and then is connected to differential relay? I cannot understand why the CTs are to be connected in delta even if the transformer is star-star and with proper CTs on both sides so that there is no phase shift and no CT ratio error. Please explain.

 

[dpc]

The use of wye-connected CTs on the delta transformer winding serves a couple of purposes. In addition to correcting the transformer phase shift, the delta CT connection helps prevent mis-operation due to zero-sequence currents during external faults.

Even with a wye-wye transformer, there is a possibility of unequal zero sequence current on each transformer winding due to internal flux interaction inside the transformer. If one could be absolutely certain that per-unit zero sequence current would always be exactly equal in both windings for an external ground fault, the CTs could be connected in wye, but the delta CT connection eliminates any possible risk of misoperation.

In many newer digital relays, this zero sequence current is handled digitally, and the CTs can always be connected in wye.

 

[jbartos]

Suggestion: The previous postings imply that upstream and downstream of the star-star connected transformer would have to be delta transformer connections to break paths of zero sequence currents. This situation of transformer connections is possible but very rare. Therefore, the delta-delta CT connections are appropriate as indicated in above postings.

 

[SMB1]

What mr. Munfai said is true. However, the source of the Zero Seq. current for external L-G fault which can be seen by one side not by other is coused by the tiary which is always present in Y-Y trans for balancing purposes.

Therefore, this zero seq. current shall be filtered by Del-Del connection for CT on both sides of Y-Y Xfmr

 

[bigamp]

Appunni,

It all depends on the make of relay you are using!

Transformer differential relays from some manufacturers, such as Alstom & Siemens, have internal CT ratio compensation, phase shift compensation and zero sequence filter. With such relays, you connect the CT's in star even if the transformer windings are in star. So, check the relay connection drawings and instructions provided by the manufacturer and do as they say.

 

[jbartos]

Suggestion: The present integrated protective relays sense voltage and current at necessary points only and process their outputs for functions that they have internally built in. This is in agreement with the previous posting.