Differential Relay

 Question is a 3 phase delta-wye 30MVA, 33kv/11kV transformer is protected by a differential relay

Calculate the relay current setting for faults up to 200 percent of the rated current.  the CT current ratio is 500/5 for primary and 2000/5 on secondary.

The solution is attached.  I understand everything except why the WYE CT current is multiplied by 1.73.  

It could be the question is poorly written, but to me i read it as a single 3phase transformer and the CTs are measuring line currents.  So to me the 1.73 multiplier is not necessary.

There is a buried assumption there.  In the case of a mechanical differential relay, the CTs on the WYE side of the transformer will be connected in delta to compensate for the 30 degree phase shift across the transformer.  It's still a good practice to connect them that way but it can also be dealt with in the software of a programmable (SEL, Beckwith, ...) relay.

Seen this exact problem before. You have to compensate for the phase shift on the wye side so, multiply the current by root 3. It's also done this way in one of my protection books I have. If it were delta-delta or wye-wye then there's no root 3 multiplication, I think. Just another little thing to keep in mind when going these problems.

Drewism, which protection book do you have? I have the Fundamentals of Power Systems Protection...is it in there. I haven't seen it if it is.

Nashi said:

Drewism, which protection book do you have? I have the Fundamentals of Power Systems Protection...is it in there. I haven't seen it if it is.

Click to expand...

Fundaments of Power System Protection by Y.G. Paithankar and S.R. Bhide.