CT Wiring Problems - Need Help Explaining Results

[coloradog1]

Hello guys. Need some help. We are currently getting the following readings off our relay. Phase rotation is acb.

Ia=100A < 0 deg
Ib=100A < 180 deg
Ic=100A < -120 deg

The voltage readings from the relay are correct. This is obviously giving us both zero sequence and negative sequence currents. Both phase A and C currents look correct. My question is, can someone explain why phase B is giving the angle of 180 degrees. It should be giving about 120 deg. Our thought was that the CT polarity might be reversed, although we don't know this yet. However, if it was reversed I would assume this would change the angle from 120 deg to -60 degrees (120 minus 180), not 120 to 180. Any help anyone can give would be appreciated.

On a similar note, if anyone has a document or spreadsheet that helps simulate CT wiring problems and the corresponding currents that would result, I would appreciate that also.

Thank you very much.

 

[davidbeach]

Are you sure that you really have Ib into the relay and not -Ia? If there are shorting blocks near the CT, short out the real Ib current and see if you still have Ib at the relay.

 

[pwrtran]

First, as long as your voltages are okay, your system is good.

Second, assume your relay is an IED type that can do more algorithms internally without depending on the hard wiring, also assume your CT hard wiring is correct.

So, in the CT current compensation, if you happen to have Ib=Ia+Ic left in place, by cosine law, Ib=-|Ia|=-|Ic|, which gives you same magnitude but 180 deg on X-axis

 

[pwrtran]

Correction, I meant Ib'=Ib+Ic, sorry

 

[PHovnanian]

Ib'=Ib+Ic

So check the B phase X2 CT connection for being tied to the C phase X1 instead of the common (X2) point.

 

[davidbeach]

Anything involving Ic won't get you 100A < 180 deg for Ib. Either you won't get 100A or you won't get 180 deg.

 

[pwrtran]

Anything involving Ic won't get you 100A < 180 deg for Ib. Either you won't get 100A or you won't get 180 deg.

If given:

Ia=100A < 0 deg
Ib=100A < 120 deg
Ic=100A < -120 deg

then Ib'=Ib+Ic=100A <180 deg

 

[davidbeach]

173A < 180 deg perhaps, but not 100A.

 

[pwrtran]

100A <180 deg, for sure. The radius of the circle is 100.

 

[davidbeach]

Oops.