Phase current Imbalance in 13.8KV Main Incoming CBs

[adityalagudu3291]

Please see the attached for the reference.

So we observed Phase current Imbalance in our 13.8KV Main Incoming CBs when our Tiebreaker is closed condition. The maximum difference we observed between IA and IC is 36.9A. But when we Open out Tiebreaker, all three phases become balance.

As our Incoming CBs has 2000:5 CTs and no neutral or ground CTs installed? So what settings are acceptable for the 50G/50N on these Main Incomers?

And why there is no imbalance in the currents when the Tie breaker is OFF? These Main incomers are fed from a Utility substation using a 4000ft Power cable.

 

[iceworm]

Do you have a one-line?

 

[adityalagudu3291]

Do you have a one-line?

yes, please find the attached.

 

[powerpete69]

A three phase current imbalance is perfectly normal. Not everything is three phase, hence the difference.
Why it balances out when you turn on the tie breaker? Good question.
Hard to say what you should set your relays at without more info. Depending on your wire size, Looks like a long time of 400 or 500 amps might be ample depending if that load goes up in the summer or what not due to extra cooling or chillers or whatever. Again, amps will be limited by wire size.
As far as instantaneous and so forth, you are going to have to read up on that. then apply to your specific application.

 

[jim dungar]

So we observed Phase current Imbalance in our 13.8KV Main Incoming CBs when our Tiebreaker is closed condition.

Are you paralleling the two utility sources when you close the Tie? Maybe your sources are not completely in sync. Have you looked at the positive, negative, and zero currents?

 

[powerpete69]

As far as the instantaneous setting for you main relay, if you remove or disable the instantaneous setting, your main will not trip when a feeder breaker is short circuited leaving the rest of the branch feeders turned on. This is critically important for a hospital with people on life support for example. One of the negatives of disabling the instantaneous is more arc flash potential, so that is a balancing act. I believe it is code for hospital applications like this to disable the instantaneous on the main, but you can look that up for exact details.

I'm assuming this is a factory, not a hospital. Looking at our single lines for our 4160V model, 2 of our instantaneous settings and turned on in one substation building and two are turned off in the other substation building!! So what does that tell you? Right.

 

[adityalagudu3291]

Are you paralleling the two utility sources when you close the Tie? Maybe your sources are not completely in sync. Have you looked at the positive, negative, and zero currents?

Both the utility sources are fed from the same 15KV Utility switchgear, which is like 4000 ft away from these Incoming CBs. I believe when Tie is closed on both sides, it becomes a closed-loop and impedance of each phase in one loop becomes a little different than the other loop. I believe the impedance is due to differences in impedance in each phase. When Tie opens it becomes two different radial feed circuits and there will be no imbalance.