Phase current Imbalance in 13.8KV Main Incoming CBs 1

[Aditya Lagudu]

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.

 

[davidbeach]

The two source lines are electrically connected?

With the tie closed you have a loop and the impedances of the phases on each side of the loop are different. Doesn't take much difference, but the current splits between the two sides of the loop will be different on different phases. Open the tie and it's two radial feeds and no current split.

 

[Aditya Lagudu]

Yes, the two source lines are electrically connected and fed from a Utility 15KV switchgear with Tie closed.

What are the factors that change the impedance of each phase? We are using the same cable size and model.

How much unbalance is acceptable on the Main breakers?

 

[davidbeach]

You're seeing the effects of milliohms of difference if not microohms. Many different things can have those kinds of impacts including the simple matter of how the conductors of each phase relate to each other in each conduit. Connectors with slightly different contact pressure might be enough.

 

[Aditya Lagudu]

David,

Okay, that seems to be the reason. We also did the spicing for the one phase of the cable and which could also make little change in the impedance for each phase of the loop.

 

[waross]

A related question David.
In the case where a major load was connected physically to the substation bus near one end of the bus, could the impedance difference between the horizontal bus bars be enough to cause a noticeable unbalance?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Yes, as long as there are two separate paths. The load determines the total current in each phase, the system configuration determines the current split between paths.

With a certain line-out condition we have a location on our system where a substation with two lines has power flow in on both lines; it's at the balance point on a loop between two bulk power transformers. A- and C-phase currents are much higher from the south than the north, but the B-phase current is much higher from the north than from the south. Slight impedance imbalances can do that. That difference can be due to how the phases are arranged on the poles or to a "bad" connection somewhere or to a switch somewhere with a phase that isn't seated as well as the other phase. Subtle enough that IR scanning has never found it.

 

[waross]

I am thinking about transmission lines with an poorly spaced or lacking transpositions.
Yes, I realize that this is cable fed, but I was wondering if there may be enough difference in reactance between the bus bars (at either end of the circuit) that a load concentrated at one end of the bus may be part of the cause of the unbalance.
Is that reading at one of the incoming breakers or at the tie breaker?

Some interesting notes:
A phase is lagging 16 degrees
B phase is lagging 19 degrees
C phase is lagging 18 degrees

The phase with the least lag has the highest current.
The phase with the lowest voltage has the highest current.
Assuming that the loads are balanced, the greater lag on B phase and C phase may indicate a higher impedance that is causing a greater share of the current to be carried by the other circuit.
For a more complete discussion it would be valuable to see the values for incomer A, incomer B and the tie breaker.

Note: With differing phase angles the sum of the currents in both feeders may be greater than the total load current.
A related effect is paralleling transformers with differing X:R ratios.
When transformers with differing X:R ratios but the same % impedance are operated in parallel, they will still share the load in proportion to their KVA ratings but the sum of the transformer KVAs will be greater than the load KVA.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Reactance losses cause phase shifts.
Resistive losses do not cause phase shifts.
Other things besides line reactance cause phase shifts so it can become complicated.
But it would be interesting to compare the current phase angles with the tie ope and with the tie closed.
A phase angle that shifts when the tie breaker is closed may be a strong indicator of unbalanced reactance.

Bill
--------------------
"Why not the best?"
Jimmy Carter