Electric Utility with balanced phase to neutral voltages and unbalanced phase to phase voltages

[k2panman]

I am an electric utility engineer. On a 20 mile long 25kV distribution feeder, 336.4kcmil AAC Tulip phase conductor with a 7#8 Alumaweld neutral, peak load about 9MW around 200A/phase. We are having trouble with voltage unbalance. Eighteen miles out on the feeder we have a set of three single phase voltage regulators. Phase to ground voltage magnitudes are all being held well matched, but the phase to phase voltages are unbalanced and causing problems for some of our three phase customers. This problem has been here longer than I have, but we've never addressed it as the end of line customers are usually fed from a different source.

We have capacitor banks along the feeder. They all check out to be operating ok.

Our primary neutral conductors are grounded at every pole. The feeder runs down a chain of very narrow low elevation porous rock islands (Florida Keys), so lots of our pole grounds are in salt water.

What are all the probable causes for the phase to phase voltages to be unbalanced while the phase to ground voltages are all ok? I understand that the phase angles between voltages are not equal - is this all caused by unbalanced load, or should I be looking for other issues also?

K2ofKeyLargo

 

[PHovnanian]

Post numbers, so we can get some ideas of how bad this is. Also, what is the line configuration? Its possible that a long, untransposed line will exhibit different impedances on each phase.

What do the phase to phase and phase to neutral voltages look like before the voltage regulators? Do they typically operate on the same (or close to it) taps?

If that's not it, start working your way back upstream toward the substation feeding this circuit looking for imbalances. What is the substation transformer configuration and impedance?

 

[waross]

Unbalanced phase to neutral loads will cause a voltage drop on the neutral. Your voltage regulators will correct the phase to neutral voltages but may make the neutral voltage drop worse. So, your phase to phase voltages are equal past the voltage regulators but the voltage drop on the neutral causes the wye point to shift away from the neutral point. This causes phase angle errors and unequal phase to phase voltages. Three phase induction motors act as induction generators and develop a back EMF almost as great as the applied voltage. The problem is, this back EMF is equal in phase angles and voltages. When the motor is energized from a line with unequal voltages the motor tends to take more current from the higher voltage phase. The phase angle errors make things worse.
The current unbalance is much greater than the voltage unbalance that caused it.
I was a consultant to a sawmill in Central America where this was a BIG problem. We were out on a long line with a small town about 15 miles ahead of us. To make things worse, we were the end of the three phase line and only two phases carried on past our mill.
On the two phases were several villages and a small town. There was also a small mill and a dairy running open delta transformer banks. This caused a neutral current of similar magnitude to the two phase currents on the line past our mill. This in turn led to a arge neutral voltage drop.
We had a lot of issues and a lot of motors running very hot.
A four-wire-wye:delta bank will cause a neutral current such as to tend to correct the issues, however the four-wire-wye:delta bank has its own issues. The delta voltage is not important.
The unbalanced voltages and angles cause a circulating current in the delta which causes a neutral current and phase currents that tend to correct the issues.
Balancing the load ahead of the voltage regulators will help but it is sometimes impossible as different users may have peak usage at different times.

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

 

[davidbeach]

Ya know, Bill, you ought to write a FAQ about that. I've seen you answer that same question over and over again. With a FAQ the rest of us could just point to it to and save you a bunch of typing.

 

[waross]

Thanks for the comment David.
Do you have this issue on any of your distribution circuits?
I ran into it several times on long distribution circuits with a lot of single phase loading and then a three phase load.
As I remember, the shortest distance out to the start of the issues was about 15 miles. Some circuits were much longer.


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

 

[davidbeach]

I've not heard of us having that issue, but my focus is much more on the transmission system rather than distribution.

 

[jghrist]

It is unusual in a distribution feeder to have a relatively high impedance neutral like alumoweld. I would expect maybe #4/0 AAC to be used with 336.4 kcmil AAC phase conductors. If you have much unbalanced load, then you will get excessive voltage drop (and losses) in the neutral. Neutral voltage drop will cause phase angle shifts. Voltage regulators will make the Ø-n voltage magnitudes equal, but will not change the voltage angles and you will have unbalanced Ø-Ø voltages.

I suggest that you model the feeder with a distribution analysis program. Then you can see what effect different mitigating measures will be, like balancing the loads or reconductoring the neutral. Make sure that you use a program designed for distribution systems with unbalanced loads and that can model voltage regulators.

 

[prc]

Waross, what is the preventive action for such a situation? Will delta /star transformer connection mitigate the problem?

 

[waross]

Mitigation:
The first step is to balance the single phase loads on the distribution line that are causing the unbalance. This is not always possible.
Different customers may peak at different times of the day.
Look for large open delta loads. The issues typically arise on long rural distribution lines. A large irrigation pump or similar load on an open delta transformer bank will cause these issues. Changing from open delta banks to three transformer Y:Y banks will balance these loads.
Floating a Wyeelta transformer bank on the line will do a good job of balancing the circuit. I have seen this happen inadvertently, and when the unused Wye;Delta bank was disconnected, the phase to phase voltages became badly unbalanced.
HOWEVER, due to the unintended consequences that often develop with Wye;Delta banks I don't recommend this approach.
The usual solution is to live with the problem. You may have to oversize the transformer bank and the three phase motors at locations experiencing issues.
The motors are trying to correct the phase unbalances (both voltage and angle) with back EMF. Changing the %Imp of the customers transformers may help somewhat. I suspect that higher impedance transformers may be the way to go, but I am still mulling this over.
Know that by leaving the unbalance and trying to correct downstream, we are treating the symptoms, not the problem and each treatment of a symptom may have unintended effects.
I am still thinking about this problem and I may post more later.
BOTTOM LINE:
First step, try to balance similar loads as well as possible.
Second step, if need be, oversize transformers and motors at problem locations. Use the highest impedance transformers available and live with the greater voltage drop.



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

 

[prc]

Thank you Waross.

k2panman, what is the rating and connections of the transformer at sending end of the feeder?