Voltage Unbalance Problem

[fjrivera]

Greetings to all,

For the past two weeks, we are experiencing voltage unbalance after few days of bad weather.

Our main transformer is a 5000KVA delta to wye with primary voltage 38kv and secondary voltage 8.32kv. Then, each building have step down transformer from 8.32kv to 480/277v.

To please the power authority, we tested our main transformer (insulation, turn ratio, etc), feeders megger, performed thermal images to the substation, etc resulting in satisfactory result.

After this, the Power Authority began testing the primary voltage resulting in a voltage unbalance of approx. 5.5% in the entrance of our facility (with and without the load of our plant). Also, they performed thermal imaging from their substation to ours resulting only in one hot spot in one feeder connection at the entrance of our facility. They corrected this issue but the unbalance persists.

The voltage unbalance is affecting several motors (i.e chillers, centrifugal pumps) and not permitting to start because of protection.

Another facility in the same primary line has the unbalance but not phase to phase but phase to ground

Any suggestion of any other possible cause?

 

[LarryFine]

After this, the Power Authority began testing the primary voltage resulting in a voltage unbalance of approx. 5.5% in the entrance of our facility (with and without the load of our plant).

Clearly on their side of the jurisdiction dividing point, it seems. What could you do about it?

 

[fjrivera]

What about this situtation: "Another facility in the same primary line has the unbalance but not phase to phase but phase to ground"?

At least from our side I think there is nothing else to do.

For high voltage, what is the maximum permissible voltage unbalance? 5%?

 

[zog]

What about this situtation: "Another facility in the same primary line has the unbalance but not phase to phase but phase to ground"?

At least from our side I think there is nothing else to do.

For high voltage, what is the maximum permissible voltage unbalance? 5%?

Usually +/- 5% is allowed, and shouldnt cause you any problems. Also I hate to say it but in my experience, when the utility does the testing, there is never a problem, you may want to consider hiring an independent testing company to check it out. www.netaworld.org

 

[Lcdrwalker]

I would agree that a 5% umbalance would be acceptable, however, if you are experiencing starting problems, the unbalance is probably worse under load. I would find the worst case and start working my upstream.

 

[beanland]

Voltage Regulators!

Voltage Regulators!

The POCO probably has a bad voltage regulator or transformer LTC. Because your transformer is delta primary, you are seeing the problem as line-line. The other customer has wye-wye transformer so is seeing only one phase low.

 

[fjrivera]

Thanks for your responses

Now Part II

Early this week, power authority measured the primary service with a high voltage voltmeter/phaser with hot sticks. Voltage unbalance persists. Now they are saying that the magnetism from the system is afecting the voltmeter/phaser measures and this measures are not reliable. Can this be true? I have always used these kind of meters without any problem in the past.

Also, we performed all kind of testing to main transformer like winding resistance and dissolve gas in oil (still waiting for results) loking for hot spot in the transformer.

Megger to main switchgear were performed resulting in several issues with surge arrester where few were leaking current to ground. Surge arrester were changed but problem persist.

Any suggestion?

 

[robbietan]

here in our utility, measurement of unbalance is done at the metering point, since it is where the law says unbalance should be measured. we have a 2.5% unbalance limit, which is slightly above the ANSI C84.1 recommended 3% unbalance.

however, measurements at the transformer secondary could result in a higher or lower unbalance reading than the result at the meter. as there is no perfectly balanced utility distribution, there is no perfect facility load balance.

the best thing you could do is to bypass an equipment's unbalance protection during starting. and restore it when the motor is at full speed. motors can take this unbalance voltage when they are just starting.

 

[beanland]

Phasing Sticks

Phasing Sticks

Early this week, power authority measured the primary service with a high voltage voltmeter/phaser with hot sticks. Voltage unbalance persists. Now they are saying that the magnetism from the system is afecting the voltmeter/phaser measures and this measures are not reliable. Can this be true? I have always used these kind of meters without any problem in the past.

High voltage phasing sticks are low accuracy devices. They would not be good for measuring a 5% difference. The magnetic fields are unlikely to cause problems with the inaccurate measurements.

Can you check voltage at a nearby facility fed from a power authority transformer bank? Especially if the load on the bank is light, this will tell you the primary voltage unbalance.

It still could be a bad voltage regulator, a capacitor bank that has some fuses blown, unbalanced loading on the line, or even unequal conductor sizing on the POCO line.