facility current unbalance 1

[lrohn]

When checking equipment capacities and equipment utilization efficiencies at a recently acquired facility found that phase current to motors was unbalanced. e.g. 25 hp motor with phase current of 32,32, and 20 amps. This type of unbalance is at every motor ranging from 3hp to 100 hp. Power is provided thru 3- 500kva xfmrs connected delta primary and delta secondary with one phase grounded. Xfmr secondary and motor voltages are approx 459, 459 and 472 VAC which is 2.8% unbalance. Had the utility check their side and they say is ok. I checked the few small 1ph. lighting xfmrs and single ph. loads are fairly evenly divided. The distance to the elect rm equipment from the xfmrs is about 80 feet with wire lengths equal. Located the old electricial contractor that did work for the previous owners and they said they noticed problem never found a good solution or even the cause. They changed one of the 500 kva xfmrs but did not help. In effect just de-rated the motors and operated in that manner. They were eager to come in again and run up another bill. Any ideas or other suggestions of other things to look at?

 

[Alwaysexciter]

The current unbalance is the result of the 2.8% voltage unbalance. The 25 HP motor you gave as an example will have 4 amps of Negative Sequence Current as a result. This current causes additional heating in the motor windings mostly around the end turns. Typically, motors are designed to handle up to 20% Negative Seq. current.

Most large motors (500+ HP) will have relays that monitor this current and trip if say 16% NSC is exceeded.

The cause of your voltage unbalance seems to me will be the result of 1 phase of your utility feeder coming in being a little low. This may however be within the allowable veriation that they garrentee. It could also be from a loose connection on the line. Have you done a IR scan lately?

I would have the actual incoming voltage measured accuratly. If voltage is over 600 volts and you don't have a meter & PT's to do this it could be done at the Utilities WHM if it is on the incoming line but they will have to do it.

Also, Verify that the transformer taps are all set the same.

 

[busbar]

This has been a topic in electrical fora at Eng-Tips. Per-unit voltage imbalance in 3ø induction motors may cause on the order of six per-unit current imbalance. One thing to try is “lead rolling” on some larger motors to determine if the imbalance follows “line” or “load;” e.g., by swapping T1-T2-T3 from L1-L2-L3 to L2-L3-L1.

 

[lrohn]

Have not yet done an IR scan but have it scheduled.
Tried lead rolling and problem appears to originate on the line side so may have to get power comany involved again.
thanks

 

[busbar]

Out West, utilities and PUCs typically call out a 2½% voltage-unbalance limit, but that is at the facility service entrance or “point of common coupling.” Looks like your case is a very close call. ANSI Std C84.1, Annex D lists a motor derating factor of ~0.9 for 2.8% imbalance.

Search this site for “unbalance / imbalance / balance” to read significant discussion on the subject.

 

[GamezBeCJ]

One solution would be to take care of the imbalance with the transformers. If, as I understand, you have 3 one phase transformers conected in delta-delta, you may move the tap changer of one phase to overcome the imbalance in the line feed. Usually, at this kVA, transformers have 2.5% step taps, which is very close to what you need. The only thing to consider, is that you would have a circulating current in the transformer bank delta, so you would have to derate the transformers, but you would have on the other hand a balanced supply of power for your whole facility. Hope this helps.

Carlos Gamez, P.E.
Industrial Consultant
Transformer Specialist