Current imbalance on 3 phase on all motors at sewer plant

[hockeyoligist2]

We have a 6% or more current imbalance at one of our sewer plants, every motor in the plant has lower amperage on C phase.

It started on Nov. 21 as far as we know, the blowers started shutting down every few hours because of it. The operator just reset them from a remote computer and didn't call in any of our electricians since the backups would start and we all went home early for the holiday. Then there was a power outage on the 22nd for about 1 hour, the plant switched to generator power,no electricians were called in. So we didn't know about the imbalance or outage until Fri. after I got called in.

The the voltage had went up to as much as 507v phase to phase, with spikes as high as 511v according to our trend monitor. It's a 480v system and the voltage has been a little high for 2 years, since the new generator and transformer was installed, around 488v.

We had them to do a tap change today to lower the voltage and we are back to 480v with a load on it, 490v with no load.

We hadn't been able to do a closed transition, the generator wouldn't sync with the utility because of the high voltage. And due to DEHEC regulations we didn't want to do an open transition. We did get it to sync before he made the tap change.

The current imbalance went away on generator power.

POCO has been saying it's on our side and that they are within their perimeters. The guy that did the tap change left as soon as he was finished so I didn't get to talk to him and let him know that the imbalance went away on generator power. It only took him 2 or 3 minutes.

If it is on our side it would have to be in the 2 year old breaker for the utility or the 2 year old wires running 20' from their transformer. Doubtful.

I don't have a clue what could be causing it on the POCO side and I doubt that they will admit that it is on their side since they are within their perimeters.

Any ideas?

 

[kaichosan]

I am not sure of your WWTP configuration. I am guessing that you have two main transformers feeding a main 480V switchgear with tie breakers. And when on generators, the incoming feed from the transformers will be opened and the phase balance is OK under standby-generators.

You could try each transformer feed to see if one transformer is contributing to the imbalance.

Do you infrared your equipment for hot spots?

 

[SG-1]

If the trouble is on your side it has to be between the bus & the utility or you would see the imbalence while running on generator. Maybe a cap bank going bad on the utility side?

 

[kwired]

Is utility voltage unbalanced even with no load on it? Kind of hard to blame the plant if it is.

 

[brian john]

What is the phase to phase voltage and phase to neutral for each parameter.

I was told that a 1% voltage unbalance can result in a roughly a 10% current imbalance resulting higher motor operating temperatures and of course a shortening of motor life.

A-B
B-C
C-A
A-N
B-N
C-N

 

[hockeyoligist2]

I am not sure of your WWTP configuration. I am guessing that you have two main transformers feeding a main 480V switchgear with tie breakers. And when on generators, the incoming feed from the transformers will be opened and the phase balance is OK under standby-generators.

You could try each transformer feed to see if one transformer is contributing to the imbalance.

Do you infrared your equipment for hot spots?

It only has one transformer and one generator, a small plant.

We infrared and plan to do it again tomorrow.

"If the trouble is on your side it has to be between the bus & the utility or you would see the imbalance while running on generator. Maybe a cap bank going bad on the utility side?" The POCO was saying no, but they also don't know about the balanced current when running on generator. They will be informed tomorrow. We may be helping them find their problem because they have been getting complaints from residential customers about blips.

"Is utility voltage unbalanced even with no load on it? Kind of hard to blame the plant if it is."

It is a little unbalanced, after the tap change, varying about 6v now, but I only monitored it for about 1 hour while we were on the generator. It seems to be very unstable late at night, under load, about 3 AM during the past week.

 

[brian john]

At 1% 480 VAC would be 4.8 VAC, Though if I remember voltage imbalance is calculated as a average of the measurements and the difference between high and low (I'll try to locate this)

Not what I was looking for but this might help????

http://www.cncexpo.com/VoltageImbalance.aspx

 

[kwired]

It only has one transformer and one generator, a small plant.

We infrared and plan to do it again tomorrow.

"If the trouble is on your side it has to be between the bus & the utility or you would see the imbalance while running on generator. Maybe a cap bank going bad on the utility side?" The POCO was saying no, but they also don't know about the balanced current when running on generator. They will be informed tomorrow. We may be helping them find their problem because they have been getting complaints from residential customers about blips.

"Is utility voltage unbalanced even with no load on it? Kind of hard to blame the plant if it is." It is a little unbalanced, after the tap change, varying about 6v now, but I only monitored it for about 1 hour while we were on the generator. It seems to be very unstable late at night, under load, about 3 AM during the past week.

Is there a large load that switches around 3 am in the area? Maybe a plant starts first shift production around that time, something else that may be in area that changes around that time, closing time at all the bars

 

[Hv&Lv]

It seems to be very unstable late at night, under load, about 3 AM during the past week.

Sounds like a regulator is stuck, and the loads during the daytime are keeping the voltages within tolerances. As the loads diminish during the night, the voltage goes high because the regulator won't step down. Just my guess... Ask them if they will monitor voltage at a house served from the problem phase, or check the regulator for operations.

 

[zog]

Have you tested the transformer per NETA specs? How about the breakers associtaed with that transformer?

 

[hockeyoligist2]

Is there a large load that switches around 3 am in the area? Maybe a plant starts first shift production around that time, something else that may be in area that changes around that time, closing time at all the bars

I don't know if a plant is starting production up at that time. But I know it's not the bars, city law requires them to close at 12!

 

[hockeyoligist2]

Have you tested the transformer per NETA specs? How about the breakers associtaed with that transformer?

The transformer belongs to Mr. Duke Power so we can't test it. Only one breaker associated with it in our switch gear. We plan to infrared it in the morning.

 

[brian john]

The transformer belongs to Mr. Duke Power so we can't test it. Only one breaker associated with it in our switch gear. We plan to infrared it in the morning.

I would also perform a fall of potential test from the utility CT cabinet to a motor starter, with the motor operational.

 

[zog]

The transformer belongs to Mr. Duke Power so we can't test it. Only one breaker associated with it in our switch gear. We plan to infrared it in the morning.

Thats Mr Progress now

 

[broadgage]

I would suspect that the voltage from the utility is unbalanced, with one phase higher than the others.
As others post, it takes only a small voltage difference to cause significant current inbalance on 3 phase motors.

Although most motors will tolerate a voltage between 90% and 110% of the nameplate, this presumes that each phase is the same, or very nearly the same.

I suggest carefully measuring the phase voltages on load at both the service and at the motor.
If the voltage is balanced at the service, but unbalanced at the motor, then that suggests a problem within your installation such as poor connections or undersized wires on one or two phases.

If the voltage is unbalanced at the service, then it is the utilities problem, though for a small unbalance they may argue that it is within accepted tolerances.

Does the plant contain any significant single phase loads ? If so you may be able to relieve matters by transfering as much single phase load as possible to the highest voltage phase, thereby increasing the voltage drop on that phase.

If all else fails you may have to accept the imbalance. If motor overloads are tripping due to overcurrent on the highest voltage phase, then you may have to use a larger motor in order that the motor current, even if unbalanced, will still be within the nameplate rating.

Another bodge is to use different size wires on the different phases of the motor branch circuit.
Use the minimum permitted by code on the highest voltage, and say two sizes larger on the other other phases. This will help to an extent, but is poor practice and very much a last resort .

 

[Hv&Lv]

Thats Mr Progress now

Wait, there some buzz around Charlotte that Progress bought Duke? It was my understanding there is no more Progress. It is all under Duke Energy now.

 

[Phil Corso]

Hockyologist...

Can you provide the following contemporaneous measurements:

A) Phase-phase Voltages?

B) Phase-neutral voltages?

c) Phase-ground voltaghes?

D) Line currents?

Regards, Phil Corso

 

[hawkeye23]

This same thing happen to a friend of mine at a plant he work at, during the middle of the night the generator came on and would not shut down after hours of running. The utility was doing work and made a wrong tap change and feed the plant over 500 volts which the gen saw as overvoltage . Utility made a correction and everything went back to normal.
Just sayin.

 

[hockeyoligist2]

Phil....

A) Phase-phase Voltages? Before tap change A-B 507 A-C 498 B-C 497 After tap change A-B 480 A-C 479 B-C 481

B) Phase-neutral voltages? No neutral

c) Phase-ground voltages? Before tap change A 288 B 286 C 282 After tap change A 279 B 280 C 278

D) Line currents? I only remember the amps at the blowers and don't have my notes with me. Before tap change, They were A 136 B 130 C 116 After tap change A 147 B 144 C 122

 

[Phil Corso]

Hockeyologist?
Before-Correction, Ph-Ph Voltage and Line-Ampere Unbalance Factors
Calculated VUFs (Voltage-Unbalance-Factor) were 1.26% per NEMA and 1.27% per IEC Standards. The corresponding AUFs (Amperage-Unbalance-Factor) were 6.3% and 6.9% per NEMA and IEC Standards, respectively. The VUF indicates an incorrect tap-setting was probably the problem! Are they 1% steps!

After-Correction, Ph-Ph Voltage and Line Ampere Unbalance Factors
NEMA and IEC VUFs were 0.21% and 0.24% respectively. But the AUFs, were significantly higher, i.e, 11.3% and 11.2% respectively. Thus, if you are sure of your readings there is still a problem, but it is unrelated to supply voltage!

Before & After Correction Ph-Gnd Voltages
VUFs were 0.9% and 1.8% indicating phase-to-ground capacitances are balanced!

Regards, Phil Corso