current phase imbalance

[steve5352]

my client recently had a utility company phase loss.
After correction, I checked the phases for current.
A phase 1077A, B phase 1116A, C phase 354A
The transformer is a 750kVa 277/480v secondary.

 

[ScottyUK]

Apart from "That's nice!", what do you expect us to say?

There's clearly a lot of single phase load either connected between A-B, or A-N and B-N, or a combination of both. You could re-distribute the loads to improve balance, but is it actually causing a problem to warrant the effort?


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If we learn from our mistakes I'm getting a great education!

 

[steve5352]

There are no single phase loads to speak of.

 

[Skogsgurra]

Then there is a problem with one of the major loads. I don't feel very smart when saying that.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

Not to mention the phase A and B are overloaded. 750 kVA transformer's rated full load current at 480V is 900A. Keep an eye on the transformer for smoke!

Are there 480V single phase loads?

I would also double check the amp readings and check the method, meter, CTs or anything else that is used for the measurements.

Rafiq Bulsara

http://www.srengineersct.com

 

[racobb]

Was this a problem before the utility phase loss? Where are you getting the readings? If it is a panel meter, then it may have a problem and I would use a clip on to see if the amps match what the meter indicates. Ask the utility to see what their meter shows as well.

Rafig is correct about the loading. Somehow I feel you need to do some more checking and would love to hear the rest of the story once you get more info.

Alan

 

[alehman]

I would also be verifying the currents with another meter.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[steve5352]

I rechecked the loads yesterday and they were normal,
A 738A B 723A C 696A.
I had the utility company set up ma five day monitor to see what is going on. I checked all larger motors and loads were normal.
Interesting! Any thoughts?????

 

[rbulsara]

Yes, earlier readings were erroneous.

You never mentioned how were they measured and if the second time measuring method was any different.

Rafiq Bulsara

http://www.srengineersct.com

 

[steve5352]

Same Greenlee CM-950 clamp on,same conductors,same method.
Also, voltage was normal both times.

 

[ScottyUK]

Did you use a tong-tester / clamp-on ammeter? Tong-testers can produce seemingly inexplicable results if the split cores do not close fully. Normally it's caused by a little bit of dirt which gets on the face of the magnetic core. If it is an ammeter then the readings are wrong - usually low in my experience, but I ain't seen everything - and if it is a scope probe then the amplitude and the phase displacement is affected.


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

Operator error seems the most plausible cause. Happens all the time. I would not scratch my head.

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

Yeah, my explanation certainly falls into 'operator error', it's just not an immediately obvious one!


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If we learn from our mistakes I'm getting a great education!

 

[steve5352]

Clamp on meter was used. Same meter was used both times.

 

[steve5352]

Clamp on meter was used. Same meter was used both times.