Utility Transformer Clarification

RRoc · Feb 5, 2017

My Pump Station Load is listed below and the Main Utility Transformer Supplying this load is 150KVA. From adding up all the current draw. The Pump Station Load is greater than the Secondary Transformer FLA. The Utility Engineer informed although the FLA has been exceeded the Operational Limits of the transformer has not. Can someone explain why this is ok?

Pump Station Load
50HP, Current Draw = 65
50HP, Current Draw = 65
50HP, Current Draw = 65
15HP, Current Draw = 21
15HP, Current Draw = 21
25HP, Current Draw = 34
2HP, Current Draw = 3.4
Total = 274.4

Utility Transformer
Pri Volt: 16.34 KV, Secondary Volt: 480VAC
Rating: 150KVA, Secondary FLA: 180 FLA

T Gibbs · Feb 6, 2017

Can all the motors run at the same time?

James Mcquade · Feb 6, 2017

Don't forget to derate the transformer based on ambient temperature.
What is the transformer type of cooling?
Waiting on the previous post answer.
james

RRoc · Feb 6, 2017

@T_Gibbs: Yes all motors are running at the same time.
@James_Mcquade: See picture attached. This is all the info that was given to me. It's a typical Utility Transformer that is owned by the Electric company that supplies the pump station I am analyzing.

Tom Jenkins · Feb 6, 2017

My guess is that the motors are not drawing rated power. This is common. Pumps only draw full power at rated flow and head, and the motor is usually sized above the rated power draw.

James Mcquade · Feb 6, 2017

RRoc,

I see where the transformer is not derated, but i'm not Utility Engineer and cannot second guess what he did.

james

T Gibbs · Feb 7, 2017

Tom Jenkins said:
My guess is that the motors are not drawing rated power. This is common. Pumps only draw full power at rated flow and head, and the motor is usually sized above the rated power draw.

That was my thought, too. The amps that the OP has listed are from the NEC chart. Motors rarely pull as much as the chart.

AustralIan · Feb 7, 2017

Do you have VSDs on your pumps?

It is possible that the rating is to keep the temperature of the transformer down but also to keep the voltage distortion low.

As you are the only customer, and you are just running motors, they probably don't care about voltage distortion as much.

Helliana · Feb 7, 2017

Your motors have a total of 207 horsepower, which is 154KW. I doubt all your motors are 100% loaded and as long as your power factor is not aweful, you are right at the rating of the 150KVA transformer. Generally, the utilities only stock specific sizes of transformers, so i bet the next size they install is 300KVA which costs more a lot more and is most likely not needed.

Brandon_K · Feb 7, 2017

I ran this past a lineman friend of mine.

He said pole cans can generally run 200% of name plate, sustained with a good breeze. 300% for short spans.

I suspect the utility engineer knows this and knows that even if you pulled 100%, you're barely breaking the name plate on the can, that can supply a good bit more.

Helliana · Feb 7, 2017

Brandon_K said:
I ran this past a lineman friend of mine.

He said pole cans can generally run 200% of name plate, sustained with a good breeze. 300% for short spans.

I suspect the utility engineer knows this and knows that even if you pulled 100%, you're barely breaking the name plate on the can, that can supply a good bit more.

My father worked for a utility company that sized all their oil filled pole cans at 200% as well. Pad-mount were still only 100% though.

Also, he used the following method for sizing transformers:

If it burned up and had been installed for 20 years, he replaced it with the same size transformer.
If it burned up and it had been installed for very long, he replaced it with the next size up.

V0N_hydro · Feb 7, 2017

If you are pulling over 150 kVA you may also need to be concerned about the cables from the transformer secondaries to your building's power distribution center, they are probably sized to the transformer and may not have the same access to free air as the transformer and not take as kindly to the excess current.

Utility Transformer Clarification

RRoc

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T Gibbs

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James Mcquade

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RRoc

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Tom Jenkins

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James Mcquade

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T Gibbs

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AustralIan

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Helliana

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Brandon_K

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Helliana

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V0N_hydro

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