Faulty Transformer?

Richie V6 · Jul 17, 2019

Hello All. We recently moved into a new commercial faculty. We are trying to figure out our high power consumption. Our first bill showed a use of 3400kWh while we were moving. During the move we did not use any equipment other than computers, lights and AC for a 1500SQFT office.

We did some digging and found that our Delta-Delta 480V to 240V transformer was putting out a lot of heat. We shut off all the loads on the secondary and measured input current on the primary.

Primary input current (No load)
L1 = 8A
L2 = 8.4A
L3 = 11.5A

Is this typical input current for a unloaded 150kVA transformer?

I just did some more detailed measurements with equipment running and it appears the primary input current is unbalanced. However the voltages are close.

Primary input current (Loaded)
L1 = 29.2A
L2 = 11.4A
L3 = 34.5A

Primary phase to phase voltages (Loaded)
L1-L2 = 494V
L2-L3 = 491V
L1-L3 = 491V

Primary phase to neutral voltages (Loaded)
L1 = 284V
L2 = 285V
L3 = 283V

Secondary phase to phase voltages (Loaded)
L1-L2 = 247V
L2-L3 = 244V
L1-L3 = 243V

Secondary phase to neutral voltages (Loaded)
L1 = 122V
L2 = 213V
L3 = 122V

I called the utility out and they claim the power outside of the building is fine. My electrician thinks the utility is sending us unbalanced power causing the transformer to heat up.

Any ideas? Any help would be appreciated.

Wirenuting · Jul 17, 2019

Welcome to ET,

Please finish filling out your profile.

frenchelectrican · Jul 17, 2019

Please fill your profile as you will see it on user CP on upper left corner of the screen that you can fill it in.

( you may have to use the desktop verison to get this running correct )

that is a 4 wire delta transformer.

Richie V6 · Jul 18, 2019

Profile filled out. Thank you gentlemen.

CoolWill · Jul 18, 2019

3400 kWh... As in 3.4 MWh?

Your loaded transformer drawing 35 amps would use that in about 117 hours. How long were you moving and are you sure there was no previous balance on the bill?

joebanana · Jul 18, 2019

Looks like your panel is out of balance. What is your electrician smoking? "sending unbalanced power"? It's not what they're sending, it's what you're drawing. 11.5 A unloaded on L3 along with 34.5 A loaded on L3 (with 11.4 on L2), needs further investigation. Have you done a visual on the xfmr.?

Cow · Jul 18, 2019

It's a 240v delta configuration on the low side. I'm not surprised to see A and C phases more heavily loaded than B. That's typical.

I can't help with the idle current though, I don't have a 150kva to measure.

If I take your primary current amp readings with no load:

8+8.4+11.5=27.9/3=9.3 amp avg/phase

9.3 x 480 x 1.732=7732 w/hr=7.732 kw/hr

3400 kw/hr / 7.732 kw/hr=439 hrs/24 hrs day=18.3 days to get 3400 kw/hr of power consumed.

I also think your electrician may be out of his depth when he thinks the utility is sending unbalanced power even though you posted it's all within 3 volts under load(491-494)......

You may be able to request another service to get your lower voltage house power, with service installation and monthly meter charges, the payback may be a ways out there.

Richie V6 · Jul 19, 2019

Thank you all for the help. They sent out another electrician yesterday to do some measurements. They are going to call the transformer manufacture and see what the transformer losses should be. Without knowing the power factor we can't come to any conclusions.

Weasel · Jul 19, 2019

Is the transformer bonded to a good earth ground terminal X0

Richie V6 · Jul 22, 2019

I will have to check and get back to you.

Southeast Power · Jul 22, 2019

I would say to shut down the transformers or put a load on them.

micromind · Jul 23, 2019

The figures in the first post sound pretty close to what I'd expect from a 150 KVA transformer.

The difference between them doesn't surprise me at all.

Faulty Transformer?

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