Transformer Excitation 1

[rjb83]

I've got a question concerning XFMR ratios and excitation. Assume a Wye/Delta 138kV to 13.2kV GSU (138kV/13.2kV = 10.455 turns). Are there any issues with using a Wye/Delta 138kV to 13.8kV GSU, if I adjust the taps to closely match the 10.455 (set tap at 144,900kV to yield 144.9kV/13.8kV=10.5 turns). Matching ratios seems the obvious thing to watch for, but are there any possible issues with under exciting the core, operating it at 13.2kV instead of the designed 13.8kV? Losses may change?

 

[electricpete]

I'm a little rusty so take it with a grain of salt.
Your transformer nameplate is 138kV/13.2kV.
You have high side taps but no low side taps.
You want low side system voltage around 13.8 and high side around 145kv.
You want turns ratio to be 145/13.8
So your high side voltage tap Hshould be set so that
H/13.2 = 145/13.8
That means H = 145 *13.2/13.8
You should adjust your high side tap down (not up) shouldn't you?


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(2B)+(2B)' ?

 

[electricpete]

Sorry, I didn't do the calculation:
H = 145 *13.2/13.8 = 138.7
Isn't that (138.7) where you want to set your tap?
Maybe there is transformer voltage drop to consider as well.

Better to wait for someone else to weigh in.


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(2B)+(2B)' ?

 

[rjb83]

To clarify, the low side will be operated at 13.2 so to maintain 138 high side, I would need to tap at 144.9. There are obvious concerns if you overexcite a transformer, but I am curious as to any things to watch for under exciting it, other than the need to retap to maintain needed high side voltage. I assume there would be a change in losses.

 

[magoo2]

I believe the transformer standards permit 5% overvoltage at full load. I think you'll be OK since you're looking at a 4.5% overvoltage.

 

[rcw retired EE]

The 13.8 kV transformer is probably designed to operate at 13.8 kV near or above the knee of the excitation curve. Operating above nameplate voltage pushes the core further into saturation. Operating at lower voltage moves the operating point down the curve away from the knee and most likely improves the no load losses.

From factory test data on 378 MVA GSU, 18 kV, Voltage and No Load Losses:
90% V 111 kW
100% V 146 kW
110% V 207 kW
115% V 256 kW.

 

[rjb83]

That's what I was looking for regarding losses. The 13.8kV XFMR would be operated at or near 13.2kV.

 

[waross]

You may want to recalculate the KVA at the lower voltage. Remember that the rated current will remain the same and the KVA rating will drop slightly at the lower voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

ok, I guess I'm lost. Maybe I can learn something. Please help me understand this.

You have high side taps but no low side taps.
You want low side system voltage around 13.8 and high side around 145kv.
You want turns ratio to be 145/13.8

Is what I stated above correct?

If so, why isn't the high-side tap set to:
H = 145 *13.2/13.8 = 138.7

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(2B)+(2B)' ?

 

[electricpete]

I meant to also verify this piece:

Your transformer nameplate is 138kV/13.2kV.

(at middle tap of high winding). Correct?

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(2B)+(2B)' ?

 

[electricpete]

OK, I saw your 2nd post and re-read your first.
I had it all wrong.
Never mind.

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(2B)+(2B)' ?