LTC for paralleled transformers 1

[EECougar]

Is a load-tap-changer (LTC) required for all paralleled transformers? If so, why? If not, what are the circumstances that one will not be required?

 

[dpc]

If the transformers have the same voltage ratings and can be place on the same tap, there is no need for an LTC. This assumes two transformers in the same location.

You just need the effective turns ratios to be the same, regardless of how you get there.

But to equally share load, the two transformers need to have the same impedance.

David Castor

http://www.cvoes.com

 

[waross]

In the unlikely event that two transformers have the same pu impedances but unequal X:R ratios, They will still share the load but the total current may be a little less than the arithmetic sum of the individual currents.
If one transformer of a paralleled set has an OLTC then the second transformer should have an OLTC slaved to the first OLTC. Different taps mean different turns ratio and that results in circulating currents.

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

 

[EECougar]

So, basically, if the transformers have the same characteristics (voltage rating, impedance, X:R ratio, etc.) and placed on the same tap then a LTC is not required. Otherwise, circulating currents and unequal load sharing will result. Is this correct?

 

[waross]

Unequal impedances result in disproportionate load sharing. Different turns ratios resulting from unmatched tap settings causes circulating currents.

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

 

[dpc]

As Bill said, if the transformers have different impedances, they will not share load equally, regardless of LTC or not.

David Castor

http://www.cvoes.com

 

[stevenal]

Perhaps my understanding of the question is different from the others, but I don't see that paralleled transformers affect the need for LTCs. LTCs or regulators are needed when voltage will unacceptable for the loading conditions otherwise. The choice of regulators versus LTCs is an economic one. If LTC transformers or regulators are paralleled, they must have controls set up for this to avoid recessive circulating current. Everything you will ever need to know about it can be found at beckwithelectric.com.

 

[EECougar]

Typically, power transformers are paralleled because the load growth has exceeded the capacity of the existing transformer and the age of the transformer precludes scrapping or relocating them and purchasing a new larger one. With that said, is a LTC only necessary if the existing transformer has a LTC? Or maybe a better question would be, what are the scenarios in which a LTC is required?

 

[catserveng]

Try taking a look at this,

http://www.beckwithelectric.com/docs/tech-papers/advxpara.pdf

I did some control and wiring work on several transformers using the Beckwith controls last year, this paper provided some answers I couldn't get from the people I was working with, hope it helps you too.

Mike L.

 

[TurbineGen]

If your existing transformer has an active LTC and you are going to parallel a new transformer with that, I would think you would want an LTC to control circulating currents between the units. This is one scenario where I would think it would be required.

If the LTC is inactive and thus remains on the same tap all the time, then I wouldn't think that the second would need an LTC. However I would probably still opt for one.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[stevenal]

How did the word recessive get in my post above? Excessive was what I meant to say.

No, it is not required. You could lock off the existing LTC and go unregulated. You could install (a) regulator(s) beyond the interconnection point (LTC still locked off). Or you could pair the new transformer with a regulator so the transformer regulator pair is paralleled with the LTC transformer. Matching impedances and ratios might be tricky for the last choice. I would opt for an LTC transformer closely matched in ratio, impedance, no of steps, and voltage per step.