Parallel Operation of Transformers

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VBelmonte_DCDC1001

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Dear All,

I would like to ask if it is possible for 3 units of 120kvA transformer to be connected in parallel (Note: 2 Units are new, while the other 1 is existing and running for about 1 year as of today)

Regards,

Victor Belmonte
 
Re: Parallel Operation of Transformers

It is possible to operate transformers in parallel, but a great deal needs to beknown to see if yours will.

Is this a single phase installation? If so transformers with nearly identical impedances, with the same turns ratio, with the same tap settings, with the same polarity, and the same frequency ratings, can operate successfully

Is this 3 phase operation? If so in addition to the above concerns, you must have banks with the same angular displacement.

Overcurrent protection will become more of a challenge.

Post some additional information, and you should find out lots more.

Jim T
 
Re: Parallel Operation of Transformers

There is also an issue with available fault current. With three transformers in parallel, the amount of fault current the system can put out will triple.

Why do you want to do this anyway?
 
Re: Parallel Operation of Transformers

Transformer specs:
120 kVA,3Phase,60Hz
Primary: 480V
Secondary:380V

Our client is a call center and they want their power to be redundant. They want their 3-80kVA UPS to operate in parrallel. We need to install transformers to convert the building supply from 480V to 380V because the UPS is 380V.
The designer is want to us install transformer common output panel board before going to the UPS

regards,
Vic
 
Re: Parallel Operation of Transformers

Charlie B, would the available fault current triple or is it more than that? Not only are you tripling your system VA but your impedence is about 1/3, wouldnt the available fault current be more like 9 times as high or am I missing something here?
 
Re: Parallel Operation of Transformers

Not a bad question, Zog. The answer is that it would triple, and the reason is that the resistance drops to 1/3 of the value for one transformer.

The VA rating does not come into play here. A component's VA rating is based on its ability to reject heat. That is a long term consideration. A fault is a very short term event. The amount of fault current comes for nothing more complicated than Ohm's Law. With the three transformers in parallel, the voltage is the same as it would be with just one transformer. The resistance is 1/3 of a single transformer resistance. The current will therefore be three times higher.
 
Re: Parallel Operation of Transformers

OK, but what happens to voltage during the fault? Voltage drops, so I dont think it is as simple as using ohms law.

Wouldnt the fault current equation be Isc=(100*kVA)/(1.732*kV*%Z)?
 
Re: Parallel Operation of Transformers

If you put (3) 100KVA transformers in parallel, each with 6% impedence, you do NOT get the equivalent of a 300KVA transformer with 2% impedence.

The way %impedence is defined for a transformer, makes it directly related to the KVA capacity. You short the secondary, and increase the voltage on the primary until the RATED current flows on the secondary.

So I agree with Charlies answer, the fault current becomes 3X a single transformer, but for a different reason. The impedence of the transformer bank remains the same, but the fault current goes up because the KVA has gone up.

I guess you can claim either the KVA has gone up, or the impedence has gone down. It's really just a matter of semantics. But you cannot claim both have changed.

Steve
 
Re: Parallel Operation of Transformers

I agree it is essentially semantics. I find it easier to visualize in terms of a Thevenin equivalent circuit. That's why I prefer to say that the VA means nothing, the voltage is the same, and the impedance has gone down.
 
Re: Parallel Operation of Transformers

Zog:

I think you said it better than I did. The impedence of 3 parallel transformers is lower than just 1, but the %Z stays the same.

Steve
 
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