Transformer rating

[H1H2N3F4]

Hi every one;
I am a new engineer and I have a question to the group;
I have a small project where we need a three phase 225 KVA transformer.

This transformer size is not available in our yard and the installation is urgent. A friend of mine advised me to use three single phase 75 KVA transformers instead.

My Question, is it okay to use a three single phase transformers instead as per my friend advise? In other word, is a bank of three single phase (75KVA) transformers adds up to one three 225KVA transformer?

Regards

 

[jghrist]

Yes. The single phase transformers have to be compatible with the connection desired. For instance, if you want a delta-wye connection, the single phase transformers must be two-bushing type with line-to-line primary voltage rating.

 

[dpc]

It would be nice if the transformer impedances were about the same, as well.

 

[steelerfan28655]

Is the load going to be a constant 24 hour a day load, or will the transformer have time to cool off between peaks? I work for a utility and we are able to use up to 140% of a transformers name plate rating. Depending on the utilization.
Also is the 225 kVA a connected load or have you already applied diversity to your load calculations? Do you really need 225 kVA. We size by load not by code.
If you are bound by the NEC then your best bet may be to parallel the three 75's. As was posted before, try to keep their impedences within 5% of each other.

Happiness is a way of travel, not a destination.

 

[waross]

You don't want to parallel the transformers, you want to connect them in a three phase bank.
Your friend has given you good advice.
Re impedances. If you use the percent impedance voltage shown on the transformer name plate, this figure also equates to the voltage drop under full load. A 2.7% will have 2.7% drop at full load, a 3% will have 3% voltage drop at full load etc.
If you use a 2.7% transformer with a 3% transformer you will have a voltage unbalance of 0.3% with equal primary voltages.
If you use a 3% with a 7% transformer you will have a 4% voltage difference under full load.
Consider the stability of the primary voltage. How are your motors loaded, a lightly loaded motor will withstand voltage unbalance better than a fully loaded motor.
If you find 3 similar transformers in the yard, great. If you don't, do the best you can and check any heavily loaded motors for over heating for a day or so.
I hope your secondary connection is wye. With a delta secondary the impedance matching becomes more important to avoid circulating currents.
Respectfully

 

[H1H2N3F4]

Thank you all and I appreciate your time and advice;
I think I understand now that I can use three Identical Transformers (luckily they are available) to feed my non motor loads.

To answer, steelerfan28655,
No my load will not be constant.
I already considered the de-rating factors during my transformer sizing.

Regards



MMQ

 

[rbulsara]

A 3 phase tranformer bank can also be used to feed motor load. May be it will help you know that, a "three phase" trasfromer in nothing but effectively three single phase transformers in one enclosur and shared (linked) magnetic path.

As waross said, this is different than paralleling, they are either connected in "delta" or Wye configuration.

Read up a good text book on AC Machine design or Poly phase circuits.