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Paralleling two small transformers of different sizes and impedences

Paralleling two small transformers of different sizes and impedences

Paralleling two small transformers of different sizes and impedences

(OP)
We have a new 50kVA UPS system that has a maintenance bypass as a second source. The batteries synch to the primary power provided through a 45kVA, 480V-208Y/120V transformer. Should the UPS be bypassed there is a make before break contactor that temporarily parallels the two primary and bypass sources.  We were going to buy a new 45kVA transformer identical to the primary, but...

We are using less than 25% of the UPS capacity and have on hand a 30kVA, 480V-208Y/120V transfomer. It's been suggested to use this 30kVA on the bypass.  Other than limiting the UPS capacity, is there an electrical reason not to do this? The 45kVA has a IZ% = 2%, the 30kVA IZ% =4.5%?  

RE: Paralleling two small transformers of different sizes and impedences

Harmonics.
Some UPS systems have a fair bit of harmonics, in which the transformer is part of the function to reduce them.
 

RE: Paralleling two small transformers of different sizes and impedences

The 30 KVA may be re-rated to 2%Z. 30/4.5% x 2% = 13.3KVA
In parallel with a 45 KVA transformer it will take 13.3KVA/(13.3KVA + 45KVA) of the load. If the total load is more than (45KVA + 13.3KVA) or more than 58.3KVA the 45KVA transformer would be overloaded.
When the transformers are paralleled the 30KVA will take some of the load. When the 45KVA is disconnected the 30KVA will assume all the load.
With two equal transformers the same thing happens but the load sharing when the transformers are in parallel is equal.
If the %impedance of the larger transformer is greater than the %impedance of a smaller transformer you can have a situation where the two transformers in parallel have less capacity than the larger transformer has by itself.
In your application you should be OK.
 

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

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