Critical factors to consider when paralleling power transformers 2

[mpc]

I have a small project that will require installing a 2000KVA, 13800v/ 480-277y transformer on the same bus as another unit the same size. This parallel operation will be controlled by a tie breaker on the LV bus. What is the critical level of impedance difference that will determine which transfoemer to use? The load on both will be Approx. 500amps @480V. [Ponder]

 

[mpparent]

My thoughts are there really isn't an issue with impedance as far as paralleling is concerned. I could be wrong, but my concern would be are you going to use closed transition or open? The reason is your available fault current will be more obviously if you go closed transition, and you'll want to be sure you're braced for that condition.

Mike

 

[peterb]

Is the parallel operation meant to be continuous, or only for transition? If continuous, you will need to match the impedance closely, as the two transformers will share the load in the inverse proportion of the impedances - the lower impedance unit will take more load. This won't necessarily be a problem unless you are loading close to rating, where one unit taking proportionally more load will leave capacity unused - you would not be able to get 2x2000 kVA from the combination.
That said, you must, as mpparent points out, consider the effect on the available fault current. Should not be a problem if you have fused switchgear, but could be a major headache if you have circuit breakers.

 

[mpparent]

Just something to think about,

If you go parallel, closed transition, with two 2000kVA transformers, your available fault current(keep in mind this is the maximum with no motor contribution, you'd have to do an actual analysis), is approximately 83kA(that's assuming a Z% of 5.75). This is also assuming the fault is right at the switchgear bus. Take a look at your mains and feeder breakers to make sure you don't have an issue with bracing.

Mike

 

[Andiri]

If the 2 trf's are to operate simultaneously, the difference of impedances should be less than 10% of the lower trf's imp. value.
Example:
trf1 = 8.35; trf2 = 7.67
8.35- 7.67 = 0.68
0.68 / 7.67 *100 = 8.87%
<10% difference, thus can do.