derate 2500kVA, 60Hz transformer for 50Hz service

[antigfk]

Hello all,

I have a 2500kVA, 60Hz, 5.75%Z, 480V-wye to 7200V-delta transformer. I will be using a couple of 440V, 50Hz gensets to start and run a customers 6600V, 50Hz, 650HP motor.

I am aware that typically a 60Hz-rated transformer can be derated for 50Hz service by derating the voltage to the ratio of 5/6, or 80%. I however will be running the transformer at 91.6% voltage.

-Is total transformer impedance reduced to 80% of rated, due to 80% rated frequency, because Z=j?L?
-What is the proper method to determine the derated transformer full-load kVA in my situation?
-Normally let-through kVA would be calculated by 2500/0.0575. How would I calculate the let-through kVA in this situation running 50Hz at 91.6% voltage?

Thanks in advance!

 

[edison123]

At 440V, 50 Hz, the trafo will go into saturation (unless you have taps on LV winding) resulting in a high no-load current.

If your motor can produce enough torque at 6 KV to meet the load, then you should run your DG at 400 V, 50 Hz.

 

[edison123]

And your derated KVA will be 2000. (80% of old voltage and 100% of old current)

 

[prc]

If you operate at 91.6 % voltage the transformer will be under overfluxing at 15 % continuously which normally is not possible.Usually if flux density adopted in transformer is 1.7T, then transformer can be overfluxed up to a maximum of 10 % only.You may contact manufacturer to get actual designed flux density.

Percenatge impedance also will come down in the ratio of frequency and final derated KVA rating.

KVA possible is new voltage X old current as edison explained.

 

[waross]

Back to KVA basics.
The limiting factor is the current. Calculate the maximum current from the nameplate values. The current is the maximum under all conditions. Do not exceed the rated current. The Actual KVA will be the rated current times the Actual voltage. This also applies to transformers used at other than the 100% tap.
You may check your saturation knee point by ramping the generator voltage up slowly while monitoring the transformer excitation current. When the current starts to rise disproportionally, you have reached the saturation point and are just past the working voltage limit.

Bill
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