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Looking For Oil Filled Transfromer Impedence

Looking For Oil Filled Transfromer Impedence

Looking For Oil Filled Transfromer Impedence

(OP)
I have an old Oil Filled 150 kVA 3 phase Transformer.  I have no Documentation and need a rough idea of what the impedence would be. The namplate is long gone however based on the age of this plant and upgrades done on it I belive it is  likely from the 40's or 50's. I would also like a way to solve these type of problems in the future.    

RE: Looking For Oil Filled Transfromer Impedence

A good reference is GE Publication GET-3550, if you can find one.  It doesn't seem to be available on their website any longer.  It has a lot of typical impedance data for various types of transformers and other things.  

What are primary and secondary voltages for this transformer?

RE: Looking For Oil Filled Transfromer Impedence

(OP)
it's 480-240

RE: Looking For Oil Filled Transfromer Impedence

I'll take a stab at it, the impedance is less than 10%.

For regulation, you should probably assume about 6% and for fault currents assume about 1.5%.  If you need better numbers than those you will likely need to test the transformer.

RE: Looking For Oil Filled Transfromer Impedence

The approximate parameters values available for a three phase transformer are:

   1-150kVA:   X=3.0% ;  R=1.5%
   3-50 kVA:   X=2.2% ;  R=1.3%

Impedance also may be measured by method specified in the standards with the low voltage winding shorcicuited:

              Z% = measured voltage/rated voltage
    

RE: Looking For Oil Filled Transfromer Impedence

Normally impedance of 150 kva transformer is 4 %.

RE: Looking For Oil Filled Transfromer Impedence

Conrad St. Pierre's book A Practical guide to short-circuit calculations state:

typical load center transformer impedances (primary 15kV or less) for 150kVA liquid filled trsfs as 4%
typical distribution transformer impedances (primary 15kV or less) for 150kVA liquid filled trsfs as 3.7%

This data is based on average data for transformers made in the late 1960's


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RE: Looking For Oil Filled Transfromer Impedence

(OP)
Thanks you guys you have been realy helpful.  I'm going to cycle through a few values and see which one gives me the worst arc flash.  I belive it will be the largest impedence but better safe the sorry.  

RE: Looking For Oil Filled Transfromer Impedence

You could measure the load current, power factor, primary voltage, and secondary voltage under heavy load to calculate the impedance.  You would have to assume some X/R ratio; 2.5 would be close.

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