Looking For Oil Filled Transfromer Impedence

[MJJBEE]

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.

 

[dpc]

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?

 

[MJJBEE]

it's 480-240

 

[davidbeach]

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.

 

[cuky2000]

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

 

[prc]

Normally impedance of 150 kva transformer is 4 %.

 

[RalphChristie]

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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[MJJBEE]

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.

 

[jghrist]

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.