impedance of a 500kVA pad mounted transformer

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mshields

mshields

Senior Member
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Boston, MA
I'm putting in a new switchboard which will be fed via an existing 13.8 to 208Y/120V pad mounted transformer. I don't know how old it is (yes it might be a good idea to take a look) but at this point I'm told by the onsite electrician that there is no indication of it's impedance on inside nameplate.

This same electrician assures me that there is no way that transformer has an impedance lower than 4 percent. I know this is probably correct. Yet, it seems to me, anything is possible! How good an assumption do you think this is?

Thanks,

Mike
 
A 500 kVA PMT has an impedance that ranges from 1.5% to 7% with a 10% possible variance. That makes the range from 1.35% to 7.7%. You have to get access to the actual nameplate or use the lowest impedance for sizing your service equipment after taking into account the length of your service conductors.

By the way, you will not be able to use this information for determining the PPE. :)
 
080530-1819 EST

mshields:

If you have the components and tools you can estimate the source impedance as follows:

Pick a suitable load that you can switch on and off. The load needs to be large enough to get moderately accurate readings. On my 200 A service I am playing with a 10 A load. In your case a 50 kva load would be nice, but 20 KVA might do.

Measure the load current of this test load, or the change in the total current resulting from this load. At the same time measure the change in the source voltage at the transformer output. You probably want to make the measurements assuming the source is a Y. So if the voltage change is read between legs convert the change to the Y value. Impedance is Vchange/Ichange.

.
 
gar said:
080530-1819 EST
Measure the load current of this test load, or the change in the total current resulting from this load. At the same time measure the change in the source voltage at the transformer output. You probably want to make the measurements assuming the source is a Y. So if the voltage change is read between legs convert the change to the Y value. Impedance is Vchange/Ichange..
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This is only true if the X/R of the load equals the X/R of the transformer. The voltage drop through a transformer is very dependent on the power factor of the load.

VD = I?(R?cos? + X?sin?)
where R and X are the tranformer resistance and reactance and cos? = pf.
 
080531-0754 EST

jghrist:

Note, I used the word estimate at the start of the post.

It is true I do not have an easy way to measure the voltage drop across the transformer internal impedance, and that simply measuring the change of voltage across the load might give very incorrect results. However, the likely load used for the test may not be too out of balance with the internal impedance in terms of the ratio of reactive and resistive components.

With more sophisticated test equipment than uncorrelated voltage and current measurements the measurement accuracy could be improved.

.
 
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