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Impedance and Various Load Levels of Transformers

Feb. 9, 2014
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Q: The percent impedance of a transformer is also the voltage drop of the transformer at full load. What is the voltage drop of a transformer at various other load levels, i.e. 25%, 50%, 75% or in excess of 100%. Is it linear or non-linear?

A: A useful approximation is that the full-load voltage drop through a transformer is essentially the percent impedance of the transformer. A further approximation is that the drop at other load levels is directly proportional to the load – in other words, it is linear.

It should be recognized that the percent impedance of a two-winding transformer is often determined by short-circuiting one winding of the transformer and raising he voltage on the other winding until the rated load current flows through the transformer. Thus, the voltage drop during the impedance test is actually the drop for a load having a power factor as determined by the X/R ratio of the particular transformer. For load with a power factor significantly different from that existing during the impedance test it will be more accurate to determine (from transformer test reports) the resistive (R) and inductive (X) components of the transformer impedance. Then the voltage drop can be calculated for whatever power factor may be of interest. There can be relatively large differences in the results if the load – such as locked-rotor current of a motor-- has a particularly low power factor.

The Westinghouse Distribution System Reference Book, 1959, page 218, gives a thorough explanation of how to calculate voltage drop for any power factor using the data available on the typical transformer test report.

Edward S. Thomas, PE
Utility Electrical Consultants, PC

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