Treatment of Resistance for single phase transformers

[siberhusky]

Hi...

Given that one uses the series (winding) resistance values from the transformer test reports to estimate* the resistive part of transformer impedance...

...And for a 3-phase transformer, one uses the sum of the resistance values for the 3 phases... what does one do if the given transformer is actually made up of 3 single-phase transformers?

- john

* (I know this isn't complete, but assume that's all one has available.)

 

[jghrist]

I've always used the load loss divided by the rated power to determine the resistive part of the impedance in per unit. It's easier, it includes stray losses, and it works for single-phase as well as three-phase transformers.

 

[siberhusky]

Just to be clear, when you say the load-loss method works for single-phase and three phase transformers:

To get one value for three single-phase transformers used together, would I add up the load loss values for each of the three phases and divide by the sum of the MVA values, to get an average resistance value?

- john

 

[jghrist]

//To get one value for three single-phase transformers used together, would I add up the load loss values for each of the three phases and divide by the sum of the MVA values, to get an average resistance value?\
Correct. Losses would have to be expressed in MVA. You would get the per unit resistance at the transformer rating base (same base as the impedance).

 

[jbartos]

Suggestion: The short circuit transformer test should provide an input for the transformer resistive and inductive parts of the transformer series impedance calculations. The series transformer impedance is used in power distribution modeling, calculations and analyses.
If the transformer is three-phase, the short circuit transformer test is a three-phase transformer test. This way the transformer connections can stay intact. If the transformer is a single phase, than the results will be obtained for a single phase transformer. If the three phase transformer consists of three single phase transformers, than there is a choice. The three phase connected single phase transformers will be more representative for the measurement, test and calculation results.