Short version: How can I convert transformer resistance values from Watts to Ohms?

Long version: In PSS/E, transformer resistance can be entered as either per unit values (system base or transformer base), or as a MW-value (I believe this represents loss at full load).

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How can I convert the values from I/O code 3 to I/O code 1? That is from pu based on winding kV and system MVA-rating to full load loss and per unit impedance (for reactance).

Voltage level: 125/11 kV
System MVA rating: 100 MVA
Transformer rating: 35 MVA

1 Answer 1


I looked at some transformer test results for a 60 MVA 132/33kV transformer, and found this in the test results:

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This seems to suggest that the resistance, in percent on transformer MVA base (60 MVA), is simply Load Loss [W] ÷ Transformer MVA Base [MVA]. In this case, that's 381,000 W ÷ 60,000,000 MVA = 0.635%. Enter this for 'I/O Code 2'.

Similarly, the percent impedance on system MVA base (typically 100 MVA) would just be 381,000W ÷ 100 MVA = 0.381%. Enter this for 'I/O Code 1'.

  • \$\begingroup\$ Yes, you are right :) I figured it out a few days ago too. I thought I'd answer the question myself once the bounty expired, but you got to it first. Thanks for validating the answer with the test report! \$\endgroup\$ Nov 5, 2014 at 7:57
  • \$\begingroup\$ I stumbled upon this again today. I guess the answer is correct, but I find it strange. I would think that the resistance should correspond to the R in "I2R Loss", not Load Loss %. The load loss includes the no-load losses, which is not dependent on R. The no load losses is a separate input parameter. I know that when entering Load Loss (W), then the no load loss is automatically included, but why it should be included when entering the resistance is beyond me. \$\endgroup\$ Mar 9, 2020 at 9:38

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