**Per unit resistance**

- It is defined as the resistance drop at rated current and frequency expressed at rated voltage.

_{1}R

_{01 }/ V

_{1}

= I

_{2}R_{02 }/ V_{2}
Where

- R
_{01}= Transformer winding resistance as referred to primary - R
_{02}= Transformer winding resistance as referred to secondary - I
_{1}= Rated primary current - I
_{2}= Rated secondary current - V
_{1}= Primary voltage - V
_{2}= Secondary voltage

% Resistance = [
I

_{1}R_{01}/ V_{1 }] × 100%
= [ I

_{2}R_{02}/ V_{2 }] × 100%
= [ I

_{2}^{2}_{ }R_{02}/ I_{2}V_{2 }] × 100%
= [ Full load copper
losses / transformer rating ] × 100%

**Per unit reactance**

Per unit
reactance = I

_{1}X_{01}/ V_{1}
= I

_{2}X_{02}/ V_{2}
% Reactance = [
I

_{1}X_{01}/ V_{1 }] × 100%
= [ I

_{2}X_{02}/ V_{2 }] × 100%- It is not very easy to calculate total resistance of the transformer winding.
- The primary winding resistance and secondary winding resistance is added directly if the voltage ratio of the transformer is unity but it is not possible.
- The important advantage of expressing resistance and reactance of a transformer in per unit / percentage is that both resistance and reactance have same values whether determined as referred to primary side or secondary side.
- When they expressed in ohms, they have different values as referred to primary side and secondary side.

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