Per unit resistance & Per unit reactance
Per unit resistance
- It is defined as the resistance drop at rated current and frequency expressed at rated voltage.
= I2R02
/ V2
Where
- R01 = Transformer winding resistance as referred to primary
- R02 = Transformer winding resistance as referred to secondary
- I1 = Rated primary current
- I2 = Rated secondary current
- V1 = Primary voltage
- V2 = Secondary voltage
% Resistance = [
I1R01 / V1 ] × 100%
= [ I2R02
/ V2 ] × 100%
= [ I22
R02 / I2V2 ] × 100%
= [ Full load copper
losses / transformer rating ] × 100%
Per unit reactance
Per unit
reactance = I1X01 / V1
= I2X02
/ V2
% Reactance = [
I1X01 / V1 ] × 100%
= [ I2X02
/ V2 ] × 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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