Transformer Short Circuit Impedance (3 phase transformer , 1 phase test supply)

[warickwrx]

Hi All

Upon reading the IEEE Std 62-1995 which indicates that a three-phase transformer may be tested for impedance using a single-phase power source . The test is done by short-circuiting the three line-leads of the low-voltage windings and applying a single-phase voltage at rated frequency to two terminals of the other winding. Readings are taken on the three pairs of leads, (e.g., H1 and H2, H2 and H3, H3 and H1), with the test current adjusted to the same level for each reading. Then the %Z of the three-phase transformer is given by

Z% = 1/60 * [( E12 + E23 + E31)/Im ] * [3 ph Kva / (Vline * Vline]

E12, E23, E31 are measured test voltages

Im is the current mesurement (not sure if this is single value measurement or all 3 separate measurements multiplied by 3 )

3 ph Kva is the trfr rated kva

Vline is the rated line to line voltage

I would like to know what the purpose, significance of the 1/60 constant is for, also the value of Im is this from only one measured loop or all three ?

 

[prc]

IEEE Std 62 was revised and new standard is IEEE C57.152-2013.

When E1+E2+E3 are measured,average voltage value will be 1/3. Since the voltage is applied across two phases,average voltage per phase = 1/2 .
E/6I = ohmic value. 100x OhmsxkVA/kV/(V/1000) = %Z. That brings out 1/60
Im is the primary line current where the voltages are measured.