The value mention on name plate for % Z is the percentage of voltage dropped by the
transformer impedance.
Z=R + j(XL+XC)
Z - Impedance
R - Transformer coil resistance ( Fixed Value)
XL - 2∏fL
XC - 1/2∏fC
The impedance of the Transformer is depending upon voltage applied.
Example:
33 / 11 KV
87.5 /262.5 A 6.81 %Z
The value of voltage drop in nominal voltage = 33000X6.81/100 = 2247 V
The value of impedance on full load condition = 2247.3/87.5 = 25.82 Ω
Conform this % impedance by site testing
1.Short the secondary side terminals
2. Apply the 380 V 3 phase in the HV side.
3. Before that calculate the primary current for applied voltage
If applied voltage 2247.3 v to HV side then full loade current 87.5 A will flow on primary side ( Secondary shorted).
Calculate the drop voltage if we are apply 380v
X /380 = 87.5 / primary current
This current can measure in primary side = 14.8A.
X /380 = 87.5 /14.8
X = 2246.62 Volts ( This is the total voltge drop for transformer total impedance)
Then we have to calculate this value how mach % in the Rated primary voltage
33000 * x / 1000=2246.62
x=6.8
This value is % Z of the transformer the error not more than
± 0.1 %
