
Transformer 157
If we work in the per unit system—expressing the values in pu of their respective
base values—the voltage ratio multiplier n
2
does not apply, and the total impedance
looking from the HV or LV side is merely the sum of all pu impedances, that is.
Z
Total.pu
= Z
HVcable.pu
+ Z
LVcable.pu
+ Z
Load.pu
and I
pu
= V
pu
÷ Z
Total.pu
(6.13)
At full 100% primary voltage, in per unit system, I
pu
= 1.0 ÷ Z
Total.pu
(6.14)
In amperes, I
HVamp
= I
pu
× I
HVbase
and I
LVamp
= I
pu
× I
LVbase
(6.15)
Example 6.9
Consider a transformer rated 15 kVA, 1-phase, 480/120 V supplying 9.6 Ω
load as shown in the figure below (top). In calcula ...