A transformer’s leakage impedance is determined empirically through a short-circuit test. Leakage impedances provided by equipment manufacturers are usually expressed in percent using the device’s rated power as the base. To convert the impedance from percent to per unit, just divide it by a hundred.
| (7) |
To express the impedance in per unit on the system base rather than the transformer base
| (8) |
where
Sbase is the transformer power base.
Vbase is the transformer voltage base.
is the system power base.
is the system voltage base.
When the transformer voltage base coincides with the system voltage base, this equation simplifies to
| (9) |
The transformer’s impedance may also be expressed in ohms
| (10) |
where
Vrated is the transformer’s rated voltage. The computed impedance is referred to the transformer’s primary or secondary by the appropriate choice of Vrated.
Srated is the transformer’s power rating.
c is a constant that keeps the units straight. The value of c depends on how the transformer’s power and voltage ratings are specified. Variations in c over a wide range of unit systems are found in Table 1.
| Voltage Unit | Power Unit | Constant |
|---|---|---|
| Vln | VA1 | 1 |
| Vln | VA3 | 3 |
| Vln | kVA1 | 1/1,000 |
| Vln | kVA3 | 3/1,000 |
| Vll | VA1 | 1/3 |
| Vll | VA3 | 1 |
| Vll | kVA1 | 1/3,000 |
| Vll | kVA3 | 1/1,000 |
| kVln | kVA1 | 1,000 |
| kVln | kVA3 | 3,000 |
| kVln | MVA1 | 1 |
| kVln | MVA3 | 3 |
| kVll | kVA1 | 1,000/3 |
| kVll | kVA3 | 1,000 |
| kVll | MVA1 | 1/3 |
| kVll | MVA3 | 1 |