Question

The per unit impedance of a transformer is:
1. larger if computed from primary side than from secondary side
2. the same whether computed from primary or secondary side
3. always zero
4. always infinity

Answer 1

Correct Answer - Option 2 : the same whether computed from primary or secondary side

Per unit system:

It is usual to express voltage, current, voltamperes and impedance of an electrical circuit in per unit (or percentage) of base or reference values of these quantities.

The Per Unit value of any quantity is defined as

PU value = actual value/base value

Per unit system in transformers:

The per unit impedance of a transformer is the same whether computed from primary or secondary side so long as the voltage bases on the two sides are in the ratio of transformation (equivalent per phase ratio of a three-phase transformer which is the same as the ratio of line-to-line voltage rating).

Transformation ratio of transformer is given by K = V2/V1 = E2/E1 = N2/N1.

Where N1 is the number of primary turns

V1 is the primary voltage

N2 is the number of secondary turns

V2 is the secondary voltage