Transformer Impedance

Transformer percentage impedance (Zk%) is measured by short-circuiting the secondary winding and gradually increasing the primary voltage until rated current flows — the voltage required, expressed as a percentage of rated voltage, is the impedance. Typical values range from 4% to 6% for distribution transformers. A lower impedance allows higher fault currents at the secondary terminals (Isc = In × 100/Zk%), requiring protective devices with greater breaking capacity, but provides better voltage regulation under load. Higher impedance limits fault currents — reducing equipment ratings and costs — but increases voltage drop under load and may cause problems with motor starting. The impedance comprises resistance (causing copper losses and heating) and reactance (dominating in larger transformers). For parallel operation, transformers must have impedances within 10% of each other to share load proportionally. When calculating short-circuit levels in an installation, the transformer impedance is converted to ohms referred to the secondary side and forms part of the total fault loop impedance.