Impedance of Relay Protection Transformer

The impedance value of a transformer, expressed as percent impedance, is crucial for relay protection as it determines fault current levels and influences relay settings.

Transformer Impedance Overview

Transformer impedance represents the opposition to current flow within the transformer, primarily due to winding resistance and leakage reactance. It is typically expressed as a percentage of rated voltage and indicates how much voltage is required on the primary side to drive full-load current through the transformer under short-circuit conditions. Higher impedance limits fault current more effectively, while lower impedance allows higher fault currents, which can affect protective relay coordination.

The impedance is measured during a short-circuit test, where the secondary winding is shorted and a reduced voltage is applied to the primary until rated current flows. The resulting voltage, expressed as a percentage of rated voltage, is the transformer's percent impedance. This value is essential for calculating fault currents using the formula: Isc = Irated / Z% where Isc is the short-circuit current and Z% is the percent impedance.

Role in Relay Protection

In high-impedance restricted earth fault (REF) protection, the relay is connected through a stabilizing resistor to control current flow and maintain stability during slight CT mismatches or saturation. The transformer impedance directly affects the spill current through the relay, ensuring that the relay operates only for internal faults and remains stable for external faults. The minimum relay operating voltage is calculated considering the transformer impedance, CT secondary resistance, and any series resistors.

This ensures that the relay reliably detects internal faults while avoiding false trips due to through faults or CT saturation. Non-linear resistors like Metrosil may also be used to limit excessive secondary voltage during severe faults, protecting the relay and CT insulation.

Practical Considerations

Parallel Operation: Transformers operating in parallel must have closely matched impedance values to ensure even load sharing and avoid overloading. Fault Current Limitation: Percent impedance determines the maximum fault current the transformer can supply, which is critical for selecting protective devices like fuses, breakers, and relays. Temperature Correction: Winding resistance changes with temperature, so impedance values are often corrected to a reference temperature (e.g., 75°C for Class A insulation) for accurate relay coordination.

In summary, the impedance value of a transformer is a key parameter for relay protection, influencing fault current magnitude, relay stability, and coordination. Accurate knowledge of this value, along with proper CT and resistor selection, ensures reliable and selective protection of the transformer.