An electrical impedance relay is used in motor differential protection schemes to detect electrical faults or imbalances in the motor windings. The main objective of this protection scheme is to provide quick and sensitive protection against internal faults that can occur within the motor.
Here's how an electrical impedance relay operates in motor differential protection:
Measurement of Currents: The protection scheme requires current transformers (CTs) to measure the current flowing through the motor windings. These CTs step down the current to a level suitable for the relay's operation. The currents from the CTs are then fed into the electrical impedance relay.
Differential Current Comparison: The electrical impedance relay continuously calculates the difference between the currents entering the motor (input currents) and the currents leaving the motor (output currents). Under normal operating conditions, the input and output currents should be equal, and the differential current should ideally be zero.
Impedance Calculation: The relay calculates the impedance based on the differential current and the applied voltage. Impedance is the complex ratio of voltage to current and has both magnitude and phase angle components. The impedance relay uses this calculated impedance as a reference to compare against the set protection thresholds.
Setting the Protection Thresholds: For motor differential protection, the impedance relay is typically set with two impedance thresholds: the operating impedance (or restraint impedance) and the pickup impedance (or trip impedance). These thresholds define the range within which the motor is considered to be operating normally. If the impedance falls outside this range, it indicates an internal fault within the motor.
Fault Detection and Operation: When the impedance relay calculates that the impedance is outside the operating range (pickup impedance threshold), it sends a trip signal to the motor circuit breaker or contactor. This action disconnects the motor from the power supply, preventing any further damage due to the fault.
Time Delay: To avoid unnecessary tripping for transient or temporary faults, impedance relays typically include a time delay function. If the impedance falls outside the operating range but then returns within the specified range within the time delay period, the relay will not trip the motor.
It's important to note that motor differential protection schemes are very sensitive and require proper CT sizing, connection, and calibration to ensure reliable and selective operation. Additionally, some modern protection schemes may use more advanced numerical relays that incorporate additional features such as thermal modeling and adaptive settings, enhancing the overall protection performance.