An electrical distance relay, also known as a distance protection relay or impedance relay, is an important component in protection schemes for distributed generation systems. Its main function is to detect faults (short-circuits) in the electrical network and to initiate the appropriate actions to isolate the faulty section, ensuring the safety and reliability of the overall power system.
Here's how an electrical distance relay operates in protection schemes for distributed generation systems:
Measuring Impedance: The electrical distance relay measures the impedance (resistance and reactance) between the relay location and the point where the fault occurs. Impedance is calculated based on the voltage and current phasors at the relay's location.
Setting the Reach: The relay is set with a specific impedance value, which defines the distance over which it will protect the power system. This value is called the "reach" of the relay and is typically set according to the characteristics of the protected transmission or distribution line.
Impedance-Reach Zone: The relay establishes an impedance-reach zone around itself. Any fault that occurs within this zone will be detected by the relay.
Fault Detection: When a fault occurs within the impedance-reach zone, the impedance measured by the relay will change significantly from the normal operating impedance. This change is detected by the relay as a deviation from the set value.
Operation Time: The electrical distance relay operates based on the principle of measuring the fault impedance and comparing it to the pre-set reach value. If the impedance measured is within the reach of the relay (i.e., inside the impedance-reach zone), the relay will issue a trip signal to the circuit breaker.
Circuit Breaker Operation: Upon receiving the trip signal, the circuit breaker associated with the relay will open, isolating the faulted section from the rest of the power system. This quick action prevents the fault from spreading and causing further damage.
Zone Selectivity: In distributed generation systems, there might be multiple protection zones, and it's essential to ensure selectivity in fault isolation. This means that only the nearest relay to the fault should trip, leaving other parts of the power system unaffected. Coordination between different relays and their reach settings ensures proper zone selectivity.
Backup Protection: In case the primary distance relay fails to operate or if a fault occurs outside its reach, backup protection schemes, like overcurrent relays or directional relays, are employed to provide additional layers of protection and ensure system reliability.
In summary, electrical distance relays operate by measuring impedance and comparing it to a pre-set reach value. When a fault occurs within the relay's reach, it issues a trip signal to the associated circuit breaker, isolating the faulty section and maintaining the stability and safety of the distributed generation system.