A distance relay, also known as a impedance relay or distance protection relay, is a protective relay used in electrical power systems to detect and locate faults in transmission lines and distribution networks. It operates based on the principle of measuring the impedance or distance to the fault location and comparing it to pre-set impedance or distance settings.
Distance relay working:
- Impedance Measurement: The distance relay measures the impedance of the protected electrical circuit, typically by using voltage and current transformers to obtain voltage and current phasor information. The impedance is calculated by dividing the measured voltage by the measured current.
- Impedance Settings: The relay is configured with pre-set impedance settings based on the characteristics and length of the protected line. These settings determine the reach and sensitivity of the relay. The impedance settings are typically provided in terms of impedance per unit (pu) or as an impedance versus distance characteristic curve.
- Fault Detection: The relay continuously compares the measured impedance with the pre-set impedance settings. If the measured impedance falls outside the impedance settings, it indicates a fault condition. The relay determines the distance to the fault based on the deviation from the pre-set settings.
- Protective Actions: When a fault is detected, the distance relay initiates protective actions, such as tripping the associated circuit breaker or contactor to isolate the faulty section. The relay may also activate alarms or indicators to alert operators or maintenance personnel about the fault.
Distance relays offer several advantages in power system protection:
- Selectivity: Distance relays provide selective protection by operating based on the impedance measured at the fault location. This allows them to distinguish between internal faults and faults outside the protected zone, ensuring that only the appropriate circuit breakers are tripped.
- High-speed Operation: Distance relays can quickly detect and locate faults, enabling fast fault clearance and minimizing the impact on the power system. They are particularly effective in detecting faults on long transmission lines.
- Adaptability: Distance relays are adaptable to various system configurations and fault conditions. By adjusting the impedance settings, they can be optimized to provide accurate protection for different line lengths, fault types, and system configurations.
- Sensitivity: Distance relays are sensitive to changes in impedance, allowing them to detect even small faults and disturbances in the power system. This sensitivity helps ensure effective fault detection and protection.
Distance relays are commonly used in transmission systems, distribution networks, and industrial power systems. They provide reliable and selective protection for overhead transmission lines, underground cables, and other power system components. Distance relay coordination and proper settings are important for ensuring reliable fault detection and coordination with other protective devices in the power system.