An electrical differential relay is a protective relay used in line protection for transmission lines. Its primary function is to detect and clear faults that occur within the protected section of the transmission line. The differential relay operates based on the principle of current balance between the currents flowing into and out of the protected zone. Here's how it generally works:
Current Transformers (CTs) and Connections: The transmission line currents are measured using current transformers (CTs). These CTs are placed at both ends of the protected section. The CTs step down the line currents to a level suitable for the relay's operation.
Current Comparison: The differential relay continuously compares the current entering the protected zone (from one end) with the current leaving the zone (from the other end). It does this by calculating the phasor differences between the currents.
Balanced Currents: Under normal operating conditions, where there are no faults within the protected section, the currents entering and leaving the zone should be almost equal (some minor differences may exist due to losses).
Fault Detection: When a fault (short circuit or other abnormal condition) occurs within the protected zone, the balance between the currents is disturbed. This causes a significant increase in the difference between the currents.
Operation and Trip Signal: The differential relay is designed with a pre-defined threshold setting. When the difference between the currents exceeds this threshold, the relay operates, indicating the presence of a fault within the protected section. Upon operation, it sends a trip signal to the associated circuit breaker, instructing it to open and isolate the faulty section.
Zone Selectivity: Differential protection can be applied selectively for different sections of the transmission line. By using multiple differential relays, each covering a specific zone, faults can be pinpointed and cleared only in the affected zone. This helps to minimize the impact on the overall system and improves system reliability.
Backup Protection: While differential protection is very sensitive and fast in detecting internal faults, it may not be suitable for all fault scenarios (e.g., external faults). Therefore, backup protection schemes, such as distance protection or overcurrent protection, are often used in conjunction with differential protection to provide comprehensive line protection.
Overall, electrical differential relays are essential elements in ensuring the reliable and secure operation of transmission lines, providing rapid fault detection and clearance, which helps to maintain the stability and continuity of the electrical power grid.