An electrical differential relay is a protective device used in transformer protection schemes to detect internal faults and prevent damage to the transformer. It operates based on the principle of current balance, where the sum of currents entering a system (primary side) should be equal to the sum of currents leaving the system (secondary side) under normal operating conditions. When a fault occurs inside the transformer, this balance is disturbed, and the differential relay detects the fault.
Here's how an electrical differential relay works in transformer protection:
Current Transformers (CTs): CTs are used to step down the currents on both the primary and secondary sides of the transformer to a measurable level. The currents flowing through the primary and secondary windings of the transformer are sensed using these CTs.
Summation of Currents: The differential relay continuously calculates the algebraic sum of the currents on the primary and secondary sides. Under normal operating conditions, this sum should be zero or very close to zero, indicating that the total current entering the transformer is equal to the total current leaving it.
Current Comparison: The relay compares the summation of currents on the primary side with the summation of currents on the secondary side. If there is no internal fault, the difference between the two summations should be negligible.
Setting a Threshold: The relay is typically designed with a setting threshold. If the difference between the summations of the primary and secondary currents exceeds this threshold, it indicates an internal fault within the transformer.
Operation: When the differential current exceeds the set threshold, the relay operates and sends a trip signal to the circuit breaker connected to the transformer. The circuit breaker then opens, isolating the transformer from the faulty system and protecting it from further damage.
It's important to mention that the electrical differential relay should be designed carefully to avoid false tripping due to magnetizing inrush currents during transformer energization or other external disturbances that could momentarily create a current imbalance.
Additionally, modern transformer protection schemes may employ more advanced differential relays, such as numerical differential relays, that utilize digital signal processing techniques for enhanced accuracy and additional features like adaptive settings, communication capabilities, and fault recording for post-event analysis. These advanced relays provide more reliable and selective protection for transformers.