A differential protection relay is a protective device used in electrical systems to detect and respond to faults or abnormal conditions that may occur in power transformers, generators, motors, and other sensitive equipment. The primary function of the differential relay is to compare the currents entering and leaving the protected zone (e.g., a transformer winding) and trip the circuit breaker if a significant difference in currents is detected. This difference indicates the presence of an internal fault within the protected zone.
Here's a general overview of how a differential protection relay operates:
Current Measurement: Current transformers (CTs) are used to measure the current flowing into and out of the protected zone. These CTs convert the primary currents to lower magnitudes that are suitable for relay measurements.
Current Comparison: The differential relay continuously compares the incoming and outgoing currents. It calculates the difference between the two currents, often referred to as the "differential current" or "differential quantity."
Setting Threshold: The relay is typically set with a predefined threshold for the differential current. During normal operating conditions, the incoming and outgoing currents should be nearly equal, resulting in a small or negligible differential current.
Operation: If a fault occurs inside the protected zone, such as a winding short-circuit or turn-to-turn fault, the differential current will exceed the preset threshold. When the differential current exceeds the set value, the relay activates its trip coil, which, in turn, trips the associated circuit breaker, isolating the faulty equipment from the power system.
Applications of Differential Protection Relays:
Power Transformers: Differential protection is widely used to protect power transformers, especially in high-voltage substations. Transformers are critical and expensive equipment, and timely fault detection can prevent catastrophic damage.
Generators: Differential protection is applied to generators to safeguard them against internal faults that could lead to severe damage and costly downtime.
Motors: Large motors used in industrial applications can also be protected with differential relays, ensuring prompt isolation during faults.
Busbars: Differential protection is utilized to protect important busbars and interconnected switchgear in electrical substations.
Cables: Differential protection is employed in high-voltage cable systems to quickly locate and isolate faults, reducing the impact of outages.
Line Protection: In some cases, differential protection may be used for short transmission lines, especially those with high fault currents.
It's worth noting that differential protection is sensitive and can be affected by unequal CT ratios, CT saturation, or inrush currents during transformer energization. Therefore, proper coordination and testing are essential to ensure reliable and selective operation. Differential protection relays are usually combined with other protective schemes, such as overcurrent and distance protection, to provide comprehensive protection for power systems.