Which protection relay compares currents to protect zones of an electrical system?

The differential relay is designed to compare the currents entering and leaving a specific zone of an electrical system, such as a transformer, generator, or transmission line. The fundamental principle behind a differential relay is that, under normal operating conditions, the current entering the zone should equal the current leaving it. When there is a fault, such as a short circuit or equipment failure, the currents will differ significantly.

This difference is detected by the relay, which then initiates a protective action, such as tripping a circuit breaker, to isolate the faulty section from the rest of the system. The differential relay's sensitivity to current imbalances makes it a critical protective device in ensuring the safe operation of electrical systems, effectively preventing damage to equipment and enhancing system reliability.

Other relay types serve different protective functions: directional relays detect and respond to faults based on the direction of current flow; voltage relays monitor voltage levels rather than current; and impedance relays analyze the relationship between voltage and current to assess the system's condition. Each of these relays plays a role in system protection, but the unique function of the differential relay in comparing currents makes it the correct choice for this question.