What is the primary function of a #21 relay?

Study for the NERC Transmission Operations (TOP) Exam with our multiple choice questions and comprehensive explanations. Prepare effectively and boost your confidence for test day!

The primary function of a #21 relay, also known as a Distance Relay, is to provide protection for transmission lines by measuring the impedance between the relay and the fault location on the line. This relay operates by calculating the distance to the fault based on the voltage and current to determine if it falls within its specified zones of protection.

Distance relays are configured with different zones, typically three, which enable them to provide graded protection. The first zone offers fast clearing for faults occurring close to the relay, while the subsequent zones offer additional protection for more distant faults, thus allowing for coordinated operation with other protective devices.

This capability makes the #21 relay particularly critical in transmission operations, as it can help isolate faults effectively and minimize the impact on the overall electrical system. The precision and speed of this relay are vital for maintaining system stability and reliability.

The other options do not correctly describe the function of a #21 relay. Ground fault protection pertains to relays that specifically detect ground faults, monitoring system voltage involves measuring and regulating voltage levels, and controlling reactive power is focused on managing voltage stability and phase angle, none of which align with the main purpose of the #21 relay.

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