Why are pilot relay schemes necessary in the context of transmission line protection?

Pilot relay schemes are essential in transmission line protection primarily because they facilitate instantaneous tripping for faults that occur within a specific distance from the relay, often within 10%-20% of the line distance. This capability is critically important during fault conditions to minimize damage to the transmission infrastructure and maintain system reliability.

When a fault occurs, the speed of response is vital in preventing further damage and ensuring the safety of the network. Traditional protection schemes, which rely on time-delay mechanisms, may not react quickly enough, potentially allowing fault currents to rise to damaging levels. Pilot relaying schemes, on the other hand, utilize communication between relays at both ends of the transmission line to determine the fault's location quickly and accurately. This allows the relays to trip almost instantaneously for faults detected within their designated zone, reducing the risk of prolonged fault conditions.

Furthermore, while the other options provide varying degrees of functionality and support for transmission line management, they do not capture the primary purpose of pilot relay schemes as effectively as the instantaneous response for close-in faults. For instance, providing backup for remote terminal protection and activating alarms are important tasks but do not directly address the immediate need for swift action during fault situations. Reducing maintenance frequency is a desirable outcome but is not the