Pilot relaying schemes are used for the protection of transmission lines. These schemes involve the use of pilot wires or communication channels to transmit signals between the relays located at different ends of the transmission line. The relays use these signals to coordinate their actions and provide effective protection to the transmission line.

Transmission lines are critical components of the power system that carry bulk power over long distances. They are subjected to various types of faults such as short circuits, overloads, and ground faults. It is essential to detect and isolate these faults quickly to minimize the damage to the transmission line and ensure the stability and reliability of the power system.

Here is why pilot relaying schemes are specifically used for the protection of transmission lines:

1. Communication and coordination: Pilot relays enable communication and coordination between the relays located at each end of the transmission line. This allows the relays to exchange information about the fault conditions and coordinate their actions to isolate the faulted section of the line. This coordination ensures that only the faulty section is disconnected while keeping the rest of the line in service.

2. Distance protection: Pilot relaying schemes are commonly used for distance protection of transmission lines. Distance relays measure the impedance of the line and compare it with the pre-set values to determine the fault location. The pilot signals transmitted through the pilot wires or communication channels provide the necessary information for accurate fault location and discrimination.

3. High-speed operation: Transmission line faults need to be detected and cleared quickly to prevent damage to the line and minimize the impact on the power system. Pilot relays are designed to operate at high speeds, allowing them to detect faults and initiate protective actions within a few milliseconds.

4. Flexibility and adaptability: Pilot relays can be easily configured and adapted to different transmission line configurations and fault conditions. They can be programmed to respond differently to various fault types, ensuring optimal protection and minimizing unnecessary tripping.

In conclusion, pilot relaying schemes are specifically used for the protection of transmission lines due to their ability to provide communication, coordination, high-speed operation, and flexibility. These schemes play a crucial role in maintaining the reliability and stability of the power system by detecting and isolating faults on transmission lines efficiently.