30.9 : Pilot and Numeric Relaying

Pilot relaying is a type of differential protection used in power systems. It compares electrical quantities at the terminals of equipment via a communication channel instead of direct relay interconnection. This method is essential for transmission lines where the terminals are far apart, typically up to 80 km for lines with 69 to 115 kV ratings. Four types of communication channels are used for pilot relaying:

1. Pilot Wires: These are separate electrical circuits operating at DC, 50 to 60 Hz, or audio frequencies.
2. Power-line Carrier: This type utilizes the transmission line itself as the communication medium, with frequencies between 30 and 300 kHz.
3. Microwave: Uses 2 to 12 GHz signals transmitted via line-of-sight paths with dish antennas.
4. Fiber Optic Cable: Employs light modulation through non-conducting cables, eliminating electrical insulation and interference issues.

There are two fault detection methods: directional and phase comparison. The directional comparison method compares power flows at line terminals, while the phase comparison method compares the relative phase angles of currents at terminals.

Numeric, or digital relaying, uses microprocessor-based systems with software for fault detection. Numeric relays reduce panel wiring, simplify settings changes via remote updates, and minimize burdens on transformers. They offer greater sensitivity, faster tripping times, and improved reliability through fewer components. These relays can analyze post-fault data, check system recovery, and adapt settings to changing conditions.

Numeric relays are sensitive to cybersecurity risks, so it is crucial to comply with North American Electric Reliability Corporation (NERC) security standards.

In decentralized protection, each relay receives information from the local current and voltage transformers, only tripping local breakers. This approach ensures reliability without relying on pilot relaying. Numeric relays can integrate with centralized systems to enhance overall protection coordination and system reliability.