Communications systems for protective relaying

In normal power system operation, a variety of communications links are used to provide this information transfer, including: pilot wire channels operated by the power utility; pilot circuits rented from national communications utilities; power line carrier over the transmission lines; radio channels operating either at very high frequencies or over microwave; and optical fiber communications systems. These schemes were originally intended for electromechanical and static analog relays but have subsequently been applied to digital relays. Following a description of the ISO communications model, these communication links are discussed. Modulation techniques, asynchronous and synchronous communication, error detection, and protocols and profiles are also discussed.