Protective relays act as the brain of a protection system responsible for protecting power system equipment against faults. The protected equipment includes generators, transformers, feeders, buses and minor equipment, like motors, capacitor banks, diesel-generator set etc.
The first-generation protective relays are electromechanical in nature and use either electromagnetic or bimetallic operating elements. The second-generation relays are static in nature (that is, have no moving parts) and use analog electronic circuits for their operation. The third-generation relays are built around a microprocessor, which performs numerical calculations on digital input data; and have several significant advantages over the first and second generation relays. The latest, that is, the fourth-generation, relays are the extension of the third-generation microprocessor-based relays in that they carryout multiple protective functions to provide a comprehensive protection of the equipment or even offer complete management, including protection, of the equipment.
The market share of first- and second-generation relays is shrinking while that of third- and fourth-generation relays is fast increasing, thanks to the substantial benefits and fast reducing costs of the latter.
An overview of the numerical relays was presented by Prof. H.K. Verma at the Institution of Engineers (India), Andaman and Nicobar Islands State Centre, Port Blair in December 2006. A freely down-loadable PDF version of the presentation is given here. While some of the commercial relay versions included in the presentation by way of examples may have been superseded in the meantime, this would hardly affect the general concepts and principles dealt with. Click to open the presentation.