Different Types of Protective Relays

Different Types of Protective Relays Different Types of Protective Relays Electrical relays are used in many types of technological systems from cars and appliances to medical equipment and aircraft. Power devices in power plants must have protective relays to operate. Protective relaying is a crucial part of electric power systems. Three important aspects of power systems in relation to relaying are: Normal operation Failure prevention Mitigating the effects of failure Relays A relay isolates or alters an electric circuit. Relay classification depends on what they’re used for. Types of relays include: Monitoring Regulating Auxiliary Reclosing Protective Protective Relays It is the job of the protective relay to monitor power, voltage, and current and detect problems. If any of those parameters go beyond set limits, an alarm is tripped or the circuit is isolated. This device causes a circuit breaker to isolate the bad element from the rest of the system. Protective relaying results in the removal of abnormal or short-circuiting power system elements. To protect against short circuits there is primary relaying, the first line of defense, and back-up relaying, which springs into action when primary relaying fails. Protective relaying equipment is described with the words “sensitivity,” “selectivity” and “speed.” These characteristics allow the relay to accurately detect problems and quickly disconnect faulty elements. Types of Protective Relay There are two operating principles: electromagnetic attraction and electromagnetic induction. Basic classification of protective relays includes: Electromagnetic Relays: Armature Induction cup / induction disc Static Relays: Analog input signals are processed by solid state devices Digital / Numerical Relays: Uses programmable solid state devices based on digital signal processing Electromagnetic Relays Electromagnetic relays have mechanical, electrical, and magnetic elements as well as operating coil and mechanical contacts. The mechanical contacts are opened or closed when a coil is activated. DC relays de-energize the coil with a diode. AC relays have laminated cores that prevent losses. Attraction electromagnetic relays attract metal if the coil is supplied power with a plunger moving towards a solenoid or the attraction of an armature toward an electromagnet’s poles. Induction relays are used in AC systems by developing actuating force when a conductor (cup or disc) moves into the electromagnetic fluxes interaction. Solid State Relays The switching operation is performed with solid state components without moving the parts. The less necessary control energy compared to the output power results in higher power gain than with an electromagnetic relay. Types of solid state relays include reed relay coupled SSR, transformer coupled SSR, and photo-coupled SSR. Thermal Relays Thermal relays use the effects of heat. Temperature increase makes the contacts switch positions. These relays have elements like temperature sensors and control elements. Thermal relays are often used in motor protection. Hybrid Relays Hybrid relays combine electromagnetic relay and electronic components. Rectification is performed by electronic circuitry, while the electromagnetic relay is the output part. Reed Relays These relays have magnetic strips (“reeds”) in a glass tube acting as a contact blade and armature. Switching occurs when a magnetic field causes the reeds to move. Relay Differentiation There are a few ways relays can be classified: Dimensions: Miniature Subminiature Micro miniature Construction: Open Sealed Hermetic Load operating range: High power Intermediate power Low power Micro power To keep up with power system components and industry trends and innovations, as well as regulatory compliance, you need a comprehensive, convenient training program. Consider the Power and Utilities Subscription Library from 360training.com. Get unlimited access to 61 on-demand courses and save with group enrollments.