Protection relays are used in generators, transformers, feeders, transmission lines, etc. The primary function of these relays is to continually monitor a specific parameter such as current, voltage or power and to generate alarm/isolate the system or device in the situation of deviation from set limits for the parameter or a fault. For instance, an overcurrent relay may be programmed to operate when the current in a feeder exceeds a certain predetermined limit. These relays generally obtain their feedback from current or voltage transformers.
These relays are used to regulate a specific parameter such as the output voltage of a transformer. These relays operate a control equipment such as the tap changer of a transformer. These relays are not designed to respond to fault conditions.
These relays are used to put the system into operation. These relays are used to synchronize lines and feeders. These relays usually are used in connecting different components of an electrical distribution system such as generators, feeders, transformers, etc. They also come into play when restoring the system after a fault.
These relays are used to monitor conditions in a system such as the direction of power flow and generate alarms when there are deviations. Examples include the low forward power relay which generates an alarm when the power in a direction falls below the minimum set points. They are also used to monitor the continuity of systems such as pilot wires.
These relays are used generally for contact multiplication. The single contact available in a relay is used to trip a number of breakers. Besides, these relays are also to isolate the relay from other equipments such as breakers.