STATIC RELAYS

A static relay refers to a relay in which there is no armature or other moving element and response is developed by electronic, magnetic and other components without mechanical motion. The solid-state components used are transistors, diodes, resistors, capacitors and so on. Static circuits accomplish the function of comparison and measurement. A relay using combination of both static and electro-magnetic units is also called a static relay provided that static units accomplish the response.

In static relays, the measurement is performed by electronic, magnetic, optical or other components without mechanical motion. Additional electro-mechanical relay units may be employed in output stage as auxiliary relays. A protective system is formed by static relays and electro-mechanical auxiliary relays.

BASIC STATIC RELAY

The essential components of static relays are shown in fig 20. Rectifier rectifies the relaying quantity i.e., the output from a CT or PT or a transducer. The rectified output is supplied to a measuring unit comprising of comparators, level detectors, filters, logic circuits. The output is actuated when the dynamic input (i.e., the relaying quantity) attains the threshold value. This output of the measuring unit is amplified by amplifier and fed to the output unit device, which is usually an electro-magnetic one. The output unit energizes the trip coil only when relay operates.

In a static relay the measurement is carried out by static circuits consisting of comparators, level detectors, filter etc while in a conventional electro-magnetic relay it is done by comparing operating torque (or force) with restraining torque (or force). The relaying quantity such as voltage/current is rectified and measured. When the quantity under measurement attains certain well-defined value, the output device is triggered and thereby the circuit breaker trip circuit is energized.

Written by arjun on March 17th, 2009 with 4 comments.
Read more articles on classification of relays and Power System and Relays and Switchgear&Protection.

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