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The function of a relay is to detect abnormal conditions in the system and to initiate through appropriate circuit breakers the disconnection of faulty circuits so that interference with the general supply is minimized. Relays are of many types. Some depend on the operation of an armature by some form of electromagnet.
A very large number of relays operate on the induction principle. When a relay operates it closes contacts in the trip circuit which is normally connected across 110 V DC supply from a battery or internal made itself by relay.
The passage of current in the coil of the trip circuit actuates the plunger, which causes operation of the circuit breaker, disconnecting the faulty system. In the laboratory, a contactor simulates the operation of the circuit breaker.
The closure of the relay contacts short-circuits the ‘no-volt ‘ coil of the contactor, which, in turn, disconnects the faulty system.
The protective relaying which responds to a rise in current flowing through theprotected element over a pre-determined value is called ‘overcurrent protection’ and the relays used for this purpose are known as overcurrent relays.
Earth fault protection can be provided with normal overcurrent relays, if the minimum earth fault current is sufficient in magnitude.
The design of a comprehensive protection scheme in a power system requires the detailed study of time-current characteristics of the various relays used in the scheme. Thus it is necessary to obtain the time current characteristics of these relays.
The overcurrent relay works on the induction principle. The moving system consists of an aluminum disc fixed on a vertical shaft and rotating on two jeweled bearings between the poles of an electromagnet and a damping magnet.
The winding of the electromagnet is provided with seven taps (general 0, which are brought on the front panel, and the required tap is selected by a push-in -type plug. The pick-up current setting can thus be varied by the use of such plug multiplier setting.
The pick-up current values of earth fault relays are normally quite low. The operating time of all overcurrent relays tends to become asymptotic to a definite minimum value with increase in the value of current. This is an inherent property of the electromagnetic relays due to saturation of the magnetic circuit.
The torque of these relays is proportional to f1 f2 Sina, where f1 and f2 are the two fluxes and a is the angle between them. Where both the fluxes are produced by the same quantity (single quantity relays) as in the case of current or voltage operated, the torque T is proportional to I2, or T = K I2, for coil current below saturation. If the core is made to saturate at very early stages such that with increase of I, K decreases so that the time of operation remains the same over the working range. The time -current characteristic obtained is known as definite -time characteristic.
If the core is made to saturate at a later stage, the characteristic obtained is known as IDMT. The time-current characteristic is inverse over some range and then after saturation assumes the definite time form.
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