Relay protection circuitb

The operating power supply of relay protection devices can be divided into two categories: DC operating power supply and AC operating power supply. Since AC operating power supply has
the advantages of low investment, convenient operation and maintenance, and simple and reliable secondary circuit, it is widely used in small and medium-sized factories.
    (1) The direct-action protection circuit uses
the over-current release
(trip coil) YR in the manual operating mechanism of the circuit breaker as a direct-action over-current relay
, which is connected to a two-phase one relay type or a two-phase two relay type.
Device type. During normal operation, the current flowing through YR is much
smaller than its operating current, so it does not operate. When a phase-to -phase short
circuit occurs in the primary circuit , YR acts to trip the circuit breaker QF. This operation method is simple and economical , but the protection sensitivity is low and is rarely used in practice, as shown in the figure.     (2) Remove the shunt trip protection circuit   as shown in Figure b. During normal operation, the normally closed contact of current relay KA will short-circuit and shunt the trip coil YR . No current flows through YR, so circuit breaker QF will not trip. When a phase-to-phase short circuit occurs in the primary circuit, the current relay KA operates, and its normally closed contact opens, so that the short- circuit shunt is removed, which is the so-called "de-shunt", thereby reducing the secondary current of the current transformer. All of them pass through YR, causing circuit breaker QF to trip, which is the so-called "de-shunt trip". The wiring of this operation method is also relatively simple, sensitive and reliable, but it requires the breaking capacity of the current relay KA contacts to be large enough. The current relays     of GL15, 16 and GL25, 26 models on the market have quite large contact capacities and a short-term breaking current of up to 150A, which can fully meet the requirements of "de-shunt tripping" during short circuit . Therefore, this method of de-shunt tripping is now widely used in factory power supply systems .