The protection principle wiring of current direction is shown in FIG. 14, current relays 3 and 5 are starting elements, power direction relays 4 and 6 are directional elements using 90 ° wiring (UbcIA and UabIc). The contacts of each phase current relay and the corresponding power direction relay contact are connected in series to achieve phase activation.
The time relay 7 is the necessary operation time limit for the protection device, its contacts closed and sends a trip pulse through the signal relay 8 to trip the circuit breaker QF.
Direction overcurrent protection, because the power direction relay is installed, although the short circuit of the line may act, only when the current flowing into the power direction relay is consistent with the power direction relay (when the line, the primary current from the bus to the line), so as to make the circuit breaker trip.
When the current flowing into the power direction relay is opposite to the direction specified by the power direction relay (i. e., when the primary current flows from the line to the bus), the power direction relay does not act, locking the direction over-current protection to ensure the selectivity of direction over-current protection.
In normal operation, the direction of the load current may also meet the action direction of the power direction relay, and its contact is closed, but the current relay does not move, so the overcurrent protection in the whole direction still is not locked.
The action time limit of direction overcurrent protection is determined by protecting the same action direction according to the principle of reverse ladder.
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