After the power supply is disconnected by the protection action, the arc at the fault point is automatically extinguished, the insulation strength is restored, and the fault is eliminated by itself. If the circuit breaker is closed again, the normal power supply can be restored. It is mainly caused by the scattering of the insulator surface caused by lightning, the discharge of the line to the branches, the short-term contact with the line caused by strong wind, and the short circuit caused by the discharge through the body of a bird. This type of failure accounts for more than 80% to 90% of the total number of failures.
After the power supply of the fault line is disconnected, the insulation strength of the fault point cannot be restored, and the fault still exists. Even if the circuit breaker is closed again, it will be disconnected again by the relay protection device such as inverted pole, broken wire, insulator breakdown or damage and so on.
An automatic device that automatically re-inserts a circuit breaker that has been tripped by an abnormal operation. After the line adopts automatic circuit recloser, if there is a transient fault, after the protection action removes the fault, the reclosing action can be successful and the power supply of the line can be restored. For a permanent fault, after the reclosing action, the protection will act again, the circuit breaker will trip, and the reclosing will not be success. The advantages of using automatic 11kv auto reclosers, and 33kv vcb breakers, and lbs 24kv on transmission lines are as follows.
1. The automatic circuit recloser improves the reliability of the power supply of the transmission line and reduces the loss caused by the instantaneous fault and power outage.
2. For high-voltage transmission lines with double-ended power supply, the automatic circuit recloser can improve the stability of the parallel operation of the system, thereby increasing the transmission capacity of the line.
3. The automatic circuit recloser can correct the erroneous trip caused by the faulty circuit breaker itself or the malfunction of the relay protection.
The regulations stipulate that overhead lines and cables and overhead mixed lines of 1kV and above, under the condition of a circuit breaker, if electrical equipment allows and no standby power is automatically turned on, automatic reclosing shall be installed. Bypass circuit breakers and automatic reclosing should be installed for the bus circuit breaker or section circuit breaker that also serves as a bypass. The step-down transformer on the low-voltage side without power supply should be equipped with automatic reclosing. If it is necessary, the bus can adopt automatic reclosing.
After the automatic circuit recloser is adopted, it will have an adverse effect on the system. When the reclosing is in a permanent fault, the system will be impacted by the short-circuit current again, which may cause the system to oscillate. At the same time, the circuit breaker cuts off the breaking current twice in a short period of time, deteriorating the working conditions of the circuit breaker. Therefore, the use of automatic circuit reclosers is sometimes restricted by system and equipment conditions.
The automatic circuit recloser is mainly used for overhead lines. For cable lines, because of its low probability of failure, even if a failure occurs, the insulation is often permanently damaged, so automatic reclosing is not used.