At present, the stroke of the moving iron core (that is, the air gap between the moving iron core and the yoke) of the permanent magnet mechanism (bistable) produced at home and abroad is relatively small (usually no more than 25mm), much smaller than the conventional electromagnetic, spring, hydraulic and pneumatic The stroke of the operating mechanism. Therefore, it can only be used on medium voltage vacuum circuit breakers with small contact strokes at present. So what is the selection and determination of the opening and closing of the vacuum circuit breaker of the bistable permanent magnet mechanism? Let’s introduce it to everyone!
1. The selection of the holding force of the permanent magnet mechanism in the opening and closing state
The permanent magnet mechanism is simple in structure and highly reliable in action. It does not require mechanical holding and off-line devices for the closing and opening positions. It is the suction force generated by the magnet to keep the circuit breaker in the opening and closing positions.
The vacuum circuit breaker requires a certain contact pressure. Therefore, the suction force of the permanent magnet mechanism must not only overcome the reaction force of the contact spring and other reaction forces, but also have sufficient closing position to effectively maintain the force to prevent it from being affected by the outside world. Under the action of factors (mechanical vibration, electric power, etc.), an automatic error opening occurs. The size of the holding force not only determines the closing and holding performance of the circuit breaker, but also determines important parameters such as opening power and opening speed.
According to the test, the effective holding force of closing is obtained: when the breaking current is 20kA and below, it should be 500～700N; when the breaking current is 31.5kA and above, it should be 700～1000N.
Because the double-coil permanent magnet mechanism does not need to be equipped with a full-stroke opening spring, the operating mechanism only needs not too large holding force to reliably maintain the circuit breaker in the opening state. Consider that the permanent magnet mechanism must have sufficient closing power to ensure that the circuit breaker has sufficient closing capacity. Therefore, the opening holding force of the permanent magnet mechanism cannot be too small. According to the test, it is concluded that the effective holding force of the permanent magnet mechanism should be selected at 2000～3000N.
The magnet circuit of the permanent magnet mechanism is usually designed according to the side (closing side) that requires large suction, and the other side must take measures to reduce its suction, adding a magnetic isolation between the moving iron core and the suction surface of the yoke To reduce the effective pull-in area; install a short-range spring on the opening side to offset part of the opening retention force, so as to meet the characteristic requirements for opening and closing of the circuit breaker at the same time.
2. Determination of the opening and closing time of the vacuum circuit breaker
The opening time of vacuum circuit breaker equipped with permanent magnet mechanism can usually reach about 30ms (excitation time 25ms plus contact overtravel movement time 5ms). Starting from the requirements of 12～40.5kV power distribution network, there is no need for circuit breakers to have such a short opening time. Switching time, usually 50ms opening time is sufficient.
When the opening time of the circuit breaker is too short, it is difficult to take measures to extend the opening time on the circuit breaker or the operating mechanism. Usually, only a delay relay is installed at the exit of the opening operation circuit to extend the opening time of the circuit breaker. To about 50ms, the closing time of the circuit breaker is usually 40～45ms (excitation time 30～35ms plus contact opening movement time 15～20ms). It is also rare for a 12～40.5kV medium voltage circuit breaker to have such a short closing time. The shorter closing time has no negative effect on the circuit breaker and the power system, and there is no need to take measures to extend it.
The decrease of the power supply voltage has a great influence on the opening and closing time of the circuit breaker, because the power supply voltage is low, and the excitation time is long.