At present, the permanent magnet mechanism (bistable) moving iron core stroke produced at home and abroad (that is, the air gap between the moving iron core and the yoke) is relatively small (usually not more than 25mm), much smaller than conventional electromagnetic, spring, hydraulic and air pressure. Operate the organization’s itinerary. Therefore, it can only be used on medium voltage vacuum circuit breakers with small contact strokes. So what is the selection and determination of the switching and closing of the bistable permanent magnet mechanism vacuum circuit breaker? Let me introduce you to the following!

1. Selection of retaining force for the separation and closing state of permanent magnet mechanism

The permanent magnet mechanism has a simple structure and high operational reliability, and does not need to be mechanically held and off-line by the opening and closing position. It is a suction force generated by the permanent magnet to keep the circuit breaker in the branching and closing positions.

The vacuum circuit breaker requires a certain contact contact pressure. Therefore, the suction force of the permanent magnet mechanism must not only overcome the reaction force and other reaction forces of the contact spring, but also must have sufficient closing position to effectively maintain the force to prevent external exposure. Under the action of factors (mechanical vibration, electric power, etc.), automatic mis-breaking occurs. The magnitude of the holding force not only determines the closing performance of the circuit breaker, but also determines important parameters such as the opening work and the opening speed.

According to the test, the effective holding force of the closing is obtained: when the breaking current is 20kA or less, it should be selected from 500 to 700N; if the breaking current is 31.5kA or more, it should be selected from 700 to 1000N.

Because the double-coil permanent magnet mechanism does not need to be equipped with a full-stroke trip spring, the operating mechanism only needs less holding force to keep the circuit breaker reliably in the open state. Consider that the permanent magnet mechanism must have a large enough closing work to ensure that the circuit breaker has sufficient closing capacity. Therefore, the holding force of the permanent magnet mechanism cannot be too small. According to the test, the effective holding force of the permanent magnet mechanism should be selected from 2000 to 3000N.

The permanent magnet circuit of the permanent magnet mechanism is usually designed on the side where the suction force is the most demanding (closing side), and the other side must take measures to reduce the suction force, and the gap between the movable iron core and the yoke suction surface is added. The magnetic sheet and the effective absorption area are reduced; a short-range spring is installed on the opening side to offset a part of the opening holding force, so that the characteristic requirements of the circuit breaker opening and closing can be satisfied at the same time.

2. Determination of the opening and closing time of the vacuum circuit breaker

The vacuum circuit breaker with permanent magnet mechanism can usually reach 30ms (excitation time 25ms plus contact overtravel time 5ms). Starting from 12~40.5kV distribution network requirements, there is no need for circuit breakers to have such short points. The gate time, usually with a 50ms trip time is sufficient.

When the circuit breaker opening time is short, it is difficult to take measures to extend the opening time on the circuit breaker or the operating mechanism. Usually, only the delay relay is installed at the exit of the opening operation circuit to extend the opening time of the circuit breaker. Up to 50ms, the closing time of the circuit breaker is usually 40~45ms (the excitation time is 30~35ms plus the contact opening distance movement time is 15~20ms). It is also rare for a 12 to 40.5 kV medium voltage circuit breaker to have such a short closing time. Shorter closing times have no negative effect on the circuit breaker and the power system, and no measures are required to extend it.

The reduction of the power supply voltage has a great influence on the switching and closing time of the circuit breaker, because the power supply voltage is low and the excitation time is long.

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