Switching overvoltage of the hottest vacuum switch

Operating overvoltage and protection of vacuum switch at present, vacuum switch and SF6 switch are the two leading products of oil-free switch, which are similar in performance, but vacuum switch has no greenhouse effect of SF6, and its process level is suitable for the manufacturing status of Chinese enterprises, with relatively low price. Therefore, the production and use of vacuum switches are much higher than SF6 switches, especially in 10kV indoor products, vacuum switches have taken an absolute advantage. According to statistics, the following items of the instrument were tested with the help of relevant special identification tools. In 1996, among 10kV oil-free switches, vacuum switches accounted for about 70%. With the oil-free transformation of city switches and the large-scale application of vacuum switches, the problem of switching overvoltage has become increasingly prominent, which must be paid attention to and corresponding solutions must be taken

1 ﹐ structural characteristics of vacuum switch

the contact of vacuum switch opens and closes the circuit in a sealed vacuum chamber. When the contact cuts off the current, only the arc formed by metal vapor ions is formed, and there is no gas collision and dissociation. Because the diffusion and recombination process of metal vapor ions is very rapid, it can hardly play a role. It is inevitable to encounter some small problems in the working process, so it can quickly extinguish the arc and restore the original vacuum degree, It can withstand multiple opening and closing without reducing the breaking capacity. Its main features are as follows:

(1) compact structure, small volume, light weight, fast action, and small power required for opening and closing

(2) the electrical and mechanical life is long, the contact life is generally 50 times longer than that of the low oil switch, and the maintenance workload is less

(3) with large breaking capacity and many allowable breaking times, it is suitable for frequent operation

(4) no high-pressure gas and toxic gas, no fire and explosion hazards, no environmental pollution

(5) switching off small inductance current is easy to cause cut-off overvoltage and arc reignition overvoltage. In order to ensure the safe operation of electricity, the operation management is strengthened and protective measures are taken to suppress the switching overvoltage

2 ﹐ operating overvoltage of vacuum switch

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(1) cut off overvoltage vacuum switch cuts off the inductive circuit and extinguishes the arc before the current crosses zero, resulting in a very high voltage - cut-off overvoltage. Taking cutting off the no-load transformer as an example, this paper analyzes the mechanism and related factors of overvoltage

the attached figure shows the transient equivalent circuit of the no-load transformer, where vs is the power supply voltage, QF is the vacuum switch, GT is the transformer winding

group ground capacitance, lt is the transformer excitation inductance. If the current of QF disconnection is I, the electromagnetic energy stored in the transformer winding before disconnection is wc=12lti2, and the electric field energy stored in GT is wl=12gtu2, where u is the cut-off voltage corresponding to I. When QF cuts off the circuit and quickly extinguishes the arc, the power supply is completely separated from the load, and the electromagnetic energy WL and electric field energy WC are converted to each other to form oscillation. When the capacitor voltage reaches the maximum um, the energy is

12ctu2m=12ctu2+12lti2

that is, um=u2+ltcti2