Discussion on the operation overvoltage of vacuum

Talking about the operating overvoltage of vacuum switch and its protection

the operating overvoltage of circuit breaker is caused by the existence of inductive and capacitive energy storage elements in the circuit, which release energy at the moment of switch operation and produce electromagnetic oscillation in the circuit. Due to its high-speed arc extinguishing ability, the vacuum circuit breaker is often forcibly disconnected before the current crosses zero when cutting off the circuit. The electromagnetic energy conversion between the inductor and capacitor stored in the load at the moment of arc breaking will produce Overvoltage on the load, which is more prominent than general circuit breakers, especially between the phase contacts that were first disconnected in recent years, it is possible to cause arc reignition due to overvoltage, resulting in greater overvoltage. In inductive loads, this kind of overvoltage has high amplitude, fast rise steepness and high frequency, which is undoubtedly very dangerous to the insulation of inductive loads such as motors. In short, no matter what kind of overvoltage occurs in vacuum circuit breakers, it will be detrimental to the equipment and seriously threaten the safe production and operation

when the vacuum circuit breaker disconnects the inductive load such as the motor, the wave steepness is very large and the amplitude is very high, which directly threatens the turn to turn insulation of the inductive load, and is one of the important reasons for the damage of the motor and other equipment. Therefore, it is necessary to study the overvoltage suppression measures for the operation of the vacuum circuit breaker. Only by taking appropriate protective measures, we can consider reducing the overvoltage amplitude and wave steepness (du/dt), It can effectively inhibit or reduce its harm, which will play a positive role in popularizing the application of vacuum circuit breakers. At present, there are two measures to suppress overvoltage, one is the arrester that limits the amplitude of overvoltage due to the obvious relaxation of molecular motion, and the other is the resistance capacitance (R-C) overvoltage absorber that reduces the oscillation frequency of overvoltage

I. zinc oxide arrester (MOA)

as we all know, the main advantages of this arrester are that it has very excellent nonlinear volt ampere characteristics, small aftercurrent, low residual voltage, small volume, light weight and convenient installation. However, the traditional gapless MOA has the following disadvantages in operation:

1) thermal aging

2) in order to reduce the residual voltage after discharge, its continuous operation voltage is selected to be low. When single-phase grounding operation occurs, it bears √ 3 times the phase voltage, resulting in the increase of charging rate and very vulnerable

3) the impulse discharge voltage and interphase residual voltage are high, which is not conducive to maintaining the interphase insulation of the equipment. In view of the above problems, some manufacturers have introduced zinc oxide arresters with gaps, and the author believes that priority should be given to the selection

1. MOA with series clearance. The structure of this arrester is similar or basically the same as that of the traditional valve arrester. Due to the excellent nonlinear volt ampere characteristics of zinc oxide valve slice, the freewheeling is very small, generally below 1mA, so it can be considered that there is no freewheeling. In addition, due to the existence of series gap, under the normal operation of this product, using the characteristics of high conductivity, superfine and dispersibility of graphene, zinc oxide can be ignored. In addition, the residual voltage value of this MOA is low, which can be used for overvoltage protection of equipment with high withstand voltage level such as power transformers

examples of parameter comparison between MOA with series gap and MOA without gap for power distribution:

category and model system rated voltage continuous operation voltage nominal current maximum residual voltage comparison

no gap y5ws-7.6 6434.5>29.694

y3wd-12.7 106.6 57.5>42.42

gap y5cs-7.6 6427 42.42

note: transformer insulation withstand voltage test standard (peak): 29.694kv (un=6kv), 42.42kv (un=10kv)

2 MOA with parallel clearance. The structure with parallel clearance is to reduce the activation temperature of the adhesive from 90 degrees to 45 degrees. The main valve RI and parallel valve R2 are connected in series, and there is a parallel clearance G on R2. Under normal voltage, G does not discharge, and the voltage is added to R1 and R2, so the operation is safe and reliable. When the overvoltage acts, G discharges, R2 is short circuited, and the residual voltage of the arrester is completely determined by the residual voltage on R1, so the residual voltage is relatively low and the protection performance is relatively good. It is suitable to protect the motor

3. Parameter comparison between parallel clearance MOA and gapless MOA for protection of rotating motors:

category and model motor rated voltage continuous operation voltage nominal current maximum residual voltage comparison

gapless y3wd-7.6 6 6.3 419>18.382

y3wd-12.7 10.5 6.6 31>29.694

gapped y3bd-7.6 6.3 4 16.2