Vacuum circuit breakers have the characteristics of low arcing voltage, strong arcing ability, fast breaking speed, large breaking capacity, long service life, and adaptability to frequent operations. They have been widely used in the fields of electric power, metallurgy, and chemical industry. However, during the opening process, vacuum circuit breakers will produce overvoltage, and if used improperly during operation, it may also cause accidents such as circuit breaker explosions, which will have a significant impact on safety production.
When the vacuum circuit breaker breaks the load device, it will extinguish the arc in advance before the current crosses zero, resulting in a current-limiting phenomenon, namely di/dt. Therefore, a corresponding current-limiting overvoltage will be induced on the load device, whose amplitude is related to the current-limiting level and load characteristics of the circuit breaker. The smaller the current-limiting value of the circuit breaker is under the same load, the smaller the overvoltage amplitude will be.
Multiple reignition overvoltage
When the vacuum circuit breaker opens the electrical load, the arc chamber contact will extinguish in advance when the AC current crosses zero, but due to the small opening distance of the circuit breaker contact, reignition will occur under the action of the recovery voltage. As there is an L-C circuit in all electrical loads, arc reignition will cause high-frequency current to appear in the circuit. When the high-frequency current crosses zero, the arc will again extinguish. Due to the small distance between the contacts, a new oscillation will be generated, and the contact gap will be broken again, causing the arc to reignite multiple times.
As a result, high voltage will appear in the circuit, and the amplitude of the overvoltage will increase with the increase of the number of reignitions. This reignition may occur repeatedly during the opening process, and its overvoltage value will be higher.
Actually, the overvoltage value is limited by the instantaneous working conditions of the circuit breaker operation, but its voltage amplitude is also about 4 times the phase voltage, which will cause harm to the insulation of electrical loads.
Simultaneous breaking overvoltage
When the vacuum circuit breaker is applied in the motor circuit, the current-limiting phenomenon of the circuit breaker, together with the mutual inductance and capacitance of the motor circuit, will cause arc reignition and high-frequency oscillation.
After the first phase is opened, the high-frequency current will couple to the second and third phases through the interphase mutual inductance and capacitance and superimpose with the existing high-frequency current, forming three-phase current simultaneously crossing zero. When the first phase arcs, it will force the second and third phase arcs to be forcibly cut off, resulting in a higher overvoltage.
Use a circuit breaker with a lower current limiting value: Because the overvoltage due to current limiting of the vacuum circuit breaker is related to the current limiting value of the circuit breaker, the smaller the current limiting value, the smaller the amplitude of the overvoltage. The current limiting value of the circuit breaker is related to the material of the circuit breaker contact, so when selecting the circuit breaker, special attention should be paid to selecting the appropriate contact material to reduce the current limiting value of the circuit breaker and reduce the current limiting overvoltage. At present, most arcing chamber contacts are made of copper-chromium alloy, and the current limiting value is about 1 A.
Use an R-C series circuit absorption device in the vacuum circuit breaker:
The multiple reignition overvoltage of the vacuum circuit breaker is related to the high-frequency current caused by L-C oscillation in the load circuit. Therefore, an R-C series circuit can effectively limit the overvoltage.
The R-C circuit can not only limit the amplitude of the current-limiting overvoltage but also reduce the rise steepness of the multiple reignition overvoltage. The device is composed of a resistance and a capacitance.
The role of resistance R is to increase the attenuation coefficient, consume high-frequency oscillating energy, and the capacitance C limits the steepness of the overvoltage waveform. However, the R-C series circuit cannot fully absorb the energy generated by the overvoltage.
Use an overvoltage protector: An overvoltage protector can effectively limit the overvoltage.