Vacuum circuit breakers have the characteristics of low arc extinguishing voltage, strong arc extinguishing ability, fast breaking speed, large breaking capacity, long service life, and frequent operation. It has been widely used in many fields such as electric power, metallurgy and chemical industry. However, the vacuum circuit breaker will generate over-voltage during the breaking process. If it is used improperly during operation, it will also cause accidents such as the circuit breaker explosion, which will have a major impact on safety production.
1. Current cut-off over-voltage
When the vacuum circuit breaker is used to open and break the load equipment, it will extinguish the arc before the current crosses zero, resulting in a current cut-off phenomenon, namely di/dt→, so the load equipment will induce the corresponding current cut-off over-voltage, that is, the amplitude is related to the cut-off level and load characteristics of the circuit breaker. Under the same load, the lower the current cut-off value of the circuit breaker, the smaller the over-voltage amplitude.
2. Multiple reignition over-voltage
When the vacuum circuit breaker breaks the electrical load, the contact of the arc extinguishing chamber separates the arc when the power frequency current crosses zero. Due to the small contact distance of the circuit breaker, reignition will occur under the action of the recovery voltage. For electrical loads, there is an L-C circuit, and the reignition of the arc will cause a high-frequency current in the circuit. When the high-frequency current crosses zero, the arc will be extinguished again. Due to the small contact opening distance, new oscillations will occur, which will break down the contact gap again and cause the arc to reignite multiple times. In this way, a high voltage will appear in the loop, and its over-voltage amplitude will increase as the number of reignitions increases. This kind of reignition may be repeated many times during the breaking process, and its over-voltage value will be higher.
In fact, the over-voltage value is limited by the instantaneous operating conditions of the circuit breaker, but its voltage amplitude is also about 4 times the phase voltage, which will cause harm to the insulation of the electrical load.
3. Over-voltage because of breaking at the same time
When the vacuum circuit breaker is used to break the motor circuit, due to the current cut-off phenomenon of the circuit breaker, and the mutual inductance and capacitance in the motor circuit, it will cause the reignition of the arc and high-frequency oscillation. When the first phase is disconnected, the high-frequency current will be coupled to the second and third phases through mutual inductance and capacitance between the phases and superimposed with the original high-frequency current to form a three-phase current that crosses zero at the same time. When the arc of the first phase is extinguished, the arcs of the second and third phases are forced to be cut off, resulting in a higher over-voltage.
1. Use a circuit breaker with a lower cut-off value
Since the current cut-off over-voltage of the vacuum circuit breaker is related to the cut-off value of the circuit breaker, the smaller the cut-off value, the smaller the over-voltage amplitude. The cut-off value of the circuit breaker is related to the material of the circuit breaker's contact.
Therefore, special attention should be paid to the selection of suitable contact materials when designing and selecting the circuit breaker, so as to reduce the cut-off value of the circuit breaker and reduce the cut-off over-voltage. At present, most of the contact materials of the arc extinguishing chamber are copper-chromium alloy, and the cut-off value is about 1 A.
2. Use R-C resistance-capacitance loop absorption device
The multiple reignition over-voltages of vacuum circuit breakers are related to the presence of L-C oscillations in the load circuit and the occurrence of high-frequency currents. Therefore, the use of R-C resistance-capacitance circuits can effectively limit over-voltages. The R-C circuit can not only limit the amplitude of the cut-off over-voltage, but also reduce the rising steepness of multiple reignition over-voltages. The device is composed of resistors and capacitors.
The function of the resistor R is to increase the attenuation coefficient and consume high-frequency oscillation energy, and the function of the capacitor C is to limit the steepness of the over-voltage wave head. But the use of R-C resistance-capacitance circuit can not completely absorb the energy generated by over-voltage.
3. Use over-voltage protector
Over-voltage protector can better limit over-voltage. Currently, zinc oxide arresters are commonly used as over-voltage protectors. However, zinc oxide arresters still have some problems for low-current grounding systems: first, the operating voltage is low. Secondly, they cannot withstand intermittent over-voltage and harmonic over-voltage. Thirdly, they cannot give full play to the advantages of zinc oxide arresters.
When the zinc oxide arrester is used in the motor circuit, because it is connected between the phase and the ground, its protection range is the over-voltage between the phase and the ground. The over-voltage of the motor circuit exists between phases, so it cannot protect the motor insulation well.