The overvoltage problem of vacuum circuit breakers has affected their development speed to a certain extent. Therefore, it is very necessary to study and explore the causes of overvoltage and take certain protective measures in combination with production practice.
1 Types of overvoltage generation
1.1 Cut-off overvoltage
When the vacuum circuit breaker interrupts a small AC current, due to the arc extinguishing chamber itself, when the current drops from the peak value but does not reach the natural zero point, the arc is extinguished and the current is suddenly interrupted. The residual electromagnetic energy on the inductive load will generate overvoltage, which we call cut-off overvoltage. Cut-off overvoltage is not unique to vacuum circuit breakers. Circuit breakers of other media also have it, but vacuum circuit breakers are more likely to occur, especially when breaking small inductive currents. The cut-off value and its overcurrent multiple will be higher, which may cause harm to the power system, especially high-voltage electrical appliances.
1.2 Multiple restrike overvoltage When the vacuum circuit breaker is breaking a large inductive current (such as the starting current of the motor, etc.), even if the overvoltage is not a problem, overvoltage hazards often occur, breaking down the insulation between the motor turns. This is mainly caused by the overvoltage generated by the multiple restrike of the vacuum circuit breaker, which is called multiple restrike overvoltage. For multiple restrike overvoltage to occur, many conditions must be met, so the probability of occurrence is very small, but once it occurs, its harm cannot be underestimated, so necessary preventive measures should be taken.
1.3 Capacitive load overvoltage
Vacuum circuit breakers have better performance than other types of circuit breakers in breaking capacitive loads, but when switching power capacitor banks, due to the unstable recovery strength of the vacuum circuit breaker gap after the arc and the reduced DC withstand voltage level, breakdown may occur, resulting in overvoltage.
2 Preventive measures
The overvoltage generated during the use of vacuum circuit breakers will harm the insulation of power equipment. Therefore, corresponding measures should be taken according to the type of overvoltage to reduce the generation of overvoltage and reduce the value of overvoltage. In addition to the problems in the manufacturing process of vacuum circuit breakers, protective devices can be installed to change the load parameters to achieve the purpose.
2.1 Capacitor protection Connecting capacitors in parallel to the inductive load end can effectively reduce the load impedance, thereby reducing the amplitude of the cut-off overvoltage, and can also slow down the steepness of the overvoltage front. This can not only protect the inductive load from the damage of the cut-off overvoltage, but also reduce the damage of multiple restrike overvoltage to the insulation of the motor. The vacuum circuit breaker is connected to the transformer or motor with a cable. Since the cable has a large distributed capacitance, its function is equivalent to a parallel capacitor, and the effect is very good.
2.2 Resistor-capacitor protection Connecting resistor R and capacitor C in series as protection elements in parallel at the load input end to form an RC overvoltage suppressor. The capacitor can not only slow down the rising steepness of the overvoltage, but also reduce the wave impedance of the load, thereby reducing the cut-off overvoltage. The role of the resistor is: when the current is cut off, its existence increases the attenuation coefficient of the high-frequency discharge circuit, which can reduce the number of reignitions and reduce the overvoltage caused by multiple reignitions, and can even effectively prevent its occurrence. Using an RC suppressor to protect loads such as motors is the best.
2.3 Nonlinear resistor protection
(1) Use an ordinary lightning arrester in parallel with a capacitor. The ordinary lightning arrester can limit the overvoltage amplitude, and the capacitor can be used to slow down the steepness of the overvoltage rise.
(2) Use a metal oxide lightning arrester, which uses a ZnO varistor and is an arrester without an arc-extinguishing gap. It has the stable characteristics of a semiconductor transistor. Under normal operating voltage, the resistance is large and the current is small. When the voltage increases to a certain value, the resistance decreases, showing a stable characteristic. It should be noted that when using a metal oxide lightning arrester for overvoltage protection, its model must be consistent with the system voltage and should be properly matched with the inductive load or capacitor bank capacity.
2.4 Inductive protection A LR overvoltage suppressor consisting of a series reactance coil (or saturated inductor) and a resistor in parallel is connected between the vacuum circuit breaker and the motor power supply cable to suppress the rising steepness and peak value of the overvoltage.