The overvoltage problem of vacuum circuit breaker has affected its development speed to a certain extent, so it is very necessary to study and discuss the causes of overvoltage and take certain protective measures in combination with the actual production.

1. The type of overvoltage generated

1.1 Intercept overvoltage

When the vacuum circuit breaker breaks the AC small current, due to the arc extinguishing chamber itself, when the current drops from the peak and does not reach the natural zero point, the arc is extinguished, the current is suddenly interrupted, and the remaining electromagnetic energy on the inductive load will produce an overvoltage, which we call the interception overvoltage. Interception overvoltage is not unique to vacuum circuit breakers, circuit breakers of other media have occurred, but vacuum circuit breakers are more likely to occur, especially when interrupting small inductor current, the interception value and its overcurrent multiple will be higher, which may bring harm to the power system, especially high-voltage electrical appliances.

1.2 When the multiple re-ignition overvoltage vacuum circuit breaker breaks the large inductive current (such as motor starting current, etc.), even if the interception overvoltage is not a problem, the overvoltage hazard often occurs, and the inter-turn insulation of the motor is broken, which is mainly caused by the overvoltage generated by the vacuum circuit breaker repeated re-ignition, which is called multiple re-ignition overvoltage. There are many conditions that must be met for multiple re-ignition overvoltages to occur, so the probability of occurrence is small, but once it occurs, the damage should not be underestimated, so the necessary precautions should be taken.

1.3 Load overvoltage

Vacuum circuit breakers have better performance than other types of circuit breakers in breaking capacitive loads, but when switching power capacitor banks, breakdown may occur due to the unstable medium recovery strength of the vacuum circuit breaker gap arc and the reduction of DC withstand voltage level, resulting in overvoltage.

2. Precautionary measures

The overvoltage that occurs during the use of the vacuum circuit breaker brings harm to the insulation of the power equipment, so corresponding measures should be taken according to the type of overvoltage to reduce the generation of overvoltage and reduce the value of overvoltage.

2.1 Capacitor protection is connected in parallel with the capacitor on the inductive load end, which can effectively reduce the load impedance, thereby reducing the amplitude of the interception overvoltage, and can also slow down the steepness of the front of the overvoltage, which can not only protect the inductive load from the damage of the interception overvoltage, but also reduce the harm of multiple re-ignition overvoltages to the motor insulation. The vacuum circuit breaker is connected with a cable between the transformer or the motor, and because the cable has a large distributed capacitance, its effect is equivalent to that of the parallel capacitance, and the effect is very good.

2.2 Resistance and capacitance protection: the resistor R and the capacitor C are connected in series as protection elements in parallel at the load inlet to form an RC overvoltage suppressor. Capacitors can both slow down the rise steepness of the overvoltage and reduce the wave impedance of the load, thus reducing the interception overvoltage. The function of the resistor is: when the interception occurs, its presence increases the attenuation coefficient of the high-frequency discharge circuit, which can reduce the number of re-ignition and reduce the overvoltage of multiple re-ignition, and can even effectively prevent its occurrence. It is effective to use RC suppressors to protect loads such as motors.

2.3 Nonlinear resistance protection

(1) The ordinary arrester is connected in parallel with the capacitor, and the ordinary arrester can limit the overvoltage amplitude and use the capacitor to slow down the steepness of the overvoltage.

(2) The metal oxide arrester is adopted, which adopts ZnO varistor, which is a lightning arrester without arc extinguishing gap, and has the characteristics of semiconductor transistor stability. Under the normal working voltage, the resistance value is very large, the current is very small, when the voltage increases to a certain value, the resistance value decreases, showing stable characteristics. It should be noted that the metal oxide arrester is used for overvoltage protection, and its model should be consistent with the system voltage, and the inductive load or capacitor bank capacity should be properly matched.

2.4 Inductance protection: The LR overvoltage suppressor composed of a series reactance coil (or saturation reactor) and a resistor in parallel between the vacuum circuit breaker and the motor power supply cable is formed, so as to suppress the rise steepness and peak value of the overvoltage.