1. Hazards of overvoltage on motors during operation
(1) Cut off overvoltage. Due to the excellent arc extinguishing performance of vacuum circuit breakers, when a small current is cut off, the arc will extinguish before zero crossing. Due to the sudden interruption of the current, the energy trapped in the inductance winding of the motor will inevitably charge the stray capacitance of the winding and transform into electric field energy. For motors and transformers, especially when unloaded or with small capacity, it is equivalent to a large inductance, and the circuit capacitance is small, which can generate high overvoltage, especially when disconnecting unloaded transformers, which is more dangerous. In theory, high overvoltage can be generated, but due to certain resistance losses and breakdown in the contact and circuit, it has a considerable inhibitory effect on the overvoltage value. However, this inhibitory effect is limited and cannot eliminate the overvoltage that occurs when cutting off small currents. Therefore, for inductive loads, overvoltage protection equipment should be installed when using vacuum circuit breakers as operating components.
(2) Repeated reignition of overvoltage. Multiple reignition overvoltage is caused by multiple reignition of the arc gap, and the power supply charges the motor capacitor multiple times. During the process of cutting off the current in a vacuum circuit breaker, one side of the contact is a power frequency power supply, and the other side is an oscillating power supply for LC circuit charging and discharging. If the opening distance between the contacts is not large enough, the superposition of the two voltages will cause breakdown between the arc gaps, and the recovery voltage of the circuit breaker will increase. If the contact distance is not large enough, a second reignition will occur, followed by arc extinguishing and reignition, resulting in multiple reignition phenomena. Multiple charging and discharging oscillations will cause the recovery voltage between the contacts to gradually increase, and the voltage at the load end will also continue to rise, leading to multiple reignition overvoltage and damage to electrical equipment.

(3) Three phase disconnection overvoltage. The three-phase breaking overvoltage is caused by the high-frequency current flowing through the phase gap of the circuit breaker when it first breaks and reignites, causing the power frequency current in the other two phase gaps to quickly cross zero, resulting in the disconnected phase being cut off and generating a similar large level of current interception in the other two phase gaps, resulting in higher operating overvoltage. The resulting overvoltage is applied to the insulation between phases. Three phase overvoltage is prone to occur when disconnecting small and medium-sized motors or under light loads.

2 Measures taken
Due to the large inductance of the motor winding, as well as the presence of inter turn capacitance, ground capacitance, and stray capacitance in the winding. Equivalent to an LC oscillation circuit, according to the mechanism of overvoltage generated by vacuum circuit breaker operation, overvoltage is generated when cutting off small current, which endangers the insulation of the motor and electrical equipment in the circuit. Therefore, measures must be taken to limit the operating overvoltage to protect the safe and reliable operation of electrical equipment, and to expand the application range of vacuum circuit breakers. At present, measures taken in China include installing metal oxide lightning arresters (MOA), trident overvoltage protectors (TBP), combined overvoltage protectors (JPB), etc. All three types of equipment use zinc oxide valve plates as the main components, and the main technical parameters of the protected equipment are shown in Table 1.
Table 1 Main Technical Parameters of Protection Equipment Unit: kV
Protection Equipment Parameters MOATBPJPB System Rated Voltage 610610610 Motor Rated Voltage 610610610 Generator Rated Voltage 6.310.56.310.5 Protector Rated Voltage 7.612.77.612.77.612.77.612.7.612.7 Power Frequency Discharge Voltage ＞ 11.017.816.026.0 DC Reference Voltage ＞ 11.318.910.016.017.0 Impulse Discharge Voltage ＜ 15.025.015.625.214.025.0 Lightning Impulse Discharge Voltage ＜ 19.031.018.028.817.029.0 Operating Wave Residual Voltage 15.025.015.024.814.024.0
According to the requirements of the insulation coordination regulations, the minimum withstand voltage level should exceed the protection level by 15%. At the same time, due to ungrounded or grounded through arc suppression coils in 10kV and below systems, and when single-phase grounding occurs, the sound phase voltage rises to the line voltage and is allowed to operate for 2 hours. In this case, the lightning arrester will be severely overheated and damaged. From the calculated values of motor test voltage and the protection levels listed in Table 1, the MOA lightning arrester has the worst level of motor protection. Although TBP and JPB are better than MOA, the margin is too small, and the protection performance is still not ideal. Therefore, when the vacuum circuit breaker generates operating overvoltage, it cannot effectively protect the motor.

At present, some manufacturers have developed and produced a new product, RC resistance capacitance absorber, aimed at limiting the overvoltage of vacuum circuit breaker operation and endangering motor insulation. It can reduce the operating overvoltage of the vast majority of circuits to below 2-2.5 times the peak power supply voltage. There are currently three forms of RC protectors, namely ordinary RC protectors with neutral point directly grounded; Neutral point ungrounded RC protector; Dual RC overvoltage protector. The common RC protector has the problem of excessive capacitor current during single-phase short circuit, which leads to complete tripping of the feeding circuit, especially for places with high-frequency components, causing the resistance of the RC protector to burn out. Although the ungrounded RC protector solves the problems of tripping and burning resistance due to excessive capacitor current, it does not eliminate high-frequency oscillations between phases to ground, resulting in a slightly higher accident rate. The dual RC overvoltage protector not only solves the high-frequency oscillation in the ground circuit, but also solves the problems of excessive ground current and RC device resistance burning.

3 Precautions
At present, most of the vacuum circuit breakers produced are ordinary distribution type vacuum circuit breakers, which have been widely adopted by many units. However, there are still some shortcomings and shortcomings in installing ordinary distribution type vacuum circuit breakers in the generator circuit: ① With the extension of operating time, the insulation level of the generator gradually decreases, and there is almost no margin between the operating overvoltage of the vacuum circuit breaker and the insulation level of the motor; ② The technical performance requirements for generator circuit breakers are relatively strict, and the operating conditions are harsh. It is required that the DC component value cut off by the generator circuit breaker should be greater than 60% or 80% of the rated breaking current, which is difficult for ordinary distribution type vacuum circuit breakers to achieve Due to the capacitance of the generator itself (a hydroelectric generator is larger than a steam turbine generator), as well as the capacitance generated by longer lead out and branch lines, if an RC overvoltage protector is used, the capacitance of the protector should also be added. This can cause unnecessary tripping or the addition of equipment at the neutral point (such as arc suppression coils, grounding resistance, etc.) in the event of single-phase grounding, resulting in complex power outage protection.