1. Harm caused by operating overvoltage to motors (1) Cut-off overvoltage. Because vacuum circuit breakers have good arc extinguishing performance, when a small current is interrupted, the arc will be extinguished before crossing zero. As the current is suddenly cut off, the energy remaining in the inductive windings of the motor will inevitably charge the stray capacitance of the windings and be converted into electric field energy. For motors and transformers, especially when unloaded or with small capacity, it is equivalent to a large inductor and the circuit capacitance is small, so high overvoltage will be generated, especially when interrupting unloaded transformers. Theoretically, very high overvoltage can be generated, but due to the certain resistance in the contacts and circuit, there is a certain loss and breakdown, which has a considerable suppressive effect on the overvoltage value. However, this suppressive effect is limited and cannot eliminate the overvoltage that occurs when interrupting a small current. Therefore, especially for inductive loads, when using vacuum circuit breakers as operating elements, overvoltage protection devices should be installed. (2) Repeated reignition overvoltage. Repeated reignition overvoltage is caused by repeated reignition of the arc gap, which causes the power supply to charge the motor capacitor multiple times. During the process of interrupting current in a vacuum circuit breaker, one side of the contact is the power frequency power supply, and the other side is the oscillating power supply for the charging and discharging of the LC circuit. If the opening distance between the contacts is not large enough, the two voltages will be superimposed and cause breakdown between the arc gaps, and the recovery voltage of the circuit breaker will increase. If the opening distance between the contacts is not large enough, a second reignition will occur, followed by arc extinguishing and reignition, resulting in multiple reignition phenomena. With multiple charging and discharging oscillations, the recovery voltage between the contacts increases step by step, and the voltage at the load end also increases continuously, resulting in multiple reignition overvoltages and damage to electrical equipment. (3) Three-phase interruption overvoltage. Three-phase interruption overvoltage occurs when the arc gap of the first interrupted phase of the circuit breaker causes the high-frequency current flowing through the arc gap of that phase to cause the power frequency current in the arc gaps of the other two phases to rapidly cross zero, causing the uninterrupted phase to be interrupted. A similar large-level current interception phenomenon is generated in the arc gaps of the other two phases, resulting in a higher operating overvoltage. The generated overvoltage is applied to the insulation between phases. Three-phase interruption overvoltage is prone to occur when interrupting small-capacity motors or light loads. 2. Measures Taken Due to the large inductance of the motor windings, as well as the presence of inter-turn capacitance, ground capacitance, and stray capacitance, it is equivalent to an LC oscillating circuit. Based on the mechanism of overvoltage generation in vacuum circuit breakers, when a small current is interrupted, an overvoltage is generated, damaging the motor insulation and electrical equipment within the circuit. Therefore, measures must be taken to limit operational overvoltages to protect the safe and reliable operation of electrical equipment and expand the application range of vacuum circuit breakers. Currently, the measures taken in China include installing metal oxide surge arresters (MOAs), trident overvoltage protectors (TBPs), and combined overvoltage protectors (JPBs). All three types of equipment use zinc oxide varistors as the main components. The main technical parameters of the protected equipment are shown in Table 1. Main Technical Parameters of Protection Equipment (Unit: kV) Protection Equipment Parameters MOA TBP JPB System Rated Voltage 6 10 6 10 6 10 Motor Rated Voltage 6 10 6 10 6 10 Generator Rated Voltage 6.3 10.5 6.3 10.5 6.3 10.5 Protector Rated Voltage 7.6 12.7 7.6 12.7 7.6 12.7 Power Frequency Discharge Voltage > 11.0 17.8 16.0 26.0 DC Reference Voltage > 11.3 18.9 10.0 16.5 10.0 17.0 Impulse Discharge Voltage < 15.0 25.0 15.6 25.2 14.0 25.0 Lightning Impulse Discharge Voltage < 19.0 31.0 18.0 28.8 17.0 29.0 Residual voltage of switching surge 15.0 25.0 15.0 24.8 14.0 24.0 According to the insulation matching regulations, the withstand voltage level should at least exceed the protection level by 15%. However, in 10kV and below systems, there is no grounding or grounding via an arc suppression coil. Furthermore, when a single-phase ground fault occurs, the voltage of the healthy phase rises to the line voltage, and operation is allowed for 2 hours. Under these conditions, the surge arrester will severely overheat and be damaged. Based on the calculated motor test voltage and the protection levels listed in Table 1, the MOA surge arrester provides the worst protection for the motor. Although TBP and JPB are better than MOA, their margin is too small, and their protection performance is still not ideal. Therefore, when the vacuum circuit breaker generates switching overvoltage, they cannot effectively protect the motor. Currently, some manufacturers have developed and produced new products, RC absorbers, designed to limit the switching overvoltage of vacuum circuit breakers from endangering motor insulation. These absorbers can reduce the switching overvoltage of most circuits to below 2 to 2.5 times the peak power supply voltage. Currently, there are three types of RC protectors: ordinary RC protectors with a directly grounded neutral point; RC protectors with an ungrounded neutral point; and dual-circuit RC overvoltage protectors. Ordinary RC protectors suffer from the problem of excessive capacitive current during a single-phase short circuit, causing the entire feeder circuit to trip, especially in locations with high-frequency components, leading to resistor burnout in the RC protector. While ungrounded RC protectors solve the problems of tripping due to excessive capacitive current and resistor burnout, they do not eliminate high-frequency oscillations between phase and ground, resulting in a slightly higher accident rate. Dual-circuit RC overvoltage protectors solve both high-frequency oscillations in the circuit to ground and the problems of excessive ground current and RC device resistor burnout. 3. Issues to Note Currently, most vacuum circuit breakers produced are ordinary distribution type vacuum circuit breakers, which are widely used by many units. However, installing ordinary distribution type vacuum circuit breakers in generator circuits still has some disadvantages and deficiencies: ① As the operating time of the generator increases, its insulation level gradually decreases, and the operating overvoltage of the vacuum circuit breaker and the insulation level of the motor have almost no margin; ② The technical performance requirements of the generator circuit breaker are relatively strict, and the operating conditions are harsh. It is required that the DC component interruption value of the generator circuit breaker is greater than 60% or 80% of the rated breaking current, which is difficult for ordinary distribution type vacuum circuit breakers to achieve; ③ Due to the capacitance of the generator itself (hydro turbine generators are larger than steam turbine generators), plus the capacitance generated by the longer leads and branch lines, if an RC overvoltage protector is used, the capacitance of the protector should also be added, resulting in a large capacitive current in the event of a single-phase ground fault. This can cause unnecessary tripping or the addition of equipment at the neutral point (such as arc suppression coils, grounding resistors, etc.), thus complicating the power outage protection.