The Hazard of the Vacuum Circuit Breaker's Operating Overvoltage and Its Countermeasures

1 Hazards to the motor caused by the overvoltage of the vacuum circuit breaker. There are inductance and capacitance on both sides of the vacuum circuit breaker. The inductance is the equivalent inductance of the motor and the equivalent inductance of the circuit and the transformer; the capacitance is the line to ground and phase to phase. The equivalent capacitance, the equivalent capacitance of the motor, etc. When the vacuum circuit breaker disconnects the motor circuit, there are three hazards, such as over-voltage cut-off, multiple over-combustion over-voltage, and three-phase simultaneous over-current shutdown.

1.1 Cut-off over-voltage Since the vacuum circuit breaker has good arc extinguishing performance, vacuum arc will extinguish before zero-crossing when the small current is cut off. As the current is suddenly cut off, the energy that is retained in the inductance windings of the motor and the like will inevitably be charged to the stray capacitance of the winding and converted into electric field energy. For motors and transformers, especially when they are unloaded or have a small capacity, they are equivalent to a large inductor, and the loop capacitance is small. Therefore, large overvoltages are generated, especially when the no-load transformer is disconnected. There is considerable inhibition from the values, but this inhibition is limited and does not eliminate overvoltages that occur when switching off small currents. Therefore, overvoltage protection devices should be installed especially when the vacuum circuit breaker is used as an operating element for inductive loads.

1.2 Over-burn over-voltage multiple times Over-re-ignition over-voltage is caused by multiple re-ignitions in the arc gap and the power supply is charged to the motor capacitor several times. In the process of cutting off the current in the vacuum circuit breaker, one side of the contact is an industrial frequency power supply, and the other side is an oscillating power supply for charging and discharging of the LC circuit. If the distance between the contacts is not large enough, the breakdown of arc gaps will occur after the two voltages are superimposed. The recovery voltage of the circuit breaker will increase, and a second reignition will occur, and the arc will be extinguished again. Rekindled that many reburning occurred. Many times of charge and discharge oscillation, the recovery voltage between the contacts increases step by step, and the voltage at the load end also rises continuously, resulting in multiple reburning overvoltages and damaging the electrical equipment. The experiment proves that the damage of the insulation between the motor turns is mainly caused by the voltage rise caused by repeated reignition of the vacuum circuit breaker, especially when the starting current of the motor is cut off.

1.3 Three-phase simultaneous disconnection of overvoltages Three-phase simultaneous disconnection of overvoltages is due to the fact that the circuit breaker first opens the phase-gap and re-ignites, the high-frequency current flowing through the phase-arc gap causes the remaining two-phase arc gaps. The power frequency current quickly crosses zero, resulting in the unbroken cut-off followed by a similarly large level of interception in the other two-phase arc gaps, resulting in a higher operating overvoltage.

The resulting overvoltage is added to the insulation between the phases. In the disconnection, under the condition of a small-capacity motor or a light load, three-phase simultaneous disconnection of overvoltage is prone to occur.

2 Measures to be taken in the application of vacuum circuit breakers in motor circuits The presence of large inductances in the motor windings and the presence of inter-turn capacitances, ground capacitances and stray capacitances of the windings correspond to an LC tank. According to the principle of the vacuum circuit breaker operating over-voltage, when the small current is cut off, it is easy to produce over voltage hazard to the motor insulation and circuit electrical equipment. Therefore, measures must be taken to limit the operating over-voltage in order to protect the electrical equipment from safe and reliable operation. At the same time, it expands the scope of application of vacuum circuit breaker At present, domestic measures include the installation of metal oxide surge arrestors (MOA), trigeminal overvoltage protectors (TBP), and combined overvoltage protectors (jPB). The above three types of equipment use zinc oxide valves as the main components. The main technical parameters of each protective device are shown in Table 1. Table 1 Main protective parameters of each protective device Parameter Protection Device System Rated Voltage Motor Rated Voltage Generator Rated Voltage Protector Rated Voltage Industrial Frequency Discharge Voltage> DC Voltage> Impulse Discharge Voltage Present, Some manufacturers have developed and produced a new RC resistance absorber designed to limit the operating voltage of vacuum circuit breakers and endanger the insulation of the motor. It can reduce the operating voltage of most circuits to 2 to 2.5 times the peak value of the power supply voltage. . At present, there are three types of common type RC protectors, neutral ungrounded RC protectors, and dual RC overvoltage protection. The overcurrent of the general type RC protector leads to a full trip of the feeder circuit, especially for places with high frequency components. Causes the RC protector resistor to burn out. The ungrounded RC protector solves the problem of tripping and burning the resistor due to excessive capacitor current, but it does not eliminate the high-frequency oscillation between the ground, resulting in a high accident rate; dual RC The over-voltage protector not only solves the high-frequency oscillation in the ground circuit, but also solves the problems of excessive ground current and resistance burning of the RC device.

But no matter what kind of RC protector, when it is applied in the ungrounded system, according to the regulations, the load current can run for 2h when the capacitor current is less than 34A. The capacitor in the RC loop will undoubtedly increase the capacitance current of the loop. Exceeding or approaching the regulatory value may require the installation of arc suppression or grounding resistance, adding equipment and investment. Therefore, it should be properly analyzed and selected.

According to the information of various manufacturers, the RC device has a capacitance of 0.1 垆 and a resistance of 100fi, and its capacitance reactance is 《 = 2tc/*. Its capacitance current is in the loop of 1OkV: from the above calculations, it can be seen that the capacitance of the RC device is It will be between 0.20.32A. If 510 RC units are connected to one bus, plus the capacitive current of the motor circuit, it may exceed the allowable value of the regulations. Then, arc suppression coils or resistors must be installed at the neutral point of the motor to protect the safe operation of the equipment. Therefore, when selecting the equipment in the motor circuit, especially in the generator circuit, not only the capacitance current of the motor circuit should be considered, but also the capacitance of the branch circuit to the earth and the capacitance of the RC device used to protect the vacuum circuit breaker. Problems are often overlooked by designers, manufacturers, and operations managers.

3 Some problems that should be paid attention to when using vacuum circuit breakers in generator circuit The vacuum circuit breakers currently produced are mostly ordinary power distribution vacuum circuit breakers. Many units have been widely used in small and medium-sized hydropower units, motor circuits and small-sized enterprise units. . The user also feels the advantages of simpler, more convenient, less maintenance work, smaller size, quicker installation and replacement than using less oil circuit breakers, and also considers installation of overvoltage protection devices. Even so, there are still some problems in installing the ordinary power distribution vacuum circuit breaker in the generator circuit. First, the generator gradually decreases with the running time; the vacuum circuit breaker operates over voltage and the motor is insulated. There is almost no margin for horizontal fit. Secondly, the technical performance requirements of generator circuit breakers are relatively strict and the conditions of use are severe. Such as cutting off the DC component standard requirements: Generator circuit breaker cut off the DC component value is greater than 60% or 80% of the rated breaking current. Normal distribution type vacuum circuit breakers are difficult to achieve. Finally, due to the electric capacity of the generator itself (turbine generator is greater than the turbine generator), plus the capacitance generated by the longer lead and branch lines, if an RC overvoltage protector is used, when considering the capacity, Also add the protector's capacity. It should be avoided when there is a large single-phase capacitor current. Cause unnecessary trips or add equipment (such as arc suppression coils, grounding resistance, etc.) at the neutral point, which can cause complications of power-off protection.

In the preliminary design stage of the project. In order to select the right equipment, it is necessary to make a preliminary estimate of the generator's capacitive current. There are a number of different formulae for calculating the value of the generator capacitance, and some of them require the use of electromagnetic parameters. In the preliminary design, a relatively simple calculation formula of GE company in the United States can be used: After obtaining the capacitance of the generator, the capacitor current of the motor can be obtained according to the rated phase voltage of the generator: Icv.=(CfU>l'K10' 6 The generator rated phase voltage V can be used to determine whether the generator circuit adopts a vacuum circuit breaker by calculating the capacitor current of the generator circuit and other conditions.If a vacuum circuit breaker is used, what kind of measures to limit the operation of the overvoltage and determine Generator neutral grounding method.

4 Conclusion Through the analysis of the causes of the vacuum circuit breaker operating over-voltage and the calculation of the technical parameters of the currently produced protective equipment, it is pointed out that when a vacuum circuit breaker is installed in the motor circuit, there must be perfect protection equipment to limit the vacuum circuit breaker. The overvoltage of the operation protects the main device. In this way, the scope of use of vacuum circuit breakers can be continuously expanded and the power system can be operated more safely, reliably, and economically. Especially in the use of vacuum circuit breakers in the generator circuit, we must be more cautious, not blind. In addition to perfect protection measures, it is necessary to consider factors such as the insulation level, the capacitor current of the generator circuit, and the requirement to cut off the DC component, so that the excellent performance of the vacuum circuit breaker can be fully realized.

Starch Machine

Starch Machine,Cassava Starch Machine,Starch Making Machine,Potato Starch Machine

Shuangfeng Nongjiabao Machinery Technology Co., Ltd , https://www.starchmakingmachine.com