1 Overview Circuit breakers are the most important part of high-voltage electrical appliances and are the key electrical appliances for control and protection in primary power system equipment. The circuit breaker plays two roles in the power grid: First, the control function, that is, according to the needs of the power grid operation, some electrical equipment or lines are put into or out of operation; second, the protection function, that is, when the electrical equipment or power line fails, The relay protection automatic device sends a trip signal, activates the circuit breaker, and quickly removes the faulty part of the equipment or line from the power grid to ensure the normal operation of the fault-free part of the power grid. At present, oil-less circuit breakers, vacuum circuit breakers and sulfur hexafluoride circuit breakers are commonly used in power systems to put in or out of some electrical equipment or line operations. At the same time, they are also used to cut off faulty equipment or lines in the grid. From the perspective of development, vacuum circuit breakers are widely used in substations of city grids of 35kV and below due to their frequent operation, easy maintenance, no fire, no pollution, small size, and on-column vacuum circuit breakers of rural power grids. Load transfer switch and vacuum circuit breaker for cutting electric furnace steelmaking transformers in steel companies. In summary, with the trend of no oiling of electrical equipment, vacuum circuit breakers are increasingly used in power systems. However, things are divided into two. Due to their strong arc-extinguishing performance, the vacuum circuit breaker generates a three-phase simultaneous breaking overvoltage and a high-frequency re-ignition overvoltage when the inductive load is interrupted. When breaking the no-load line and the capacitor bank, due to the re-ignition between the contacts of the vacuum circuit breaker, the line or capacitor obtains energy from the power source and accumulates to form an overvoltage V1W, which limits the wide range of vacuum circuit breakers. application.
2 Solutions and Problems 2.1 Solutions In order to make vacuum circuit breakers more widely used in power systems, it is necessary to effectively limit the operating overvoltage generated by vacuum circuit breakers. At present, RC overvoltage absorbers and arresters are commonly used to absorb the overvoltage of the vacuum circuit breaker.
2.1.1 R-C Overvoltage Absorber RC Overvoltage Absorber as shown.
Parameters: The resistance determines the decay speed is generally 50 ~ 400! . Capacitor C is the snubber capacitor. For induction motor load C is "F; for capacitor bank load, C is 0.05" F is ideal.
Principle: When the vacuum switch operation generates over-voltage high-frequency waves, the over-voltage is 1~10kHz high-frequency wave), and the capacitor impedance in the RC absorber is much lower than the power-frequency voltage. In this way, the over-voltage high-frequency wave is absorbed by the capacitor, and finally the heat energy is consumed in the resistor.
Function: The maximum advantage of the RC absorber can alleviate the steepness of the overvoltage wave head; 1<.> 2.1.2 The arrester of the arrester is mainly used to limit the atmospheric overvoltage due to the limitation of the lower limit of the power frequency discharge voltage. Generally, it is not used to limit the vacuum. Overvoltage generated by circuit breaker operation.
Zinc oxide surge arresters are widely used in vacuum circuit breaker cabinets to limit operating overvoltage due to their unique volt-ampere characteristics. The wiring diagram is as shown.
Star-type zinc oxide arrester 2.2 has problems 2.2.1 Resistor capacity RC) The problem of the absorber RC overvoltage absorber absorbs the vacuum circuit breaker operating overvoltage response speed, and theoretically, as long as the capacitor capacitance value It is large enough to be of the following formula: K transformer % device inductance; C0 device itself equivalent capacitance; C RC absorber capacitance.
It can be known from equation (1) that the operating overvoltage can be limited to the allowable range. Moreover, the operating overvoltage oscillation frequency can be lowered. However, an increase in capacitance increases the fault phase ground current at the time of single phase grounding. Therefore, the RC overvoltage absorber parameters are generally 100 at the resistor! Left and right, the capacitance is around 0.1"F. However, such a set of parameters of the RC overvoltage absorber faces two major problems in absorbing the operating overvoltage: one is the small capacity to absorb the overvoltage energy; the other is the undefined overvoltage limit value. .
The capacity of the RC overvoltage absorber to absorb the overvoltage energy, such as the steelmaking electric furnace of a machine repair factory of Maanshan Iron & Steel Co., Ltd., the electric furnace transformer V, 0% A1.66L, the electric furnace transformer is input and cut by the 35kv vacuum switch. There is a set of 35kV zinc oxide surge arresters and a set of dry R-C overvoltage absorbers between the vacuum switch and the electric furnace transformer to limit the operating overvoltage.
A machine repair 33kV electric furnace transformer primary wiring diagram Capacitor C overvoltage absorption energy is c and other special circumstances, c may be smaller. The 33kV electric furnace transformer capacity is 10000kVA. When the secondary voltage is 100V, the excitation current is 1.7L6. Therefore, the magnetic energy stored in the winding is l. If the transformer runs for a long time, L may be larger. That is to say, the L and C values ​​are relatively close, and in the primary wiring of the 33kV electric furnace transformer in the machine repair shop, the RC dry overvoltage absorber has a fast response speed, so the transformer inductive energy storage is absorbed by the capacitor for a long time, causing the capacitor to be damaged frequently. For example, the dry overvoltage absorber of the machine repair factory has been replaced twice within one year.
RC overvoltage absorber has no clear overvoltage limit value. When the vacuum circuit breaker is used to cut off the inductive load, such as cutting off the electric furnace transformer, it is assumed that the switch cutoff value is 2A, the time is 2ms, and the charging voltage value on the capacitor is: the cutoff value. 3A, the capacitor is charged to the Uc capacitor's rated voltage is only 36kV. When the vacuum circuit breaker is used to cut off the capacitor group and other capacitive loads, due to the re-ignition between the switch contacts, the formation of high peak high frequency overvoltage, and The voltage increases as the number of re-ignitions of the circuit breaker increases, eventually causing damage to the capacitors of the capacitor bank and the capacitors of the RC overvoltage absorber.
In summary, the RC absorber is used to limit the overvoltage. The capacitor voltage varies with the value of the cut-off current, the number of re-ignitions of the vacuum circuit breaker gap, and the waveform change. It is impossible to have a certain voltage limit value. Eventually it leads to damage to the protected equipment.
2.2.2 Star-connected zinc oxide arrester problems Early use of valve arresters to limit operating overvoltage, but with the popularization of zinc oxide arresters. Nowadays, the power system almost does not use a valve arrester to limit the operating overvoltage. Therefore, only the zinc oxide arrester is used to analyze the overvoltage of the vacuum circuit breaker. Take the 35kV zinc oxide arrester as an example. Zinc oxide arrester absorbs overvoltage energy:! z: ", which is a square wave 400A of 2ms," the residual pressure value of 105kV. 000 when 400A flows through the valve plate, and the capacity of the zinc oxide arrester to absorb the overvoltage energy should be sufficient. Therefore, in order to prevent the operating voltage from being generated by the vacuum circuit breaker, a zinc oxide surge arrester is generally attached. However, the operating experience shows that after the zinc oxide surge arrester is used, the high-voltage equipment is insulated and the flashover of the phase conductor still occurs. As in the above example, a machine repair factory has had phase-to-phase flashover between the current transformer and the porcelain bottle in the past two years. The theoretical and inter-phase overvoltages generated by the four-star connection MOA limit of the three series gaps are theoretically analyzed. Because the inductor has more energy storage, the RC overvoltage absorber is generally not used. If the four-series connection MOA of the three series gap has large dispersion and slow response speed, it is recommended to use a RC overvoltage absorber with fast response speed as the backup protector of the four-series connection MOA of the three series gap. Generally, the four-star type connected to the three series gap is connected to the rear side of the MOA, close to the primary side of the transformer, and absorbs the overvoltage and energy of the leakage network.
Measure 2: Limiting the vacuum circuit breaker to break the inductive load. The motor overvoltage vacuum breaker opens and breaks the motor. In addition to the current flowing through the voltage, it also generates three-phase simultaneous breaking overvoltage and high-frequency re-ignition overvoltage. Although the motor inductance is small, the overvoltage energy is not large, but the overvoltage amplitude is high, especially the high frequency re-ignition overvoltage. The insulation level of the motor is one of the lowest in high voltage appliances. It is recommended to use the three series resistance gap four-star connection MOA to limit the relative ground and phase overvoltage, so that the overvoltage protection and motor insulation achieve a good fit. The R?C overvoltage absorber is used as the backup protector of the four-star type connection MOA of the three series resistance gap, which further solves the problem that the former has the discharge dispersibility which makes the protection sensitivity decrease and improves the overvoltage tomb waveform and reduces the arrester residual. Pressure problem. At present, the 6kV high-voltage motor of a large stacker and reclaimer of a raw material plant of Maanshan Iron and Steel Co., Ltd. uses the method of measure 2 to limit the relative ground and phase overvoltage, and the effect is more obvious.
Measure 3: Limit the vacuum circuit breaker to open the capacitive load capacitor bank and the no-load line over-voltage vacuum circuit breaker to open the capacitive load, especially to break the capacitor bank, and easily form re-ignition over-voltage between the circuit breaker contacts, and The voltage increases as the number of re-ignitions of the circuit breaker increases, eventually causing damage to the capacitor. Since the RC overvoltage absorber increases the ground current flowing into the fault phase during single-phase grounding, there is no clear overvoltage limit. Therefore, the RC overvoltage absorber is not used, and only the four-star connection MOA is used to limit the relative ground and phase overvoltage generated by the vacuum breaker breaking capacitive load.
4 Conclusion The breaking of the vacuum circuit breaker not only generates a cut-off overvoltage, but also generates a three-phase simultaneous cut-off overvoltage and a high-frequency re-ignition overvoltage. These overvoltages occur not only in the opposite but also between the phases. In many cases, the phase-to-phase overvoltage is 1.52 times larger than the relative overvoltage; since the no-load transformer has more energy storage, the overvoltage energy generated is also larger.
The four-star connection MOA not only limits the relative ground and phase overvoltage generated by the breaking vacuum circuit breaker but also absorbs the overvoltage of large energy. The four kinds of four-star type MOA are commonly used to unify the contradiction between the safety and protection effects of the ZnO valve. Which wiring method should be selected according to the breaking load of the vacuum circuit breaker.
Use four-star connection MOA as the main protector to limit the operating overvoltage of large energy, and give a clear relative and phase-to-phase overvoltage limit value. Use RC overvoltage absorber with fast response and no dispersion as ZaO. In case of failure, the backup protector absorbs the overvoltage of the leakage net and uses the RC overvoltage absorber to improve the overvoltage waveform and effectively reduce the residual voltage of the arrester.
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