Through the analysis and calculation of the mechanism of operating overvoltage of vacuum circuit breaker and the technical parameters of protection equipment produced in China at present, the harm of chopping overvoltage of vacuum circuit breaker to motor and the measures taken, as well as the influence on motor after installing overvoltage protector, especially when applying vacuum circuit breaker in motor circuit, we should pay attention to other problems besides perfect protection measures in order to give full play to the excellent performance of vacuum circuit breaker.
[Keywords:] Vacuum circuit breaker operating overvoltage motor circuit harm countermeasures
In recent years, vacuum circuit breakers have been widely used in power systems, and some problems arising from them have also attracted people's attention. Because the vacuum circuit breaker will produce operating overvoltage when it is opened, rekindled or three-phase disconnected, its operating overvoltage amplitude can cause insulation breakdown of motor and other equipment and flashover of interphase conductors, which will expand the accident and cause undue losses. People gradually realized the seriousness of this hazard, so they developed various devices to limit the operating overvoltage of vacuum circuit breakers, such as metal oxide arrester, resistance-capacitance absorber, combined overvoltage protector and other products. However, due to improper selection or inapplicability of the technical performance of the protected equipment, the protected equipment is not considered.
1, the harm of operating overvoltage of vacuum circuit breaker to motor.
There are inductors and capacitors on the front and back sides of the vacuum circuit breaker. Inductance is the equivalent inductance of motor, conductor and transformer. Capacitance is equivalent capacitance of conductor to ground and phase, equivalent capacitance of motor, etc. When the vacuum circuit breaker opens the motor circuit, there are three hazards: opening overvoltage, multiple reignition overvoltage and three-phase simultaneous opening overvoltage.
Interception overvoltage of 1 and 1
Because the vacuum circuit breaker has good arc extinguishing performance, when a small current is interrupted, the vacuum arc will be extinguished before the zero crossing point. Because the current is suddenly cut off, the energy trapped in the inductance winding of the motor will inevitably charge the stray capacitance of the winding and convert it into electric field energy. For motors and transformers, especially when there is no load or the capacity is very small, it is equivalent to a large inductance, and the loop capacitance is very small, so it will produce a large overvoltage, especially when the no-load transformer is turned off. Theoretically, high overvoltage can be generated, but there are certain resistances in contacts and circuits that cause loss and breakdown, which has a considerable inhibitory effect on overvoltage, but this inhibitory effect is limited and cannot eliminate overvoltage when cutting off small current. Therefore, especially for inductive loads, when vacuum circuit breakers are used as operating elements, overvoltage protection devices should be installed.
1 2 multiple reignition overvoltage
Repeated reburning overvoltage is caused by repeated reburning of arc gap and repeated charging of motor capacitor by power supply. When the vacuum circuit breaker cuts off the current, one side of the contact is the power frequency power supply and the other side is the oscillating power supply, which is used for charging and discharging the LC circuit. If the opening distance between the contacts is not large enough, the two voltages will be superimposed to cause the breakdown between the arc gaps, and the recovery voltage of the circuit breaker will rise. If the distance between contacts is not large enough, secondary reignition, arc extinguishing and reignition will occur, resulting in multiple reignition phenomena, multiple charging and discharging oscillations, the recovery voltage between contacts will rise step by step, and the voltage at the load end will also rise continuously, resulting in multiple reignition overvoltage and damage to electrical equipment. Experiments show that the damage of motor turn-to-turn insulation is mainly caused by the gradual increase of voltage caused by repeated reburning of vacuum circuit breaker, especially when the starting current of motor is cut off, overvoltage is easy to occur.
1 3 three-phase simultaneous operation overvoltage
Three-phase simultaneous breaking overvoltage is due to the fact that when the circuit breaker first breaks the phase arc gap to produce reignition, the high-frequency current flowing through the phase arc gap makes the power frequency current in the other two-phase arc gaps quickly cross zero, so that the unbroken phase is cut off, which leads to the similar degree of current closure in the other two-phase arc gaps, resulting in higher operating overvoltage, which is added to the interphase insulation. In the case of turning off small and medium-sized motors or light loads, it is easy to have three-phase simultaneous turn-off voltage.
2, vacuum circuit breaker in the application of motor circuit measures should be taken.
Because there is a large inductance in the motor winding, there are turn-to-turn capacitance, grounding capacitance and stray capacitance in the winding, which is equivalent to an LC oscillation loop. According to the mechanism of operating overvoltage of vacuum circuit breaker, it is easy to produce overvoltage when breaking small current, so measures must be taken to limit the operating overvoltage to protect the safe and reliable operation of electrical equipment and expand the application scope of vacuum circuit breaker. At present, domestic measures include installing metal oxide arrester (MOA), trident overvoltage protector (TBP) and combined overvoltage protector (JPB). The above three devices all use zinc oxide valve plates as main components, and the main technical parameters of each protection device are shown in table 1.
Where k is the impact coefficient, and K= 1. 15.
For 6kV motors and 6.3kV generators, Us= 15.9~ 16.6(kV).
For 10kV motor and 10.5kV generator, Us=25.6~26.8(kV).
Test voltage when the motor is running: us ′ =1.5 UE.
For 6kV motor, us ′ = 9kV (effective value), and the impact value us ″ =12.7kV..
For 10kV motor, us ′ =15kv (effective value), and the impact value us ″ = 21.2kv.
According to the requirements of insulation coordination regulations, the minimum withstand voltage level should exceed the protection level of 15%. At the same time, the arrester will be seriously damaged by overheating if it is not grounded or grounded without arc suppression coil in the system below 10kV, and when single-phase grounding occurs, the acoustic phase voltage will rise to the line voltage, and it is allowed to run for 2 hours. Judging from the calculated test voltage of the motor and the protection level listed in the table, MOA arrester has the worst protection level for the motor. Although TBP and JPB are better than MOA, the gap is too small and the protection performance is still not ideal. Therefore, when the vacuum circuit breaker generates operating overvoltage, the motor cannot be well protected.
At present, some manufacturers have developed and produced a new product RC resistance-capacitance absorber to limit the operating overvoltage of vacuum circuit breakers from endangering motor insulation, which can reduce the operating overvoltage of most circuits to less than 2~2.5 times of the peak voltage of power supply. At present, there are three forms of RC protectors, namely, ordinary RC protectors with neutral point directly grounded; Neutral point ungrounded RC protector; Bidirectional RC overvoltage protection. The common problems of RC protectors are that when the single-phase short circuit occurs, the capacitor current is too large, which leads to the tripping of all feed circuits, especially where there are high-frequency components, which makes the resistance of RC protectors burn out; Although the ungrounded RC protector solves the problem of tripping and burning resistance due to excessive capacitance current, it does not eliminate the high-frequency oscillation between the two sides, which makes the accident rate slightly higher. The bidirectional RC overvoltage protector not only solves the high-frequency oscillation in the grounding circuit, but also solves the problems of excessive grounding current and burning resistance of R-C devices.
However, no matter what kind of RC protector is used in ungrounded system, it can run with load for 2 hours when the capacitance current is not more than 3~4A. The capacitance in RC circuit will undoubtedly increase the capacitance current of the circuit. If it exceeds or is close to the specified value in the code, it may be necessary to install arc suppression coil or grounding resistance, which will increase the equipment and investment, and should be correctly analyzed and selected.
According to the data of various manufacturers, the capacitance of RC device is 0. 1μF, the resistance is 100ω, and its capacitive reactance is xc =/kloc//ω c, ω= 2πfn. Its capacitance current in 10kV circuit is:
Ic=Ue/Xc=Ue2πfnC
= 10×2×3. 14×50×0. 1
=0.32(A)
In a 6kV circuit, the capacitance current is:
IC = 6×2×3. 14×50×0. 1 = 0.2(A)
According to the above calculation, the capacitance current of each RC device will reach 0.2~0.32A If 5~ 10 RC devices are connected to a bus, the capacitance current of the motor circuit may exceed the allowable value stipulated in the regulations, so arc suppression coils or resistors must be installed at the neutral point of the motor to protect the safe operation of the devices. Therefore, when selecting equipment in the motor circuit, especially in the generator circuit, not only the capacitance current of the motor circuit, but also the grounding capacitance of the branch circuit and the capacitance current of the RC device used to protect the vacuum circuit breaker should be considered, which are often ignored by designers, manufacturers and operation managers.
3. Several problems that should be paid attention to in the application of vacuum circuit breaker in generator circuit.
At present, most of the vacuum circuit breakers produced are ordinary distribution vacuum circuit breakers, which have been widely used in some small and medium-sized hydropower units, motor circuits and small units in enterprises. Users also feel that it is simpler and more convenient than the oil-saving circuit breaker, with less maintenance workload, smaller volume and faster installation and replacement, and consider installing an overvoltage protection device. Even so, there are still some shortcomings and deficiencies in installing ordinary distribution vacuum circuit breakers in the generator circuit. ① With the extension of running time, the insulation level of generator decreases gradually, and the operating overvoltage of vacuum circuit breaker is not much different from the insulation level of motor. (2) The technical performance requirements of generator circuit breakers are strict, and the use conditions are harsh. For example, the standard of cutting off DC shunt requires the generator circuit breaker to cut off the rated breaking current with DC component value greater than 60% or 80%, which is difficult for ordinary distribution vacuum circuit breakers to achieve; (3) Due to the capacitance of the generator itself (the hydro-generator is larger than the turbo-generator) and the capacitance generated by the longer outgoing lines and branch lines, if the RC overvoltage protector is used, the capacitance of the protector should be increased, so that when the single-phase grounding occurs, the capacitance current is large, which will cause unnecessary tripping or increase neutral point equipment (such as arc suppression coil and grounding resistance), which will complicate the power-off protection.
In the preliminary design stage of the project, an important task is the selection of equipment. In order to select suitable equipment, it is necessary to make a preliminary estimation of the capacitor current of the generator. There are many formulas for calculating the generator capacitance, and some related parameters that need electromagnetic calculation are limited in the preliminary design, so the relatively simple calculation formula of GE Company in the United States can be adopted:
Cf=3KdSn/ √Un( 1+0.08Un)
Wherein, the Kd of the damped salient-pole motor is 0.0317; Sn is the generator capacity; Un is the rated voltage of the generator.
After the generator capacitance is obtained, the capacitor current of the motor can be obtained according to the rated phase voltage Ux of the generator: Icr=ωCfUx× 10-6.
Where: Ux is the rated phase voltage (V) of the generator.
Through the calculation of generator circuit capacitance current and other conditions, it can be determined whether the generator circuit adopts vacuum circuit breaker, and if so, what measures are taken to limit the operating overvoltage and determine the generator neutral grounding mode.
4. Conclusion
Through the analysis and calculation of the operating overvoltage mechanism of vacuum circuit breaker and the technical parameters of protection equipment produced in China at present, it is pointed out that when installing vacuum circuit breaker in motor circuit, there must be perfect protection equipment to limit the operating overvoltage of vacuum circuit breaker and better protect the main equipment, so as to continuously expand the application scope of vacuum circuit breaker and make the power system run safely, reliably and economically. Especially when using vacuum circuit breaker in generator circuit, we should be more careful and not use it blindly. In addition to the perfect protection measures, the factors such as the insulation level coordination, the capacitance current of generator circuit and the requirement of removing DC component should also be considered in order to give full play to the excellent performance of vacuum circuit breaker.
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