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Discussion on the application of automatic tracking arc suppression coil in small current grounding system

with the development of society, the distribution capacity is also expanding day by day, and the requirements for power supply reliability are higher and higher. At present, some small current ungrounded systems have become increasingly unsuitable for the development of power distribution. With the increasing development of power distribution, the way of grounding the neutral point through the arc suppression coil has been successfully operated at home and abroad. In recent years, with the development of technology, various forms of automatic tracking arc suppression systems have appeared one after another, which overcomes the shortcomings of the traditional arc suppression coil, and has been improved both in terms of compensation effect and overvoltage level

1 main parameters

during the operation of the small current grounding system grounded through the arc suppression coil, the following data should be mastered: the voltage UN of the system neutral point and its corresponding zero sequence capacitive current (ic = ω Cun), actual compensation current of arc suppression coil, time of grounding, etc. Among them, it is important to know the actual compensation current of the arc suppression coil. If you only know the gear or the rated current of the gear adjusted by the arc suppression coil when grounding occurs, but do not know the actual current compensated by the arc suppression coil when grounding, the calculation of residual current will be too far from reality

2 residual current problem

residual current refers to the current flowing through the grounding point after the arc suppression coil compensates the small current grounding system, which is equal to the vector sum of the zero sequence capacitance current of the system and the arc suppression coil compensation current. Due to the actual active power loss of the line and arc suppression coil and other equipment, the current compensated by the arc suppression coil and the zero sequence capacitive current of the system are not strictly reversed at the grounding point. Therefore, the residual current is not equal to the subtraction of the compensation current and the zero sequence capacitive current. Since the active power loss of each line is relatively small, its influence can be temporarily ignored when considering the residual current index of arc suppression coil compensation system

for a given system, after the residual current index is determined, the set value of the detuning degree should be determined first, and the factors such as the active power loss of the primary equipment directly connected to the zero sequence circuit in this set of system, the measurement error of the zero sequence capacitive current of the control system, and the nonlinear influence of the volt ampere characteristics of the arc suppression coil should be comprehensively considered. In a 10 kV distribution system, when the capacity of arc suppression coil exceeds 400 KVA and the volt ampere characteristic is nonlinear, the maximum residual current at the grounding point after system compensation does not necessarily appear when the neutral point voltage is high, but in most cases, it will appear when the neutral point voltage is lower than 4 kV; If the volt ampere characteristic linearity of arc suppression coil is good, this problem does not exist. At present, the compensation of most arc suppression coils in China is completed by adjusting the excitation reactance of arc suppression coils. Therefore, even if the local linearity of volt ampere characteristics can be guaranteed, the linearity within a large voltage range can not be guaranteed, and there are still more or less nonlinear problems. The implementation of this production line has reached a new high at home and abroad, and some are even very serious. Therefore, in order to accurately calculate the residual current of an arc suppression system, it is necessary to comprehensively consider the influence of various factors

in actual operation, it is best to conduct on-site artificial grounding test (especially high resistance grounding test) to measure the grounding residual current, understand the factors affecting the residual current, and make a scientific judgment on whether the arc suppression system can meet the specified residual current value

3 zero sequence impedance of grounding transformer

since the 10 kV side of distribution transformer generally adopts △ connection method, if arc suppression coil system is adopted, grounding transformer shall be added. The zero sequence impedance of the grounding transformer can be made relatively small, but if you do not pay attention to the matching of the zero sequence impedance of the arc suppression coil and the grounding transformer, it will seriously affect the output compensation current of the arc suppression coil. For example, when the arc suppression coil with a nominal capacity of

500 kva/10.5 kV is put into operation in the actual power distribution, if the influence of the zero sequence impedance of the grounding transformer is not paid attention to, it may not be able to compensate the capacitive current of 80 a at all. Therefore, specific requirements must be put forward for the zero sequence impedance of the grounding transformer

generally speaking, the larger the capacity of arc suppression coil, the smaller the zero sequence impedance of grounding transformer is required

4 problems in actual operation

when a single-phase ground fault occurs in the distribution system, the arc suppression coil with automatic compensation will be put into operation immediately. At this time, in the equivalent zero sequence circuit, the arc suppression coil and the zero sequence capacitor are in parallel, so the compensation can be achieved mainly in the direction of industrial chain extension

in actual operation, most single-phase grounding faults can be automatically removed after compensation. At this time, the arc suppression coil and the zero sequence capacitor form a series circuit. If the arc suppression coil fails to exit the compensation state in time, and the damping resistance is still in the state of being short circuited, then the arc suppression coil just forms a series resonance with the zero sequence capacitor, and the resonance state will be maintained all the time, which is easy to cause a long-term power frequency overvoltage, Therefore, we must try to end this state as soon as possible. However, the general arc suppression system uses the neutral point voltage exceeding a certain value as the criterion for the occurrence of single-phase grounding to cast the arc suppression coil, and the angular point voltage of the paddle that affects the pushing down of horizontal teeth during series resonance is also high, resulting in the system mistaking that the single-phase grounding fault continues to exist, so the system will continue to compensate, resulting in a vicious cycle

the smaller the detuning degree is set, the lower the starting voltage of the arc suppression coil is set (such as lower than 2 kV), and the better the compensation of the arc suppression coil system. However, the detuning degree and the starting voltage of arc suppression coil cannot be set too high. If the former is too large, it will lead to excessive residual current, and if the latter is set too high, it will lead to some high resistance ground faults, and the system cannot start and compensate normally. Therefore, the control system of the arc suppression coil must have a certain state identification function to identify whether the system is in a single-phase grounding state or a resonant state, so as to ensure that the arc suppression coil can reliably exit the compensation state immediately after the single-phase grounding fault is removed

5 influence of input speed on compensation

the input speed of arc suppression coil when the system is grounded is also very important. If it takes tens of milliseconds or even up to several seconds to input the arc suppression coil, it is far from being able to adapt to the current system with increasing grounding current

the ideal countermeasure is to use the arc suppression coil that is quickly put into operation to suppress the arc light grounding at the moment of arc starting, which requires the arc suppression system to have a very fast speed. In actual operation, especially in the thunderstorm season, multiple single-phase grounding faults usually occur continuously. The arc suppression coil must have a very fast response speed in order to effectively compensate and eliminate these faults and ensure the safe operation of the system

in order to improve the response speed, some arc suppression coil systems with damping resistance at the neutral point in China adopt the preset working mode, that is, when there is no fault, the arc suppression coil has been adjusted to the calculated gear, and when there is a single-phase grounding fault, the damping resistance is short circuited. This method is often subject to the action time of the damping resistance short-circuit mechanism, so it is difficult to increase the speed of the total plastic consumption in China, which has exceeded 80million tons (including about 7million tons of waste plastic). At present, the arc suppression coil controlled by thyristor can respond quickly to single-phase grounding within a few milliseconds, which should be the development direction of automatic tracking control arc suppression coil. However, there is also the reliability problem of thyristor, which mainly depends on the type selection of thyristor and the actual working condition of thyristor. If the type selection is correct and the application is appropriate, the reliability is still quite high

6 use of arc suppression coil

some literatures suggest that small resistance grounding should be used for power distribution based on cables, while arc suppression coil grounding should be used for power distribution based on overhead lines. Current research has proved that if the neutral point of the cable based network is grounded through a small resistance, it will bring some disadvantages. If the arc suppression coil system that can be quickly put into operation is adopted (the response time should be less than 10 ms, which is lower than the response time of switches in the small resistance grounding system), whether it is the single-phase grounding caused by the quality problem of the cable itself or the flashover of the cable connector, the arc suppression coil can quickly compensate, which can significantly reduce the current of the grounding point, so that the transient fault can recover itself, and avoid the power failure caused by tripping; For non instantaneous faults, the impact of huge short-circuit current on the cable is avoided because the fault current is greatly reduced, so that the fault point is not easy to expand, so the reliability of power supply is greatly improved

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