Neutral grounding method comparison - Database & Sql Blog Articles

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1. The ungrounded neutral point of the neutral point has always been the most adopted method in China's distribution network. If the single-phase ground fault occurs during operation, the current flowing through the fault point is only the capacitance current of the grid to the ground. When the 35kV, 10kV grid is limited to 10A or less, if the ground current is small, the transient fault can generally be automatic. Extinguished, although the non-fault phase voltage rises, the system is still symmetrical. Therefore, when the voltage transformer heating condition permits, the power supply with fault is allowed to be continuously supplied for two hours, which has won time for troubleshooting and relatively improved power supply. Can be *. This kind of grounding method does not require any additional equipment, and the investment is saved. As long as the insulation monitoring device is installed, it can be quickly processed after the single-phase ground fault is found, and the single-phase fault can be prevented from developing into a phase-to-phase short circuit or a multi-point grounding accident.
Since the neutral point is not grounded, the neutral point is insulated from the ground. When the arc grounding occurs, the energy in the ground capacitance cannot be released, so that the arc grounding overvoltage or the resonant overvoltage is generated, and the value is high. Insulation poses a threat. In addition, under certain conditions, due to the switching operation or failure, linear resonance or ferromagnetic resonance may occur, resulting in a higher resonant overvoltage, resulting in breakdown of the voltage transformer. After a single-phase ground fault occurs, two methods are generally used. Manual pull-off with reclosing to find the ground will cause a short (reclosing time) power failure of the non-faulty line. At present, the line selection device for the small current grounding system we are using cannot accurately and 100% detect the line where the ground fault occurs, and the product needs further improvement and improvement. Also, although it is stipulated that it can operate with fault for two hours when single-phase grounding occurs, when this fault is a disconnection accident, personal injury or death may occur, especially in densely populated areas.
2. The neutral point of the traditional arc suppression coil is grounded by the arc suppression coil. When the single-phase ground fault occurs in the system, the inductor current generated by the arc suppression coil compensates for the single-phase grounding capacitor current. The reduction of the location current can automatically extinguish the arc. Neutral point arc-suppression coil grounding and neutral point ungrounding systems are collectively referred to as small current grounding systems. The traditional arc suppression coil is grounded. Technically, it not only has all the advantages of the neutral point ungrounded system, but also avoids the single phase failure that may develop into two-phase or multi-phase faults, resulting in overvoltage damage to electrical equipment insulation and burnout voltage. Harmonics such as transformers. However, under the historical conditions at the time, due to the lack of support from other technologies, the following shortcomings existed:
(1) The traditional arc-suppression coil is grounded to avoid resonance, and it is required to keep operating under over-compensation. With the development of the power grid, if the grid operation mode changes continuously, the arc-suppression coil cannot always run in the optimal gear position. In addition, the capacitor current cannot be monitored in real time, and the estimated capacitance current is greatly different from the actual one. It is possible that an arc overvoltage is generated due to the residual current not being effectively suppressed.
(2) The traditional arc-suppression coil manual tuning needs to power off the arc-extinguishing coil for a short time and loses the continuity of compensation. This method can only be applied to the normal operation mode adjustment. In case of system failure, it is too late to tune. At this time, it happens that the single-phase grounding of the power grid occurs. Because the compensation cannot keep up, the residual current is large, and it is easy to generate overvoltage, which damages the insulation of electrical equipment and causes accidents to expand.
(3) The traditional arc-suppression coil grounding method, when the single-phase grounding occurs in the power grid, the residual current varies greatly due to different compensation methods. Therefore, the microcomputer line selection device used for the grounding loop is difficult to work, which greatly reduces the accuracy of the line selection. rate. Therefore, the microcomputer line selection device is difficult to work with the conventional arc suppression coil.
3. The neutral point is connected to the neutral point of the resistor through the resistor grounding method, that is, a resistor with a certain resistance value is connected between the neutral point and the earth. This method can be considered as a grounding method between the neutral point ungrounded and the neutral point directly grounded. When the resistance value is infinite, the neutral point is not grounded; when the resistance value is zero, the neutral point is directly grounded. According to the size of the resistance value, it can be divided into high, medium and low resistance grounding. The neutral point can be effectively prevented from intermittent arcing ground overvoltage and resonant overvoltage by grounding the resistor and the neutral point is not grounded or grounded by the arc suppression coil. The neutral point is grounded by a resistor. When the system is single-phase grounded, the zero-sequence protection action is initiated by the current flowing through the grounding resistor, and the faulty line is cut off from the system. Broadly speaking, the neutral point is also grounded by a large current through the resistor grounding, so it has the advantages and disadvantages of large current grounding. The selection of the grounding resistance, that is, the range of the grounding point current value is good. It is recommended to be 100 to 200A when considering the medium resistance grounding method in Japan, and the control is considered to be around 500A when considering the low resistance grounding method in the United States.
The neutral point is grounded by resistance. The advantages of the neutral point can be effectively prevented from overvoltage. In addition, when the system is single-phase grounded, the voltage rise of the sound phase is small or does not rise, not like the neutral point. The grounding system voltage rises to the line voltage, so the insulation level of the electrical equipment can be selected according to the phase voltage; when the system is single-phase grounded, the fault current is large, the zero-sequence protection action, and the faulty line is easily removed. The disadvantage is that when a single-phase fault occurs in the system, whether the fault is permanent or non-permanent, the line trips, the number of line trips is greatly increased, and the power supply can be reduced. When the single-phase fault occurs, the ground current is large. The zero-sequence protection action fails. The electrical equipment near the grounding point is tested by dynamic heat stability, which may lead to damage and develop into phase-to-phase fault. In another case, the overhead insulated conductor is broken, and the bare conductor is in contact with the gravel and asphalt. When the concrete is dry on the ground, the grounding current is small, and the relay protection does not work, which may cause serious personal injury or death.
4. Automatic tracking compensation arc suppression coil The traditional arc suppression coil needs manual tuning, which not only makes the grid lose compensation for a short time, but also can not effectively control the fault current of single phase grounding. Now with the automatic tracking compensation arc suppression coil is different. The automatic tracking compensation arc suppression coil device can adjust the inductance value of the arc suppression coil in time and quickly according to the change of the operation mode of the power grid, so that the degree of detuning is always within the prescribed range. When the system is single-phase grounded, the inductor current of the arc suppression coil can effectively compensate the capacitance current of the grounding point, so that the residual current is minimized, and the intermittent arcing grounding overvoltage is avoided. Thanks to the new technology support of the microcomputer grounding protection or automatic line selection device, the advantages are even more prominent.
The automatic tracking compensation arc suppression coil is roughly divided into the adjustment type, the adjustment air gap type, the adjustment type, the DC bias type, and the thyristor adjustment type. A variety of different automatic tracking compensation arc suppression coils have different characteristics. Tuning type: the adjusting speed is slow. In order to make the residual current smaller, the arc suppression coil works near the resonance point, which will increase the displacement voltage of the neutral point. To limit the displacement voltage of the neutral point, a damping resistor needs to be added; Adjustable air gap type: It can realize automatic tracking stepless continuous adjustment, its shortcoming is vibration and noise; volume adjustment type: fast adjustment speed, no need to add damping resistance; DC bias type and thyristor adjustment type: with speed Fast, small residual flow, low noise and so on.