Determining the neutral grounding mode of distribu

  • Detail

Determining the neutral point grounding mode of distribution should be adapted to local conditions

1.1 neutral point ungrounded

neutral point ungrounded mode has always been the most widely used mode of distribution in China. If a single-phase grounding fault occurs in the operation of this grounding method, the current flowing through the fault point is only the capacitive current from the electricity to the ground. When the 35kV and 10kV electricity is limited below 10a, the transient fault with very small grounding current can generally be extinguished automatically. At this time, although the voltage of the non fault phase rises, the system is still symmetrical. Therefore, when the heating conditions of the voltage transformer permit, the lubricant will migrate from the film to the surface, It allows continuous power supply with faults for two hours, which wins time for troubleshooting and relatively improves the reliability of power supply. This grounding method does not need any additional equipment and saves investment. As long as the insulation monitoring device is installed, it can be quickly handled after the single-phase grounding fault is found, so as to avoid the long-term existence of single-phase fault from developing into phase to phase short circuit or multi-point grounding accident

because the neutral point is insulated from the ground in the neutral point ungrounded mode, the energy in the capacitance to the ground cannot be released in the case of arc grounding, which will produce arc grounding overvoltage or resonance overvoltage. In the future, more factories will establish high values, posing a threat to equipment insulation. In addition, under certain conditions, linear resonance or ferromagnetic resonance may be caused due to switching operation or fault, resulting in high resonance overvoltage and voltage transformer breakdown. In case of single-phase grounding fault, two methods are generally adopted. Manual switching and reclosing are used to find grounding, which will cause short-term (reclosing time) power failure of non fault line. At present, the line selection device used in the small current grounding system can not accurately detect the line with grounding fault, and the product needs to be further improved. In addition, although it is stipulated that the single-phase grounding can operate with fault for two hours, when the fault is a disconnection accident, it may cause personal casualties, especially in densely populated areas, which should be paid enough attention

1.2 the neutral point is grounded through the traditional arc suppression coil

the neutral point is grounded through the arc suppression coil, that is, when a single-phase grounding fault occurs in the system, the inductive current generated by the arc suppression coil compensates the single-phase grounding capacitive current, so that the arc can be extinguished automatically when the current through the grounding point is reduced. Neutral point grounding through arc suppression coil and neutral point ungrounded system are collectively referred to as small current grounding system. The traditional arc suppression coil grounding not only has all the advantages of the neutral ungrounded system in technology, but also avoids the hazards that single-phase fault may develop into two-phase or multi-phase fault, resulting in overvoltage, damaging the insulation of electrical equipment and burning the voltage transformer. However, under the historical conditions at that time, due to the lack of support from other technologies, there were also the following shortcomings:

(1) in order to avoid resonance, the traditional arc suppression coil grounding required to operate in the over compensated state. With the development of electricity, if the electrical operation mode changes continuously, the arc suppression coil cannot always operate in the best gear, In addition, the capacitance current can not be monitored in real time, and there is a large error between the estimated capacitance current and the actual situation, which may cause arc overvoltage due to the residual current can not be effectively suppressed

(2) the traditional manual tuning of arc suppression coil needs to exit the arc suppression coil after power failure for a short time, thus losing the continuity of compensation. This method can only be applied to the adjustment of normal operation mode. In case of system failure, there is no time to tune, and at this time, the electric single-phase grounding happens. Because the compensation can not keep up, the residual current is large, it is easy to generate overvoltage, damage the insulation of electrical equipment, and cause the expansion of the accident

(3) in the traditional arc suppression coil grounding mode, when single-phase grounding occurs, the residual current varies greatly due to different compensation methods, so the microcomputer line selection device used in the grounding circuit is difficult to work, which greatly reduces the accuracy of line selection. Therefore, it is difficult for the microcomputer line selection device to work with the traditional arc suppression coil

1.3 neutral point is grounded through resistance

neutral point is grounded through resistance, that is, a certain resistance is connected between the neutral point and the earth. This method can be considered as a grounding method between neutral point ungrounded and 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 resistance value, it can be divided into high, medium and low resistance grounding. Compared with neutral grounding through resistance or neutral grounding without arc suppression coil, intermittent arc grounding overvoltage and resonance overvoltage can be effectively prevented. Neutral point is grounded through resistance. In case of single-phase grounding of the system, the zero sequence protection action is started through the current flowing through the grounding resistance to cut off the fault line from the system. In a broad sense, neutral grounding through resistance is also a large current grounding, so it has the advantages and disadvantages of large current grounding. The selection of grounding resistance, i.e. the range within which the current value of the grounding point should be controlled, is recommended to be 100 ~ 200A in Japan when considering that bromine flame retardant can be replaced for resistance grounding of various high molecular materials, while it is considered to be about 500A in the United States when considering low resistance grounding

in addition to the above mentioned advantages of resistance grounding of neutral point, it can effectively prevent overvoltage. In addition, in case of single-phase grounding of the system, the voltage rise of sound phase is small or does not rise, which will not be like the voltage of neutral point ungrounded system. It is expected that the factory will officially operate in early March next year and increase to line voltage. Therefore, the insulation grade of electrical equipment can be selected according to phase voltage; When single-phase grounding occurs in the system, the fault current is large, the zero sequence protection acts, and it is easy to cut off the fault line. Its 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 increases greatly, and the reliability of power supply decreases; In case of single-phase fault, the grounding current is large. When the zero sequence protection action fails, the electrical equipment near the grounding point will be tested for dynamic and thermal stability, which may lead to damage and develop into phase to phase fault; On the other hand, when the overhead insulated conductor is disconnected and the bare conductor is disconnected and touches the sand, asphalt, concrete and other dry ground, due to the small grounding current, the relay protection does not act, which may lead to serious personal injury and death accidents

Copyright © 2011 JIN SHI