Mechanism Analysis And Protective Countermeasures Analysis Of 10kV Overhead Insulated Conductor Disconnection Caused By Lightning Strike

【Abstract】Lightning disconnection accidents occur very frequently in overhead insulated conductors of 10kV distribution network, which has greatly restricted the safety and stability of power supply of distribution lines. This article starts from the actual situation of a 10kV distribution line in Zhanjiang, Guangdong, and analyzes in detail the causes of lightning strike disconnection accidents of overhead insulated conductors. At the same time, it proposes some feasible protective measures and means for related issues.

【Key words】10kV overhead insulated wire; lightning strike disconnection; protective measures

introduction

During the transformation of power grid lines in many areas of our country, most distribution lines have been transformed into overhead insulated lines. As the degree of insulation of overhead lines continues to deepen, the problem of lightning strikes on insulated conductors has become increasingly complex. Therefore, how to safely and effectively deal with the problem of lightning conductor disconnection has become a difficult and key issue that must be solved for the overhead insulated lines of the distribution network.

1 Mechanism analysis of lightning strike breakage of insulated wires

1.1 Analysis of external causes

Judging from the situation in Zhanjiang, Guangdong, lightning activities are frequent in this area, and distribution lines are very vulnerable to lightning attacks. Most of the 10kV overhead lines in this area use an operation mode in which the neutral point is not directly grounded. It can be found that the grounding resistance of some towers seriously exceeds the standard. Moreover, many distribution lines are not equipped with line-type lightning arresters, but only use some similar ones. XP-7 type suspension insulator device, and the insulation level of this device is relatively low. In areas with frequent lightning strikes, it is very easy for lightning strikes to occur on wires. In addition, the soil resistivity in this area is very small. Under normal circumstances, areas with very small soil resistivity are also vulnerable to lightning strikes. This is mainly because the principle of electrostatic induction is at work. During the early discharge process of lightning, the induced current in the ground By transmitting in the direction of smaller resistivity, the area with very small resistivity on the ground is induced, and a lot of charges opposite to the thundercloud are gathered, so the lightning will continue to tend towards the area with very small resistivity. In addition, the overall structure of the lines in this area tends to be compact, and the level of resistance to lightning strikes is very low. Distribution lines are very prone to disconnection accidents of insulated conductors after being struck by lightning. Finally, another point is that the area is poorly prepared for lightning protection methods and measures, and few lightning protection measures are taken. Therefore, when distribution lines are struck by lightning, it is very easy for insulated wires to break down due to lightning strikes.

1.2 Internal mechanism analysis

If a direct lightning strikes a bare conductor and the insulator flashovers, due to the influence of electromagnetic force, the continuous power frequency short-circuit current will move rapidly in the direction of the conductor back to the power source under the action of electromagnetic force until it reaches the Protective action, cutting off the arc. The arc root of the arc moves on the conductor. When the arc root continues to move forward, it gradually floats in the air due to the effect of thermal stress. According to the temperature distribution characteristics of the arc, if the temperature of the arc root reaches the highest, then for the conductor The degree of burning loss is also the most serious. If the temperature of the arc nodule is low, there will be no damage to the conductor. This is because the arc root of the arc moves in the direction of the wire, so it usually does not burn the wire and rarely breaks the wire. If direct lightning strikes an insulated wire, the effect will be completely different. Because the amplitude of lightning is too high, it is easy for the insulation layer and insulator of the wire to flashover at the same time. Of course, although the current at this time is very strong, because the time is too short, it is generally difficult to burn out the wire. However, when lightning overvoltage flashover occurs, especially between two phases or three phases, a short-circuit channel is formed. As a result, the ampere frequency continues to flow, and the arc energy will also increase significantly. Because the insulation layer of the overhead insulated wire hinders the movement of the arc to a certain extent, the arc root with a higher temperature is fixed at the breakdown point of the insulation layer and burns. Even if the relay tripping time is adjusted to a minimum value at this time, the wire will Burned by extremely high short-circuit current.

2 Corresponding protection measures for insulated wires broken by lightning strikes

In order to better prevent 10kV overhead insulated conductors from being disconnected by lightning strikes, the main lightning protection measures currently used include installing puncture-type arc-proof fittings, installing discharge clamp post-type composite insulators, and installing overhead ground wires, etc. some measures and means.

2.1 Install puncture-type arc-proof fittings

Usually arc-proof fittings use their own functions to achieve the goal of protecting insulated wires from arc burns and disconnections. Under normal circumstances, arc-proof fittings are installed around 160-200m away from the center of the insulator. The purpose is to allow lightning overvoltage to determine flashover and continuous power frequency short circuit between the arc-proof fittings and the insulator steel feet. The arc root of the current arc burns on the arc-proof fittings, thereby preventing the insulated wires from being burned. Generally, arc-proof fittings are composed of high-voltage electrodes, low-voltage electrodes and insulation covers. See Figure 2 for details. The high-voltage electrode and the insulated wire are in contact with each other to induce high potential. The low-voltage electrode is set at the bottom of the insulator. The high-voltage and low-voltage electrodes form a gap. G1 is the lightning discharge gap. The dry arc discharge distance of the insulator must be greater than the gap distance to allow lightning surge discharge to occur. on this gap. G2 can not only provide a gap for power frequency arc burning. After lightning impact discharge, the power frequency arc arc root directly moves from G1 to G2 gap with the assistance of electromagnetic force for burning. Its purpose is to prevent the high-voltage electrode body from being burned by arc. . In addition, the safety and stability of the discharge voltage in the G1 gap can be maintained. Insulating covers usually cover high-voltage electrodes, which can provide insulation and allow space for the movement of the power frequency arc root. The arc metal tool has a safe and reliable lightning discharge voltage, which can determine the direction of lightning impact discharge and ensure that lightning appears within the lightning discharge gap. At the same time, it uses the mutual contact between the same conductors to have excellent thermal stability and can also Resistant to arc burn. In addition, arc-proof fittings do not need to separate the outer insulation layer, and the prevention effect on lightning breakage is very significant. The method of arc-proof fittings is very simple, has low investment cost, is easy to install, has good safety performance, and can effectively prevent lightning strikes from disconnection.

2.2 Installation of discharge clamp post composite insulator

Under normal circumstances, discharge clamping post-type composite insulators not only have the function of conventional post-type insulators, but can also clamp the power frequency potential of insulated conductors, determine the lightning impulse discharge direction within the range of high and low voltage electrodes, and guide power frequency arcs. The arc root leaves the conductor and burns on the load side of the high-voltage electrode. This can achieve the effect of preventing the insulated conductor from being broken by lightning strikes. Some of the following distribution lines are suitable for protective measures using discharge clamp post composite insulators. For example, during the transformation of distribution lines, newly erected insulated conductors are located in distribution line areas where lightning strike disconnection accidents often occur. But this protective measure also has certain advantages and disadvantages. It can use circuit breakers to cut off the power frequency continuous current, allowing the lightning overvoltage to establish a flashover between the hardware and the insulator, and fix the power frequency current arc to burn on the hardware to prevent burns on the wires. But it also has certain flaws. The installation and installation workload is relatively large, and the insulation layer must be broken, which to a large extent will lead to water intrusion in the core of the insulated wire, which will lead to electrochemical corrosion and disconnection.

2.3 Installation of overhead ground wires Under normal circumstances, the main function of overhead ground wires is to convert large-amplitude lightning overvoltages into currents and release them using lower tower grounding resistance, thus minimizing lightning overvoltages and ensuring insulation. Wires can be adequately protected. During a lightning strike, only induced overvoltage will occur on the conductor, and it has a very good lightning protection effect. This is generally used on 110kV distribution lines with high insulation levels. However, because the insulation level of the 10kV distribution network is generally low, lightning strikes on overhead ground wires are very susceptible to counterattack flashover, and power frequency freewheeling may still occur and burn out insulated wires. Therefore, overhead ground wires must only be established in places where direct lightning strikes are frequent.

3 Conclusion

This article investigates the actual situation of a distribution line in Zhanjiang, Guangdong, and specifically analyzes the number of lightning strikes on distribution lines in the area and the lightning activity in the direction of the distribution lines. The specific reasons, which specifically indicate that the reasons that hinder the movement of the power frequency freewheeling arc on the wire, are not only the perforation caused by direct lightning strikes on the insulated wire, but also other reasons. Finally, based on the actual specific situation of the distribution lines in the area, some methods to improve the insulation level of the lines are proposed, such as installing discharge clamp post-type composite insulators, installing overhead ground wires and other other measures and means. These protection methods can to a large extent deal with the frequent disconnection faults caused by lightning strikes on distribution lines, and these protection methods and means can also be used as a reference for protecting other distribution lines from lightning strikes.

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