Preface In many low-voltage power distribution systems, overhead lines are laid using poles, especially in remote rural areas and sparsely populated areas, to reduce construction costs. However, this overhead wiring method is highly susceptible to lightning strikes in lightning-prone areas. This article analyzes a case of a low-voltage distribution cabinet exploding due to a lightning strike. I. Accident Process and Phenomena The low-voltage distribution room in our unit's dormitory uses a 1500m overhead concrete pole directly from the main distribution room into the distribution room. In a lightning strike accident in June 2002, the main switch and more than ten meters in the distribution room were melted, causing the main switch in the main distribution room to trip. II. Accident Cause Analysis Electrical equipment is mainly affected by direct lightning strikes, induced lightning strikes, lightning surges, and ball lightning. The cause of the failure is always overvoltage, which damages the insulation of the electrical equipment. The lightning surge current generated by lightning overvoltage in the power supply system can reach hundreds of thousands of amperes, and the resulting lightning surge voltage often reaches hundreds of thousands of volts, or even millions of volts, thus causing great damage. Analysis of the accident scene reveals the following causes: (I) The distribution cabinet was installed in an unreasonable location, too close to the lightning conductor and grounding device. When struck by direct lightning, the huge lightning current flowing through the lightning protection device causes a potential rise in the device's potential. This high potential can also act on electrical lines, electrical equipment, or other metal pipes, causing discharge between them. A very high backflash voltage damages the equipment's insulation. When subjected to induced lightning, the strong electric and magnetic field changes of the lightning current generate electrostatic and electromagnetic induction, creating a high potential difference between the distribution cabinet and the metal components of the conductor, causing spark discharge and melting the metal. (II) The grounding resistance did not meet requirements. On-site measurement data shows that the resistance of the entire grounding network exceeded 30Ω. The proper grounding of various lightning protection devices is closely related to the safety of the protected objects. For lightning protection grounding, the allowable grounding resistance should be 5-30Ω. If the grounding resistance is too high, the lightning current cannot be discharged quickly, causing the lightning protection device to generate high voltage to the ground. (III) Lightning arresters should be installed on overhead service lines. Southern China is a rainy and thundery area, and overhead lines in open areas are easily struck by lightning. When an overhead line is struck by lightning, if no lightning arrester is installed at the service line, the powerful lightning current and high voltage will be applied to the power distribution equipment, causing insulation damage and short circuit accidents. III. Improvement methods. Analysis of system faults shows that overvoltage generated during lightning strikes is the main cause of equipment damage. In order to limit the rise of grounding potential and avoid overvoltage when the power distribution equipment is struck by lightning, some preventive and protective measures must be taken for the power distribution system. (1) Adjust the distance between the low-voltage distribution cabinet and the grounding wire of the lightning rod so that the air distance between the distribution cabinet and the ground wire meets the following: SK≥0.2RCH+0.1h Where: Sk is the air distance, m; RCH is the impulse grounding resistance of the independent lightning rod, Ω; h is the height of the lightning rod test point, m. (2) Improve the grounding resistance of the lightning protection wire. 1) Increase the area of the grounding grid, bury the grounding body deeply, and at the same time use chemical resistance reducing agents to artificially improve the ground resistivity. Make the grounding resistance of the system below 4Ω. 2) There shall be no less than two down conductors, and they shall be evenly and symmetrically arranged around the building, with a spacing of no more than 18m. Lightning protection grounding and electrical equipment grounding shall share a grounding device. When they are not shared, the distance between them in the ground shall meet the following requirements, but shall not be less than 2m. Se≥0.3KcRi; Where: se is the distance in the ground, m; Kc is the current shunting coefficient. The value for a single down conductor is 1; for multiple down conductors with two down conductors and a non-closed loop lightning arrester, the value should be 0.66; for multiple down conductors with a closed loop or mesh lightning arrester, the value should be 0.44. When a shared grounding device is connected to a buried metal pipe, the grounding device should be laid around the building in a ring-shaped grounding system. IV. Changing the overhead line entry method to prevent lightning surge intrusion : Before the low-voltage overhead line enters the building, replace it with a section of buried metal-armored or sheathed cable, buried in a steel pipe. The buried length should meet the requirements of the following expression, but the buried cable length should not be less than 15m. The metal sheath and steel pipe of the cable at the service entrance should be connected to the lightning protection grounding device. A surge arrester should also be installed at the connection point between the cable and the overhead line. The surge arrester, cable metal sheath, steel pipe, insulator feet, and hardware should be connected together for grounding, and the impulse grounding resistance should not exceed:
[IMG=Grounding Resistance Formula]/uploadpic/THESIS/2008/1/2008010912051265782A.jpg[/IMG]In the formula: L is the length of the metal-clad or sheathed cable buried in the ground through a steel conduit, in meters; ρ is the soil resistivity at the point where the cable is buried, in Ω·m. V. Conclusion To prevent lightning strikes and ensure the normal operation of power supply and distribution, necessary and economical lightning protection measures should be taken. Appropriate lightning protection devices should be installed on power distribution lines and various power distribution equipment installed on them, according to specific conditions and requirements. Furthermore, attention should be paid to the reasonable wiring of protection devices and the quality of surge arresters; only in this way can lightning strikes be avoided as much as possible. After the modification of the lines and power distribution equipment, the effect was very good.
