Abstract: This paper briefly introduces the lightning strike situation of power distribution lines in Shanghai urban area in the past three years. Based on the analysis of the lightning strike breakage accident of 10kV overhead insulated conductors, the causes of the breakage are identified and countermeasures are proposed. Keywords: Insulated conductor, lightning strike, arc-building rate, power frequency follow current 1. Overview 10kV power distribution lines are the longest voltage level lines in the power system and are most closely related to users. Due to well-known reasons, the insulation level of 10kV lines is generally low. Not only will flashover cause tripping when lightning strikes the conductors and tower tops, but flashover will also occur when lightning strikes nearby trees or buildings due to excessively high induced voltage. At present, more and more 10kV power distribution lines in large and medium-sized cities in China are using insulated conductors as overhead power distribution lines, which effectively solves the corridor and safety problems that bare conductors cannot solve. Compared with underground cables, it has the advantages of lower investment and faster construction, but it also brings some new technical problems, one of which is the lightning strike breakage of insulated conductors during operation. Since the late 1980s, when insulated conductors were first used in Shanghai's urban area, the 10kV urban power grid has been largely insulated. Recently, Shanghai has experienced frequent lightning activity, resulting in dozens of lightning flashover accidents on distribution lines, and incidents of insulated conductors breaking due to lightning strikes have also occurred frequently. Table 1 summarizes the statistics of lightning-induced line tripping by the Shanghai Urban Power Supply Company over the past three years (Figure 1 shows a set of actual photos of a lightning-induced breakage at pole #30 on Huangpi South Road on August 2, 2003). Therefore, the inherent problem of lightning-induced breakage of insulated lines should not be taken lightly. This article analyzes the principle of lightning-induced breakage based on relevant data and proposes some economical and effective countermeasures. 2. Principle of Lightning-Induced Breakage 2.1 Causes of Lightning-Induced Breakage The inherently insufficient lightning withstand level of 10kV distribution lines makes them unable to withstand the effects of direct and induced lightning. When a bare wire is struck by lightning and flashover occurs, due to electrodynamic forces, the several thousand amperes of power frequency follow current arc moves towards the load end until the protection trips, without causing severe burnout of the conductor. Insulated conductors are different. Insulation exists around the breakdown point, hindering the movement of the arc and causing the arc root to burn at a single point. Even with the relay tripping time adjusted to the minimum, the conductor will still be damaged by a short-circuit current of several thousand amperes, making a wire breakage accident difficult to avoid. 2.2 Shielding Effect of High-Rise Buildings The shielding of various high-rise buildings in urban areas is very effective in reducing the occurrence of direct lightning strikes. However, the lightning attraction effect of high-rise buildings increases the occurrence of induced overvoltages on adjacent lines. According to foreign statistics, induced lightning accounts for 80% of lightning strikes on distribution lines. The discharge current of induced lightning is usually less than 1kA, and the amplitude of induced overvoltages can reach approximately 200-300kV. Such a high overvoltage amplitude is difficult for a 10kV line to withstand. This explains why the flashover rate of 35kV lines is much lower than that of 10kV lines according to the statistics in Table 1. 2.3 Factors Affecting Lightning-Induced Conductor Breakage in 10kV Insulated Conductors: Line Insulation Level and Arc Building Rate. When lightning strikes an insulator, the flashover of the insulator depends on the overvoltage value and the line insulation level. The probability of arc formation depends on several parameters: rated line voltage U2, flashover path L, the time of lightning strike, the magnitude of the lightning current, and line parameters, etc. Among these parameters, the average operating voltage gradient along the flashover path is the most important. E = U2/√3·L, where L is the flashover length in meters. The arc building rate decreases as E decreases. Data analysis of the arc spark discharge process shows that when E ≤ 7～10kV/m, the arc building rate is zero. The heat generated by the power frequency short-circuit current and lightning strikes causes the arc to melt the insulated conductor. The heat generated by the arc current is related to the arc duration. The heat generated by the arc current is: Q = I²Rt, where I is the arc current, R is the arc resistance, and T is the duration. Assuming the lightning wavefront time is 2μs and the lightning amplitude is 1kA; and the power frequency short-circuit current duration is 0.2s and the short-circuit current is also 1kA, the heat generated by the power frequency follow current is 10,000 times greater than that generated by the lightning current. Therefore , induced overvoltage is the cause of lightning-induced conductor breakage, while power frequency follow current is the decisive factor causing conductor breakage. The main purpose of studying the principle of lightning-induced conductor breakage is to determine the flashover location and take corresponding measures to improve the line structure. 3. Countermeasures The lightning protection effect of installing gapless surge arresters along the entire 10kV line is self-evident. However, this requires significant investment. Furthermore, the extensive use of gapless surge arresters in parallel with overhead lines inevitably reduces the operational reliability of the overhead lines due to potential failures of the additional equipment. Installing lightning protection lines allows lightning current to be rapidly released from the protection lines. During a lightning strike, only induced overvoltage is generated on the conductor, providing good lightning protection. However, due to the low insulation strength of 10kV lines, induced lightning can still cause flashover and lead to lightning-induced line breakage accidents. Therefore, surge arresters can reduce lightning-induced line breakage accidents, but cannot eliminate them. The following solutions are economical and effective measures to prevent lightning-induced line breakage. 3.1 Using fiberglass insulated crossarms: Lightning flashover depends on the overvoltage value and the line insulation level. Studies have shown that the severity of the arc caused by lightning decreases as the electric field gradient along the flashover path decreases. Therefore, improving the insulation level of PS-15 insulators can greatly reduce the lightning flashover rate. Simultaneously, even if a lightning flashover occurs, its arc intensity is also greatly reduced. However, due to technical and economic reasons, it is difficult to significantly improve the insulation level of post insulators. Currently, fiberglass fused wire crossarms, widely used in Shanghai, have advantages such as high mechanical strength and good insulation performance. If used as a post insulator crossarm (see Figure 2), it can significantly increase the flashover path, thereby greatly improving the lightning withstand level of the line, reducing the arc-forming rate, and essentially avoiding lightning-induced line breakage accidents. Figure 2 3.2 Using Protective Insulation Gap Crossarms While the application of fiberglass insulated crossarms can reduce lightning tripping and line breakage problems, their excessive insulation may divert lightning current to other equipment, causing damage. To provide a release path for lightning current when the line suffers a high-intensity lightning strike, we adopted protective insulation gap crossarms in the line (see Figure 3). Figure 3 Protective insulation gap crossarms are constructed from spark discharge gaps and nonlinear resistance current-limiting elements. The spark discharge gap limits the amplitude of lightning overvoltage, and the position of overhead line insulation flashover can be controlled by adjusting the discharge gap. Current-limiting elements can instantly interrupt power frequency follow current, effectively protecting overhead insulated conductors. Fiberglass insulated crossarms can provide long flashover distance lightning protection for the line when current-limiting elements cannot withstand high-intensity lightning strikes, thus suppressing the generation of power frequency follow current. 3.3 Using Protective Fitting Post-Type Insulators The main functions of protective fitting post-type insulators in preventing lightning strikes are: 1) increasing the discharge distance of the insulator to reduce the lightning flashover rate of the line; 2) forming a thick component by surrounding the conductor with protective fittings to prevent the burning effect at the root of the short-circuit arc. During flashover, the arc burns between the thick parts of the protective fittings (see Figure 4), thus protecting the conductor from damage. Figure 4 3.4 Lightning Protection for Low-Voltage Power Grids Low-voltage power lines have low insulation levels and are most prone to accidents. Therefore, lightning protection for low-voltage power lines must be given full attention. We installed piercing-type gapless zinc oxide surge arresters on the cable positioning brackets at the transformer outlet (see Figure 5). Figure 5.3.5 shows that the use of NXL type tension clamps for insulated conductors began in France, Japan, Australia, and other countries in the early 1960s, with over 40 years of experience. According to relevant data, Japan replaced most of its bare overhead stranded conductors with insulated overhead conductors in the early 1990s. Lightning strikes causing line breaks have also increased. Lightning flashovers occur between two or three phases, and the power frequency current often concentrates at the insulation breakdown point, causing the insulated conductor to melt before the circuit breaker trips. Overhead insulated conductors with unstripped tension clamps are prone to insulation creep or damage under the following conditions: vibration under tension, and damage from lightning strikes. Damage mostly occurs at the point where the insulated conductor is fixed on the pole, 200-500 mm from the pole. For lightning protection of urban power grid insulated conductors, surge arresters are generally installed, or lightning-break-resistant post insulators and lightning-break-resistant tension clamps are selected. The selection of tension clamps has a certain impact on the occurrence of wire breakage accidents. One type is the stripped NXL type tension clamp, while others are the unstripped NXJ and NLL types. The advantages of the NXL wedge-shaped (stripped) tension clamp are as follows: First, it considers lightning protection and wire breakage caused by stress concentration, as well as slippage caused by insulation creep. Unstripped clamps, with their two hard plastic wedge-shaped plates at the tension clamping point, cause the insulation of the insulated conductor to creep and break due to the gripping force. In addition, the parallel sharp angles of the clamp shell structure can generate induced overvoltage, leading to lightning strikes and wire breakage. The second reason is indirect lightning strikes to the conductor. Because the backflash voltage from lightning strikes the ground or trees is on the conductor, the unstripped clamp is in an insulated state from the conductor, and the insulated conductor bears a very high residual voltage, causing it to break. Because of their thick metal bodies, stripped tension clamps serve a reservoir-like function in terms of capacity and heat dissipation. The residual voltage is borne by the tension clamp itself, similar to installing a gapped surge arrester or a long flashover surge arrester on a straight pin insulator. Thirdly, when we conducted tests on 240mm² insulated conductors using stripped and unstripped tension clamps, the stripped NXL wedge-shaped tension clamp showed no signs of sheathing under gradually increasing tension, until the conductor was damaged when the tension exceeded 3.5 tons. In contrast, the unstripped NXJ type tension clamp showed sheathing when the tension reached 1.7 tons. Many international insulated circuit design codes explicitly stipulate that insulated conductors must be stripped when installing tension clamps. (See Figure 6) Figure 6 4. Comprehensive Application of Lightning Protection Measures In response to the frequent lightning activity in our city in recent years, and the relatively weak insulation of 10kV distribution lines, we have carried out lightning protection upgrades on weakly protected lines in the areas under the jurisdiction of the urban power supply company using the above-mentioned measures. The following principles were adopted in the upgrades: 1) In open areas, lightning protection wires were installed on the original poles to prevent direct lightning strikes. 2) Insulated conductor protective insulators were used to replace the original PS-15 insulators to appropriately increase the lightning discharge voltage of the insulators and reduce the power frequency arc-building rate. At the same time, guiding the arc to the insulator fittings during flashover is beneficial for heat dissipation and can prevent the insulated conductor from breaking. 3) Fiberglass insulated crossarms were used to replace the original iron crossarms at important locations on the line to strengthen the insulation. 4) Protective insulated gap crossarms were installed at important equipment locations on the line. Protective insulated gap crossarms have many functions in the line, such as controlling the flashover location, releasing lightning current, and protecting nearby equipment, and play an important role in the lightning protection of distribution lines. 5) Replace the original NXJ, NLL, and NLD type tension clamps with NXL type tension clamps to prevent lightning strikes and overvoltage-induced line breakage. Through the comprehensive application of lightning protection measures, the lightning protection capability of our company's 10kV distribution lines has been significantly improved. For example, the Qiu18 line suffered lightning strikes and line breakage in the two years before the lightning protection upgrade, but after the upgrade, it has withstood two lightning seasons without any lightning strikes and line breakage. This example demonstrates the success of our company's lightning protection measures. Currently, our company plans to promote the above measures to ensure reliable power supply to equipment and meet user needs. 5. Conclusion Although lightning tripping of 10kV overhead lines is unavoidable, as long as leadership pays attention and measures are in place, the problem of lightning-induced line breakage of insulated conductors can be effectively solved.