Abstract: This paper studies the improvement of lightning protection measures by using composite insulated metal oxide surge arresters on the Shou-Zun 110kV line in areas with frequent lightning activity in the North China Power Grid. Through experiments and actual operation, this improvement has been proven to be successful, economical, and effective. The number of lightning-induced trips decreased from 7 in 1996 to 1 in 1997 and 0 in 1998. Keywords: Transmission line, lightning protection, grounding. Most accidents in the power grid are caused by transmission line faults, and lightning-induced trips account for a large proportion of these faults, especially in mountainous transmission lines where line faults are almost entirely caused by lightning-induced trips. According to operational records, half of the power supply faults on overhead transmission lines are caused by lightning. Therefore, preventing lightning-induced trips can significantly reduce transmission line faults, thereby reducing the frequency of accidents in the power grid. After years of exploration, my country's power transmission line lightning protection has basically formed a series of effective conventional lightning protection methods, such as reducing grounding resistance, erecting lightning protection wires, and installing automatic reclosing devices. However, for some mountainous lines, lightning strikes are very frequent, and reducing grounding resistance is extremely difficult, costly, and labor-intensive, with limited effectiveness. In recent years, the successful development of synthetic insulated metal oxide surge arresters (SIAs) for 110kV and above voltage levels has provided a new means to solve the lightning protection problem for power lines. In Chengde Power Supply Bureau, one of the two regions with frequent lightning activity in the North China Power Grid, a 110kV transmission line—the Shou-Zun 110kV line—passes through a section of towers in high mountain ridges, which are frequently struck by lightning during the rainy season, causing line tripping. To solve this problem, 20 SIAs were installed on towers 129 to 167 of this line. After more than a year of operation and a series of live-line monitoring studies, it has been proven that this improved lightning protection measure is economical and effective for lightning protection of mountainous lines. 1. Basic Information and Upgrades of the Line 1.1 Basic Information of the Shou-Zun Line The Chengde area is located deep in the Yanshan Mountains, with high mountains and ridges accounting for approximately 40% of the area. Lightning activity is extremely frequent, with more than 40 lightning days per year. Faults caused by lightning strikes account for about 60% of all operational faults annually. The Shou-Zun 110kV line is 49.40km long, with no conductor transposition. Plains account for 13.2%, general mountainous areas for 53.1%, and high mountains and ridges for 33.7%. The Shou-Zun line is a crucial connection between the Chengde area and the power grid. It is also one of the lines in the Chengde area with the most lightning strikes. Because nearly half of the poles are located on mountaintops, locating lightning strike points and replacing insulator strings is extremely difficult and involves a significant workload. According to available information, lightning strikes are selective. The 220kV Xin (Anjiang) Hang (Zhou) line, with a total length of 119.4km, was put into operation on September 28, 1960. Since 1962, numerous magnetic steel rods have been installed along the line for measurement and recording. Analysis of lightning current amplitude records from 1962 to 1988 and line lightning trip rate from 1961 to 1994 indicates that lightning strikes are selective; approximately half of the line's length has no lightning strike records, and high-risk areas and points prone to lightning strikes account for about one-third of the line. Strengthening lightning protection measures in high-risk areas and points prone to lightning strikes can significantly reduce the lightning trip rate. Therefore, we decided to install surge arresters on poles 129 to 167 of the Shou-Zun line to reduce the lightning trip rate of this line. 1.2 Improvement of poles 129-167 on the Shou-Zun line 1.2.1 Improvement of grounding There are 11 poles with high grounding resistance values ​​among poles 129-167: 129, 133, 134, 138, 139, 145, 154, 158, 162, 165 and 167, see Table 1. In this section, 42% of the towers are located in high mountains and ridges, 49% in general mountainous areas, and 9% in flat areas. We improved the grounding of this section by re-burying the grounding down conductors, replacing the soil in areas with poor grounding conditions, and burying continuously extended grounding electrodes in more severe cases. After the project was implemented, the grounding resistance of the transmission towers was significantly reduced, as shown in Table 2. [IMG=Improvement of grounding]/uploadpic/THESIS/2007/12/2007122515554863869U.jpg[/IMG] [IMG=Grounding resistance of transmission towers]/uploadpic/THESIS/2007/12/20071225155555717244.jpg[/IMG] 1.2.2 Improvement of external insulation All zero-value porcelain insulators in this section of the line were replaced, and an additional insulator was added to each phase of all straight towers (provided that the distance to the ground is sufficient), changing to using 8 XP-7 insulators. Table 3 shows the comparison between the creepage distance (hereinafter referred to as creepage distance) and leakage ratio (hereinafter referred to as leakage ratio) of the insulators after implementation and before implementation. It can be clearly seen from the table that the insulation level of the line has been significantly improved. [IMG=Improvement of external insulation]/uploadpic/THESIS/2007/12/2007122515562697240J.jpg[/IMG] 2 Selection of surge arresters and determination of parameters 2.1 Selection of surge arresters 2.1.1 Selection of zinc oxide surge arresters with composite insulation jackets Since commonly used surge arresters have porcelain jackets, they are relatively heavy, inconvenient to install, and have certain limitations in their use on lines, and if an explosion occurs, its fragments will endanger the operational safety of nearby insulators, so it is necessary to select a surge arrester that is more suitable for use on the line. With the development of domestic silicone rubber technology, composite-insulated zinc oxide surge arresters, which have been successfully developed in recent years, are a type of surge arrester suitable for suspension on power line towers. Compared with traditional porcelain-insulated surge arresters, they eliminate the bulky outer jacket and use a new type of silicone rubber composite organic jacket, thus offering advantages such as light weight. They can even allow the line to continue operating when the composite-insulated surge arrester fails, while their electrical and protection characteristics are largely equivalent to those of porcelain-insulated surge arresters. Internationally, the United States, Japan, Russia, and other countries have widely adopted composite-insulated zinc oxide surge arresters. In the United States, operating distribution transformers equipped with composite-insulated zinc oxide surge arresters are ubiquitous on highways. Statistics show that tens of millions of composite-insulated zinc oxide surge arresters are used in the power grid in the United States, and millions in Japan. With the development of silicone rubber technology in my country, 110kV and 220kV composite-insulated zinc oxide surge arresters have also been successfully developed. Table 4 shows the electrical characteristics of a 110kV composite-insulated zinc oxide surge arrester developed by a company in Beijing. [IMG=Electrical Characteristics of 110kV Composite-Insulated Zinc Oxide Surge Arrester]/uploadpic/THESIS/2007/12/2007122515565776569Z.jpg[/IMG] 2.1.2 Selection of Externally Gap-Type Composite Insulated Zinc Oxide Surge Arresters Suspended on Line Towers There are two types of composite insulated zinc oxide surge arresters suspended on line towers: one is the externally gap-type composite insulated zinc oxide surge arrester (GMOA); the other is the externally non-gap-type composite insulated zinc oxide surge arrester (WGMOA). The external gap of the GMOA can isolate the grid operating voltage during normal line operation, keeping the MOA from bearing voltage. Therefore, the rated voltage of the arrester can be selected to be lower, and the line can continue to operate when the MOA fails. However, the protection characteristics of this type of arrester are poor, and the discharge characteristics are mainly determined by the gap. Its impulse discharge voltage is much higher than the residual voltage of the arrester. Figure 5 shows the test results of the external gap impulse discharge voltage of a 110kV-level surge arrester with a series external gap developed by a company in Beijing. When the WGMOA is suspended on the line, its operation can be monitored at any time. It is easy to install and has relatively good protection characteristics, which depend only on the residual voltage of the surge arrester. Both types of surge arresters have their advantages and disadvantages. In order to facilitate installation, obtain good protection effect, and facilitate monitoring of the operation of the surge arrester, we decided to choose to use a zinc oxide surge arrester with a composite insulation jacket that has no external gap. [IMG=110 kV level surge arrester with series external gap]/uploadpic/THESIS/2007/12/2007122515570228235L.jpg[/IMG] 2.2 Selection of surge arrester parameters Since WGMOA is selected, the surge arrester operates under phase voltage for a long time, and the line operating conditions are more severe than the operating conditions in the substation. In order to improve the reliability of the surge arrester, the rated voltage of the 110kV composite insulated zinc oxide surge arrester is increased from 100kV to 120kV, the continuous operating voltage is increased from 73kV to 90kV, and the DC 1mA voltage is increased to 170kV. Considering the high probability of the surge arrester being directly struck by lightning, the high current withstand level of the surge arrester is increased from 65kA to 100kA. The specific parameters are shown in Table 6. [IMG=Specific Parameter Table]/uploadpic/THESIS/2007/12/20071225155709781244.jpg[/IMG] In addition, because surge arresters are suspended on the line for extended periods and bear the phase voltage, we added a partial discharge test during the tensile test of the surge arrester in the type test. During the test, one 110kV surge arrester was used, and a static mechanical load was applied axially with tensile forces of 500kg and 750kg respectively. Under this load condition, 1.05 times Uc was applied, and the partial discharge of the surge arrester was measured. The test results are shown in Table 7. [IMG=Test Results Table]/uploadpic/THESIS/2007/12/200712251557147922817.jpg[/IMG] The test results show that when the axial mechanical load is applied to the rated breaking load, the partial discharge does not change, so its electromechanical performance is stable and meets the design requirements. 3. Installation Status of Surge Arresters 3.1 Acceptance Test of Surge Arresters To understand the performance of the surge arresters before installation, acceptance tests were conducted on 17 composite insulated zinc oxide surge arresters from Beijing Zhongneng Ruister Company at the Shahe High Voltage Test Hall of the North China Electric Power Research Institute from October 29th to 31st, 1996. The tests included insulation resistance testing, DC testing (measurement of DC 1mA voltage and leakage current at 75% DC 1mA voltage), and AC testing. The test results were satisfactory. 3.2 Determination of Surge Arrester Installation Location After consideration and research, it was decided to install the surge arresters on both linear insulator strings and tension insulator strings, specifying the exact installation locations. Considering that the surge arrester is installed close to the insulator string on a straight-line tower (vertical insulator string), it is possible that the voltage distribution on the insulator string will affect the potential distribution of the surge arrester, thereby affecting the leakage current of the surge arrester, accelerating the deterioration process of the surge arrester, and shortening its service life. Therefore, a simulation test was conducted in the Shahe Test Hall. The test results showed that this installation position of the surge arrester has little impact on its service life and basically does not affect the test results of the live-line test. Considering the tower's altitude, topography, and the protection range of the surge arrester, and considering that the middle phase (phase B) of a horizontally arranged three-phase tower is unlikely to be directly struck by lightning, while the top phase of a triangularly arranged tower is more susceptible to lightning strikes and requires a surge arrester (such as on pole number 130), composite insulated zinc oxide surge arresters were installed on the tower. Specific installation details are shown in Table 8. [IMG=Detailed Installation Information Table]/uploadpic/THESIS/2007/12/2007122515572020077X.jpg[/IMG] 4. Operational Status and Analysis of Surge Arresters 4.1 Live-Line Tests of Surge Arresters After acceptance testing, 17 surge arresters were installed on the Shou-Zun line in December 1996, and the first live-line test was conducted in December 1996 to accumulate initial data for live-line testing of surge arresters; then, live-line tests were conducted monthly after the start of the rainy season. The results of the live-line tests showed that the surge arresters were operating normally. To verify the performance of the surge arresters, after the rainy season, two surge arresters were randomly selected, then removed and tested while energized. The test results were qualified, meaning that the surge arresters performed well after operating for one rainy season. 4.2 Surge Arrester Operation Status As of June 1998, the surge arresters operated a total of 5 times. Two of these occurred during the 1997 rainy season, both on phase A of tower 140. Three surge arresters operated in 1998: one each on phase A of tower 138, phase A of tower 140, and tower 145. Tower 138, at approximately 367.2m, has a span of 595m from tower 139, making it vulnerable to lightning strikes. Tower 140, at 464.9m, is the highest tower in this section, situated atop a high mountain ridge, isolated and highly susceptible to lightning strikes. This tower was previously struck by lightning in 1992. Tower 145, at approximately 428.2m, is also located on a mountaintop and is vulnerable to lightning strikes. The surge arrester operated five times, protecting the Shou-Zun line five times and preventing five line trips. Therefore, the effect of installing the surge arrester is obvious. 4.3 Operation of the Shou-Zun Line Since the installation of surge arresters on the Shou-Zun 110 kV line in December 1996, the line operated until June 1998, and only tripped once (August 31, 1997). The fault occurred at tower No. 117, caused by a lightning strike on the tower. This tower is outside the protection range of the installed surge arrester, which conversely illustrates that the protective effect of the surge arrester is obvious; towers within the protection range of the surge arrester are protected, while towers outside the protection range are subject to lightning strikes. Because the surge arrester on pole number 140 tripped twice in July and August 1997, protecting the line, and considering these successful experiences, as pole number 117 had also been struck by lightning in 1996, and given that the three consecutive poles (numbers 116, 117, and 118) on this section of the line had single-line surge protection, and the terrain was high and steep, making the conversion from single-line to double-line surge protection extremely labor-intensive, three composite insulated zinc oxide surge arresters were also installed on pole number 117 on November 7, 1997. Operation showed that the five lightning strikes that caused the line to trip were relatively concentrated, indicating that the installation location of the surge arresters was quite reasonable. It prevented the line from tripping five times, and the effect of the surge arresters was very obvious. A comprehensive comparison of the operation of the Shou-Zun line over the past few years reveals that since the installation of surge arresters in December 1996, the number of lightning-induced trips has decreased from 7 in 1996 to 1 in 1997 and 0 in 1998 (as of the end of June). Although lightning strikes have a certain degree of randomness, the surge arresters operated 2 times in 1997 and 3 times in 1998, effectively protecting the line and reducing the number of lightning-induced trips. Therefore, installing synthetic insulated zinc oxide surge arresters on the line can achieve a very good protective effect. 5. Conclusion The Shou-Zun 110kV transmission line, a 110kV line of Chengde Power Supply Company, frequently suffers from lightning strikes during the rainy season due to a section of towers passing through high mountains, causing line trips. Reducing the grounding resistance of this section of towers is difficult, costly, and labor-intensive, and its effectiveness is somewhat limited. To address this issue, our institute collaborated with Chengde Power Supply Company to install a total of 20 synthetic insulated metal oxide surge arresters (SIAs) on towers 117 and 129-167 of the line. After more than a year of operation, the arresters tripped five times, effectively protecting the line. These arresters were selected with parameters adapted to the line's operation. Live-line monitoring studies proved that the arresters' performance met the requirements for operation on the line. Furthermore, operational experience over more than one rainy season demonstrated that this improved lightning protection measure is economical and effective for mountainous lines. More than a year of operation has proven that the correct parameter selection and reasonable arrangement of the synthetic insulated metal oxide surge arresters effectively protect the line and prevent lightning-induced tripping. Installing synthetic insulated metal oxide surge arresters on a 110kV line is a first in my country. Operational experience shows that installing synthetic insulated metal oxide surge arresters suitable for line operation is an economical, effective, and feasible method, and a recommended and effective lightning protection method for mountainous lines.