Abstract The Huizhou-Shantou 500 kV transmission line is 268 km long. Due to the elimination of the closing resistor of the outgoing circuit breaker, the statistical operating overvoltage and line flashover rate are relatively high. To reduce the statistical operating overvoltage and line flashover rate, a line-type 444 kV MOA (zinc oxide surge arrester) was installed in the middle of the line, which is the first of its kind in China. To limit the inrush current, a set of 120 Mvar high-voltage reactors was installed on each side of the line. However, before commissioning, one set of high-voltage reactors was returned to the manufacturer for repair due to core grounding. Under the condition of only one set of high-voltage reactors and relatively serious internal overvoltage and inrush current, in order to ensure the timely and safe commissioning of the Huizhou-Shantou line, safety measures were proposed through repeated research and analysis, which enabled the smooth commissioning of the Huizhou-Shantou line and provided a basis for the development of operating procedures for the Huizhou-Shantou line and long-distance transmission lines. Keywords: Internal overvoltage research in power transmission 1. Research Process and Main Conclusions 1.1 Research Conclusions in the Design Phase In 1994, when the Huizhou-Shantou power transmission project entered the preliminary design phase, the Guangdong Provincial Electric Power Design Institute (hereinafter referred to as "Design Institute") and the former Electric Power Research Institute of the Ministry of Electric Power (hereinafter referred to as "Electric Power Research Institute") jointly conducted a calculation study on the internal overvoltage of the project. The surveyed line of this project is 293 km long. The key issue of the study is: under the condition that the closing resistors of the outgoing circuit breakers on both sides of the line are eliminated, how to take measures to limit the statistical switching overvoltage and line flashover rate within the range specified in the regulations to ensure the safety of the power transmission and transformation equipment. Since the Huizhou-Shantou line is currently the longest line in China without closing resistors, and a set of line-type zinc oxide surge arresters needs to be installed in the middle of the line (a first in China), the internal overvoltage study of this project is much more complex than that of short lines. If a conventional calculation model is used, that is, the line parameters are fixed, the statistical switching overvoltage and line flashover rate will exceed the specified values ​​in the regulations. Therefore, this study adopts the complex J.MAITI model. This model calculates based on the actual tower dimensions, average distance to ground, and soil resistivity, and considers the changes in line parameters with frequency, thus taking into account the high-frequency characteristics of the line. This precise model requires a longer calculation time, taking over ten minutes for each operating mode (compared to a few seconds for a conventional model). The precise model's calculation results can reduce statistical operating overvoltage by approximately 10% compared to the conventional model, and also correspondingly reduce the line flashover rate. In other words, using the precise model reduces operational margin by 10%. The calculation results are found in the "Study on Overvoltage and Insulation Coordination in the Huizhou-Shantou 500 kV Transmission System" compiled by the Electric Power Research Institute and the Design Institute in November 1994. The main conclusions of this study are: a) Each side of the Huizhou-Shantou line needs to be equipped with a 120 Mvar high-voltage parallel reactor (hereinafter referred to as "high-voltage reactor"), and the neutral point small reactor is taken as 750 Ω. b) Using GJ-70 type steel strand for the ground wire of the Huizhou-Shantou line is feasible. c) Under conditions where fast three-phase reclosing is not used on the line, the closing resistor can be omitted from the outgoing circuit breaker of the Huishan line. Since the flashover rate remains high even after removing the closing resistor, a set of 444 kV zinc oxide surge arresters (MOAs) must be installed on the Jieyang side of the line. After the three sets of 444 kV MOAs are put into operation, both the overvoltage of the closed line and the line flashover rate meet the requirements (the two sets of 420 kV busbar MOAs are also put into operation when the line is closed). d) Under switching overvoltage, the maximum energy consumption of the MOA is 23% of the allowable value; under fault switching overvoltage, the maximum energy consumption of the MOA is 19.6% of the allowable value. Therefore, the MOA still has a large margin when used as the main protection against switching overvoltage. e) When closing the Shantou unloaded transformer, a set of low-voltage reactors (45 Mvar) should be put into operation on the low-voltage side of the main transformer to prevent resonant overvoltage from occurring in the main transformer. 1.2 Supplementary Study Before Commissioning The 500 kV Huishan Transmission and Transformation Project was commissioned on December 18, 1997. During pre-commissioning equipment testing, it was discovered that the core of the high-voltage parallel reactor on the Shantou side was grounded. This reactor had to be returned to the manufacturer for repair and could not be commissioned simultaneously with the project. The Shantou-side high-voltage reactor had a significant impact on the safe commissioning and operation scheduling of the Huishan project. Furthermore, the actual length of the Huishan line was 268 km (not the 293 km selected on the 1994 drawings). Therefore, the 500 kV Huishan Transmission and Transformation Project Start-up Committee requested a supplementary study on overvoltage within the project. In early December 1997, relevant personnel conducted actual measurements of the line parameters. The Start-up Committee then requested another overvoltage study based on the measured parameters to ensure safe startup. In this study, we considered the actual volt-ampere characteristics of busbar surge arresters, line surge arresters, and surge arresters in the middle of the line, the actual length of the line, and the inability to simultaneously commission high-voltage arresters on the Shantou side. We used measured parameters and line parameters derived from high-frequency characteristics based on actual tower dimensions. The main conclusions of these two supplementary studies are: a) When high-voltage arresters on the Shantou side are out of service, the reclosing interval for single-phase reclosing should be extended, with a recommended interval of 1.5 s (0 s for a single-phase fault, 0.1 s for single-phase switches on both sides of the line to trip, and 1.5 s for single-phase switches to reclose). If two high-voltage arresters are operating on the line, the time interval can be 1 s. b) When closing an empty line on the Shantou side, the 500 kV bus voltage at the Shantou substation should not exceed 530 kV before closing. If system conditions permit, the bus voltage level before closing can be further reduced. c) The minimum phase-to-phase clearance between the Shantou side outgoing circuit breaker and the first tower of the Huishan line should be greater than 4.4 m (the normal distance is 7-8 m under windless conditions; this distance should be noted during switch operation if wind is present). d) When closing the Shantou unloaded transformer, at least one set of low-voltage reactors should be connected on the low-voltage side of the transformer before closing. If the Shantou station bus voltage exceeds 535 kV before closing, two or three sets of low-voltage reactors should be connected. If the Shantou station bus voltage exceeds 550 kV, or if there are capacitors connected to the low-voltage side of the unloaded transformer, it is not advisable to close the unloaded transformer to avoid resonance of the main transformer. e) Overvoltage protection devices should be activated, with a protection setting time of 0.5 s and a power frequency overvoltage multiple of 1.4 pu. f) In addition to the above requirements, during operation, other requirements should also be referred to in the research reports compiled by the Electric Power Research Institute and the Design Institute in November 1994, namely, "Research on Overvoltage and Insulation Coordination in the Huizhou-Shantou 500 kV Transmission System" and "Supplementary Research on Overvoltage and Insulation Coordination in the Huizhou-Shantou 500 kV Transmission System (Part II)" and November 1997. 2 Analysis of Measured Parameters and Equipment Parameter Calculation Results 2.1 Calculation Results of Measured Parameters In early December 1997, relevant personnel conducted actual measurements of the line parameters. Among them, the measured value of the positive sequence capacitance C1 = 0.01519 μF/km, and the calculated positive sequence capacitance C1 = 0.01317 μF/km based on the actual arrangement size of the towers, the grounding method, the soil resistivity, and considering the high frequency characteristics. Compared with the measured value, the measured positive sequence capacitance is 15.34% larger. That is to say, the charging power of the 268 km Huizhou-Shantou line, if calculated according to the measured capacitance, is equivalent to that of a 309 km line. Based on the measured parameters, the following results were obtained: a) With an intermediate MOA in the middle of the line, the maximum overvoltage value of the closed-circuit line on the Huishan line reached 2.14 pu, and the statistical operating overvoltage reached 2.06 pu, exceeding the national standard of 2.0 pu. Without the intermediate MOA (installed on the Jieyang line side), the maximum overvoltage value and the statistical operating overvoltage value would be even higher than the above values. According to the national standard, both outgoing circuit breakers on both sides of the Huishan line must be equipped with closing resistors; otherwise, the closed-circuit line will not be successfully closed. However, the project is nearing commissioning, and it is no longer possible to install closing resistors. b) Because the measured positive sequence capacitance C1 is 15.34% larger than the calculated value, the phase-to-phase capacitance increases accordingly (phase-to-phase capacitance Cφ = (C1 - C0) / 3). Under the condition that the high-voltage reactor on the Shantou side is out of operation, the residual current reaches 46.6 A, while the calculated residual current using the positive sequence capacitance is only 29.9 A. Compared with the calculated value, the residual current of the measured parameter is 56% larger, and the reclosing interval of single-phase reclosing must be extended; otherwise, reclosing will not be successful. 2.2 Analysis of measured parameters The measured positive sequence capacitance C1 of the Huizhou-Shantou line is 0.01519 μF/km, which is 15.34% larger than the calculated value. Referring to the measured values ​​of Shajiang line (0.01377 μF/km), Hezeng line (0.013 μF/km), Xuluo line (0.0134 μF/km), and Luozeng line (0.013 μF/km), the measured values ​​of Shajiang line and Luozeng line within the province are also compared. The measured C1 value for the Datong-Jinshan line in the North China Power Grid is 0.01317 μF/km, while the measured C1 values ​​for the Tianping line and Tianlai line in Guangxi are 0.01359 μF/km. This indicates that the measured positive-sequence capacitance of most 500 kV lines is very close to the calculated value, and the measured wave velocity is above 290,000 km/s (it should be 300,000 km/s, but the line has resistance damping). However, the measured wave velocity of the Huishan line is 270,000 km/s (wave velocity v=1/L1C1), indicating that the measured positive-sequence capacitance is larger than the calculated value, thus reducing the wave velocity (the measured positive-sequence inductance is close to the calculated value). The reason for the larger measured capacitance may be due to equipment limitations or formula conversion issues. For example, using distributed parameters for calculation can reduce the error by 3% to 4% compared to using lumped parameters. Since the calculations using measured parameters were conducted a few days before commissioning, to avoid affecting the commissioning schedule, the design institute and the power research institute reached a consensus to conduct further research using calculated parameters. The six conclusions from the two supplementary studies before commissioning were mainly derived from the results of the calculated parameters. The successful commissioning of the Huizhou-Shantou line on December 18, 1997, proved that the results obtained using the calculated parameters were reliable and scientifically sound. Of course, the technical conditions during commissioning of the Huizhou-Shantou line were relatively complex and challenging due to the inability to simultaneously commission the high-voltage arresters on the Shantou side. The successful commissioning on the first attempt was the result of the joint efforts of all parties, including contributions from experts at the power research institute. 2.3 The Influence of the I-V Characteristics of the Line Intermediate Surge Arresters on the Calculation Results The 444 kV line-type zinc oxide surge arresters on both sides of the Huizhou-Shantou line... The MOA (Medium-Oxide Arrester) was manufactured by Xi'an Electric Porcelain Factory, while the line-type zinc oxide surge arrester on the Jieyang side of the line was specially made by Xi'an Electric Porcelain Research Institute. Because the intermediate surge arrester needs to be placed on the tower, due to structural reasons, its volt-ampere characteristics are not entirely consistent with those of the two side arresters. The residual impulse voltage of the intermediate surge arrester at a 2 kA operating impulse current is 900 kV, while the residual impulse voltage of the two side line-type surge arresters at the same current is 862 kV, representing a 4.4% increase in residual voltage. This leads to a corresponding increase in the line overvoltage level and flashover rate. Although the relative increase is small, it is still a factor affecting overvoltage for long lines where the closing resistor has been eliminated. 3. Conclusion: The 500 kV Huishan Line is currently the longest line in China with the elimination of the closing resistor in the outgoing circuit breaker. The project was successfully put into operation on December 18, 1997, demonstrating that the calculation and research work was correct and scientific. The Huishan line has the following design features in limiting internal overvoltage: 1) The Huishan line is currently the 500 kV line in China with the longest outgoing circuit breaker without closing resistor; 2) After eliminating the closing resistor, the statistical operation overvoltage discharge line flashover rate is higher. To ensure the success of closing the line and facilitate operation, a 444 kV MOA (zinc oxide surge arrester) is installed on the Jieyang side, which is the first of its kind in China; 3) To limit the inrush current, a 120 Mvar high-resistance device is installed on each side of the line, and the neutral point reactance value is 750 Ω; 4) The calculation and research adopts an accurate model of the line parameters changing with frequency, which can reduce the overvoltage value by about 10% compared with the conventional model, thus reducing the operational margin by 10%.