Abstract: During the commissioning of 500 kV AC transmission and transformation projects in the later stages of construction, at least three single-opening and single-closing impulse closing tests are required on the 500 kV unloaded lines. However, in some areas, after successfully conducting the above tests, several circuit breaker opening and closing tests are still required on the 500 kV unloaded lines. If closing resistors and high-voltage shunt reactors are not installed at both ends of the line, conducting circuit breaker opening and closing tests on 500 kV unloaded lines carries certain risks. Based on the basic theory of reclosing overvoltage and some practical field experience, the authors have demonstrated that under specific conditions, the overvoltage of the opening and closing tests is between 2 and 3 pu (1 pu = 550 2/3 kV), confirming the existence of risks. Finally, suggestions are proposed to avoid the risks of opening and closing tests. Keywords: 500 kV AC transmission line; commissioning; opening and closing test; circuit breaker; power transmission and distribution engineering 0 Introduction The opening and closing test refers to the cycle switching of the unloaded line according to the rated operation of the circuit breaker, that is, after the circuit breaker is opened, it is quickly closed in about 0.3 s (equivalent to the reclosing interval time), and after closing, it is quickly opened in about 60 ms (equivalent to the metal short-circuit time of the circuit breaker). The purpose of this test is to assess the reclosing performance of the circuit breaker. According to Article 24.0.1 of standard GB50150-1991[1], "the test items of overhead power lines above 1 kV shall include the impulse closing test" and Article 24.0.5, "the impulse closing test of the overhead line under the rated voltage shall be carried out 3 times, and the insulation of the line shall not be damaged during the closing process". The switching test of the unloaded line is mainly to assess the insulation of the newly built line and equipment. In the no-load line switching test, the circuit breaker is usually switched on and off at intervals, which is usually called the "single-opening-single-closing" test, not the "opening-closing-opening" test discussed in this article. The purpose and process of the two tests are fundamentally different. According to the relevant provisions of the current regulations, in the later stage of the commissioning of 500 kV transmission and transformation projects, at least three single-opening-single-closing impulse closing tests are required on the 500 kV no-load line. However, in some areas, after the successful completion of this test, several circuit breaker opening-closing-opening tests are still required on the 500 kV no-load line. If the closing resistor and high-voltage parallel reactor are not installed at both ends of the line, there is a certain risk in conducting circuit breaker opening-closing-opening tests on the 500 kV no-load line. Based on the basic theory of reclosing overvoltage [2-11] and some practical experience in the field, this article proves that under certain conditions, the overvoltage of the opening-closing-opening test is between 2 and 3 pu (1 pu = 550 kV). Finally, suggestions are put forward to avoid the risks of the opening-closing-opening test. 1. Theoretical Analysis of Three-Phase Reclosing Test 1.1 Basic Theory of Three-Phase Reclosing The risks of three-phase reclosing tests are discussed in general high-voltage technology textbooks [2-3] as a typical example of "overvoltage during reclosing of unloaded lines," and the research object adopts the three-phase reclosing condition. my country's 500 kV system currently only uses single-phase reclosing [8-11], while the three-phase reclosing test is a manual reclosing under the condition of no grounding fault on an unloaded line, which is exactly equivalent to the condition of three-phase reclosing of the non-faulted phase, and is applicable to the theory of three-phase reclosing. For unloaded lines without circuit breaker closing resistors or high-voltage reactance installed at both ends, the three-phase reclosing test is theoretically equivalent to switching capacitive components. During the first opening of the circuit breaker in the three-phase reclosing test, the full breaking time occurs at the zero-crossing point (or near the zero current (highest voltage)). After the first opening of the line, since there is no fault grounding point, no closing resistor, and no high-voltage reactance, the residual charge formed by the highest voltage cannot be released in a short time. When the circuit is quickly closed, if the residual voltage happens to be opposite in polarity to the power supply voltage, a voltage superposition will occur. The power supply voltage will reverse charge the line capacitor through the loop inductance. The theoretical maximum overvoltage during the oscillation is 3 pu, which obviously exceeds the 2 pu design insulation level of the 500 kV system (1 pu = 550 2/ 3 kV). In actual test conditions, the overvoltage level is lower than the theoretical value due to the following actual factors: (1) The residual charge is released in a limited way through the line-to-ground capacitance and loop resistance during the short time between the first opening and reclosing. (2) The amplitude of the power supply voltage at the reclosing moment is random and may not reach the maximum peak value. (3) The polarity of the power supply voltage at the reclosing moment is random and may not be opposite to the polarity of the residual voltage on the line. (4) Lightning arresters are installed at both ends of the line. Despite taking into account the above-mentioned practical factors, in actual engineering, the statistical overvoltage calculation value of some 500 kV line opening and closing tests is still higher than 2 pu, and some lines are much higher. Statistically, this exceeds the design insulation level of the 500 kV system, posing a certain threat to the system insulation. 1.2 Several issues to be noted in the opening and closing tests (1) my country's 500 kV system only uses single-phase reclosing. When a ground fault occurs, the grounded phase reclosing is activated. Due to the grounding point, the residual charge is effectively released, and no dangerous overvoltage will occur. However, the opening and closing test is a reclosing operation on a 500 kV unloaded line under the condition of no fault grounding point. This condition does not exist in actual operation. This test is only to assess the reclosing performance of the 500 kV circuit breaker itself, not to assess the actual operating conditions of the system. At the same time, it also brings certain risks to the system. (2) If circuit breaker closing resistors or high-voltage reactance are installed at both ends of a 500 kV line, a loop for the release of residual charge to ground is formed, which can effectively limit the reclosing overvoltage level and effectively suppress the risk of the opening-closing-opening test. (3) At present, only single-phase reclosing is used for 500 kV lines in my country, and three-phase reclosing is not used. Some countries (such as Japan) have adopted three-phase reclosing for their 500 kV lines. If the line has no high-voltage reactance, the circuit breaker is equipped with closing resistors, which can also avoid the formation of dangerous overvoltage when reclosing without grounding (equivalent to the opening-closing-opening test). (4) The mechanism of the opening-closing-opening test is the same as that of three-phase reclosing. According to the aforementioned three-phase reclosing overvoltage principle and foreign experience, when there is no high-voltage reactance at both ends of the line, the three-phase reclosing should generally be equipped with closing resistors. However, in the early design stage, the design unit only considers the single-phase reclosing used in my country in the calculation of whether to install closing resistors. Three-phase reclosing is not used as an assessment condition for installing closing resistors. 2. Actual Situation of On-Site Opening and Closing Tests 2.1 Basis of the Tests Currently, some regional power grids, when conducting startup tests on 500 kV systems, do not consider whether high-resistance circuit breakers and circuit breaker closing resistors are installed, and conduct opening and closing tests in all cases. The current regulation GB50150–1991 stipulates that newly built transmission lines should undergo single-opening and single-closing tests, the purpose of which is to assess the insulation of the transmission system. On-site opening and closing tests are not specified in GB50150–1991; they are only listed as a type test in the laboratory as a rated operating cycle in relevant circuit breaker standards. Therefore, the purpose of on-site opening and closing tests is to assess the reclosing performance of 500 kV circuit breakers. Such tests are not based on standards and regulations, but usually on the requirements of power grid dispatching and operation departments. 2.2 Accidents during the opening and closing tests Since some 500 kV lines have high-voltage reactance or closing resistors on both sides, even without high-voltage reactance and closing resistors, the amplitude and polarity of the power supply voltage at the reclosing moment during the test are random, and the probability of encountering unfavorable conditions is limited. Therefore, most opening and closing tests are successfully completed. However, in recent years, there have been more and more short- and medium-length 500 kV lines, which often do not have closing resistors and high-voltage reactance. As a result, the probability of encountering unfavorable conditions during the opening and closing tests has increased, and some accidents have occurred, mainly in the following categories: (1) The measured bus voltage at the end of the line has exceeded 2 pu, and it will be even higher in the middle of the line (unable to be measured). (2) The voltage transformer or surge arrester at the end of the line breaks down or explodes. (3) Reignition occurs when the circuit breaker is opened for the second time. Although there are different opinions on the causes of these accidents, these accidents all occurred during the opening and closing tests, while no such accidents occurred during the single opening and single closing tests. This indicates that these accidents are closely related to the opening and closing tests. 2.3 Actual Calculation Example 2.3.1 Line Parameters and System Conditions This paper adopts a typical tower structure model. Minor differences in line parameters have no significant impact on the calculation results. The positive sequence parameters of the line are shown in Table 1. Before the test, the 500 kV bus voltage at the T station side was taken as 550 kV. 2.3.2 Overvoltage Calculation Conditions for Opening, Closing and Opening Operations The number of operations was taken as 100. The line was divided into 6 sections, each 22 km long, with 7 measuring points along the line. During the calculation, the highest value of the three-phase overvoltage obtained in each operation was recorded and statistically analyzed. The overvoltage with a probability of 2% is U2%. No closing resistors were installed on the circuit breakers, and the three-phase closing time difference was no more than 5 ms. During opening, closing and opening operations, the metal short-circuit time of the switch was taken as 60 ms, and the current-free rest time was 0.3 s. The configuration and parameters of the metal oxide surge arresters in the power grid are shown in Table 2. Note: Io is the current carrying capacity of the surge arrester. The statistical overvoltage is the relative ground overvoltage with an occurrence probability of less than 2%, and this value should not exceed 2.0 pu. 2.3.3 Calculation results of switching and closing overvoltage When switching and closing circuit breakers at the T station side, the 2% statistical switching overvoltage U2%, the maximum switching overvoltage along the line and the maximum energy consumption of the surge arrester are shown in Tables 3 to 5. [align=center] [/align] 2.3.4 Analysis of calculation results (1) In the switching and closing of 500 kV lines, the 2% statistical switching overvoltage at the beginning and end does not exceed 1.70 pu, and the maximum 2% statistical switching overvoltage along the line reaches 2.11 pu, indicating that the statistical switching overvoltage exceeds the regulation of "should not exceed 2.0 pu". (2) In the switching and closing of 500 kV lines, the maximum switching overvoltage at the beginning and end reaches 1.76 pu, and the maximum switching overvoltage along the line reaches 2.45 pu, indicating that the maximum switching overvoltage along the line is relatively high. (3) During the operation of the 500 kV line, the maximum energy consumed by the surge arrester at the end of the line reaches 2200 kJ. For a surge arrester with a rated voltage of 444 kV, it can withstand a current capacity of 5772 kJ. Therefore, the surge arrester can withstand the energy consumption caused by the operation of the line. 3 Conclusions and Recommendations (1) The dangerous overvoltage in the basic theory of three-phase reclosing is the same as that in the test of the line, which makes the test have a certain probability of danger. It is recommended that the relevant management departments formulate relevant regulations to avoid taking unnecessary risks. (2) When the closing resistor and high reactance are not installed at both ends of the line, the line should be carefully tested before the test of the line. The risk assessment of overvoltage calculation and surge arrester energy absorption verification should be carried out in advance. When the statistical overvoltage is higher than 2 pu, this test should not be carried out.