Abstract: Live-line testing of high-voltage electrical equipment insulation has significant advantages over conventional power outage testing. With the gradual development and improvement of live-line testing technology, some equipment is now ready to replace traditional power outage preventative testing with live-line testing. This paper introduces live-line testing methods for capacitive equipment and zinc oxide surge arresters, and discusses the feasibility of replacing preventative testing with live-line testing. Live-line testing of high-voltage electrical equipment insulation has significant advantages over conventional power outage testing: no power outage is required, testing is flexible and convenient; based on the operating status of the equipment, the sensitivity for diagnosing insulation defects is high; the test cycle can be flexibly arranged according to the insulation condition of the equipment, facilitating timely detection of insulation hazards and understanding the changing trends of insulation defects. Therefore, the development and application of live-line testing technology for power equipment insulation has gradually gained attention in recent years. Statistical data shows that at present, domestic high-voltage insulation live-line testing work mainly focuses on zinc oxide surge arresters, capacitive equipment (capacitive insulation current transformers, bushings, capacitive voltage transformers, coupling capacitors, etc.), and partial discharge of GIS equipment. With the gradual development and improvement of insulation live-line testing technology, some equipment has the potential to replace traditional power outage preventive testing with live-line testing. The recently published enterprise standard of China Southern Power Grid Co., Ltd., "Preventive Testing Procedures for Power Equipment", also emphasizes the importance of live-line testing and makes specific provisions for live-line testing to replace power outage preventive testing. 1. Live-line testing methods (1) Capacitive equipment Since capacitive equipment is forcibly equalized through capacitance distribution, its insulation utilization coefficient is relatively high. Once the insulation is damp, it will often cause an increase in insulation dielectric loss, leading to breakdown. Therefore, live-line testing of capacitive equipment can play a positive role in improving the operation and maintenance level of equipment, timely detection of potential accidents, and reducing the number of power outages. Changes in dielectric loss and capacitance of capacitive equipment are relatively sensitive to defects such as overall equipment dampness and insulation deterioration. Insulation dampness defects account for 85.4% of defects in capacitive equipment. Dielectric loss and capacitance measurement have always been the main items of preventive testing and are also the earliest items carried out by live (online) testing technology. In order to obtain stable measurement results, the comparative method can be used for live testing. That is, select a group of capacitive devices and use its end screen current signal as a reference standard. The end screen current of other devices is measured relative to it to obtain the relative dielectric loss difference and capacitance ratio. In this way, the fluctuation of measurement results caused by changes in external environment (such as temperature) and operating conditions (such as load capacity) will act on both the standard device and the device under test at the same time. The relative measurement values ​​between them remain stable and are more likely to reflect the true state of the equipment insulation. At the same time, since it is not necessary to use the secondary side voltage as a reference signal, the measurement results will not be affected by the change of PT angle difference. (2) Zinc oxide surge arrester During the operation of zinc oxide surge arrester, there is always a certain leakage current through the valve plate, which accelerates the aging of the valve plate. Moisture and aging are the main causes of the deterioration of zinc oxide surge arrester valve plates. Detecting the total leakage current and resistive current of zinc oxide surge arrester can effectively reflect the insulation status of zinc oxide surge arrester. The total current measurement is more sensitive to the overall moisture. In the early aging, the resistive current increases more, while the total current change is not obvious. Therefore, measuring the AC leakage current and its active component is the main method for on-site testing of surge arresters. Preventive testing procedures also include the measurement of "operating leakage current" of zinc oxide surge arresters in preventive testing items. Currently, the main methods for live-line testing of zinc oxide surge arresters include: full-current online monitoring, full-current and resistive current live-line testing, and infrared thermal imaging temperature measurement. The specific implementation methods of each testing method differ significantly, and their effectiveness and sensitivity also vary. Full-current and resistive current live-line testing uses a method similar to measuring the dielectric loss of capacitive equipment to detect the leakage current and its resistive current component of the zinc oxide surge arrester. Full-current online monitoring only measures the total leakage current. During normal operation, the valve plate of a zinc oxide surge arrester passes a certain resistive leakage current component, consuming a certain amount of power, causing slight heating of the body. Because the geometric distribution is relatively uniform, the external heating is also uniform. When using an infrared thermal imager for fault diagnosis, abnormal heating, localized temperature increases or decreases, or abnormal temperature distribution can be detected based on the thermal image characteristics, indicating an arrester malfunction. Therefore, infrared thermal imaging temperature measurement, as a non-electrical measurement method, is also an effective means of detecting defects and faults in surge arresters. 2. Feasibility of Replacing Preventive Testing with Live-Line Testing Experience from online monitoring and live-line testing conducted both domestically and internationally shows that live-line testing is more important for analyzing test results and judging development trends. (1) The current transformers were tested multiple times using the relative comparison method for live testing. The results showed that 78.9% of the current transformers had very good repeatability and were similar to the results of the power outage prevention test. 7.9% of the current transformers had very good repeatability but were different from the results of the power outage prevention test. 13.2% of the current transformers had average repeatability (but the difference between the relative values ​​of dielectric loss measured before and after the test did not exceed ±0.3%). The 90-phase 220kV current transformers were tested multiple times. The results showed that 87.8% of the current transformers had good repeatability and were similar to the results of the power outage prevention test. 4.4% of the current transformers had very good repeatability but were different from the results of the power outage prevention test. 7.8% of the current transformers had average repeatability (but the difference between the relative values ​​of dielectric loss measured before and after the test did not exceed ±0.3%). It can be seen that the relative comparison method for live testing of current transformers has a good correlation with the power outage prevention test. Moreover, since there are many differences between live testing and power outage testing, even if the results of live testing differ from those of power outage testing, as long as the data from multiple live tests remain stable and meet the requirements of the live testing guidelines, it can replace the power outage test. (2) The relative comparison method was used to test 34-phase 110kV capacitive voltage transformers multiple times. The results showed that 35.3% of the results had good repeatability and were similar to the results of the power outage prevention test, 29.4% had good repeatability but differed from the results of the power outage prevention test, and 35.3% had average repeatability. The results of multiple tests on 14-phase 110kV coupling capacitors showed that 57.1% of the results had good repeatability and were similar to the results of the power outage prevention test, and 42.9% had good repeatability but differed from the results of the power outage prevention test. It can be seen that the repeatability of the coupling capacitor's multiple live test results is very good, only some of which differ from the power outage test values. It has basically met the conditions to replace the power outage preventive test. Due to the special structure of the capacitive voltage transformer, the damping unit has a greater impact on the measurement results. The damping unit of some products can easily lead to a decrease in measurement sensitivity, and it is still difficult to completely replace the power outage test for the time being. (3) Zinc oxide surge arrester Most zinc oxide surge arresters are equipped with leakage current monitoring instruments, which can be inspected at any time during operation to observe its changes. This current is the total current passing through the surge arrester, of which the main component is capacitive current and the resistive current accounts for a small portion. This current is related to the operating phase voltage, ambient temperature, relative humidity, degree of pollution and the characteristics of the monitoring instrument. It is necessary for the staff to observe carefully and record the relevant parameters. When the reading value increases by 20% compared with the initial value, attention should be paid (the operating voltage at that time should also be taken into account); if it increases by more than 40%, a defect should be reported and tested with a special instrument as soon as possible. Infrared detection should be carried out using an infrared thermal imager and measured once a month. Since the temperature rise is not large, careful observation and differentiation should be made when measuring the temperature. Before temperature measurement, the operation record of the surge arrester current monitor should be checked, and surge arresters with problems should be carefully observed and identified. Currently, there are many instruments used for resistive current live testing of zinc oxide surge arresters, and the measurement principles of various instruments are not entirely the same. There are also many factors affecting the measurement on site. Therefore, the analysis of measurement results should focus on the comparison of historical data, as long-term test results can reflect the changes in the surge arrester insulation. During measurement, operating conditions such as voltage, ambient temperature, atmospheric humidity, and porcelain bushing contamination should be recorded. Many factors affect the on-site measurement results, such as the internal resistance of the counter and the performance of the measuring instrument. For surge arresters with a system nominal voltage of 220kV and above, the influence of the adjacent phase electric field should also be considered. Under normal circumstances, if the three testing methods of full current online monitoring, full current and resistive current live testing, and infrared thermal imaging temperature measurement are consistently used, the insulation status of zinc oxide surge arresters can be detected, replacing conventional power outage preventive testing. In 2003, the Guangdong power grid discovered several defects in 500kV surge arresters through full current inspection, live testing, and infrared detection.