Gas-insulated metal-clad switchgear (GIS) is increasingly widely used due to its advantages such as high operational reliability, convenient maintenance, and small footprint. After on-site installation, GIS undergoes a withstand voltage test to verify whether it has been damaged during transportation and installation, and to check the correctness of its reassembly. Generally, GIS that have passed acceptance testing and been put into operation perform well. However, operational experience shows that some defects within the GIS may initially be harmless and difficult to detect. But as the service life increases, factors such as vibration during switch operation, electrostatic forces, movement of foreign debris, or insulation aging can cause localized discharge phenomena, eventually leading to breakdown discharge accidents. This necessitates power outages for GIS maintenance, resulting in significant economic losses. The higher the voltage level of the GIS, the greater the losses caused by power outages. Several insulation faults have occurred in the GIS (Gas Insulator) systems operating in the Guangdong power grid, such as the 500 kV GIS at Jiangmen Substation, the 400 kV GIS at Daya Bay Nuclear Power Plant, the 220 kV GIS at Shenzhen Huanggang Substation, and the 200 kV GIS at Shajiao A Power Plant, all of which experienced insulation flashover faults. In particular, the insulation fault of the grounding switch in the No. 3 main transformer bay of the 220 kV GIS at Shajiao A Power Plant on July 15, 1989, caused a complete power outage at the plant, resulting in the shutdown of two 200 MW generator units for two days and a direct power loss of 32 GWh. Therefore, an effective means of detecting GIS faults is needed in actual operation to detect internal insulation faults early and accurately locate them, enabling planned maintenance, shortening maintenance time, saving maintenance costs, and thus improving the reliability of GIS operation. To this end, research on GIS fault detection has been conducted. [b]1 Characteristics and Signal Propagation Characteristics of Partial Discharge in GIS[/b] 1.1 Characteristics of Partial Discharge in GIS The mechanism of partial discharge can be understood as the phenomenon where only a portion of the insulation system discharges under the action of an electric field, without penetrating the conductors to which the voltage is applied (i.e., it has not yet broken down). This phenomenon is called partial discharge. Partial discharge occurs at the beginning stage of the streamer, which is the same for almost all voltage types, and is lower than the minimum leader initiation voltage and breakdown voltage. Partial discharge may occur on the conductor, or on the surface or inside the insulator. The main form of early faults in GIS insulation is partial discharge, which is mainly caused by the following defects: a) Surface defects of current-carrying conductors. Such as burrs, sharp corners, etc., causing uneven electric field intensity on the conductor surface. These defects are usually caused during manufacturing or installation, and are not likely to cause breakdown under stable power frequency voltage, but are likely to cause breakdown under operating or impulse voltage. b) Surface defects of insulators. Such as poor manufacturing quality, bubbles or cracks in the insulator, and dirt, dust, etc. left over from installation. c) Free conductive particles left inside the GIS cylinder during manufacturing and installation. d) Poor contact of conductive parts. 1.2 Propagation Characteristics of Ultra-High Frequency (UHF) Signals within GIS Partial discharge signals within GIS have an extremely wide bandwidth (greater than 1 GHz). Due to its structural characteristics, it possesses excellent waveconductivity, allowing UHF signals to propagate effectively within it. UHF signals propagate as transverse electromagnetic waves within GIS (i.e., the electric field direction is perpendicular to the propagation direction). When there are no obstructions within the GIS, signal attenuation is minimal. However, attenuation occurs when passing through discontinuous sections or obstructions, such as basin insulators, corners, and T-connections. The UHF signal strength decreases by 3–6 dB for each insulator it passes through, thus allowing accurate fault location determination based on the magnitude of the UHF signal at various points. The high frequency of UHF signals gives them strong penetrating power; when passing through insulators, they can reach the outside of the GIS through the joint between the insulator and the metal flange. Therefore, it is possible to measure the UHF signal inside the GIS from outside the basin insulator. [b]2 Design and Establishment of Partial Discharge Measurement System[/b] 2.1 Design and Manufacturing of GIS Test Section To ensure that the experimental research results match the actual operating conditions, we collaborated with a high-voltage switchgear manufacturer to design and manufacture a 110kV GIS test section. This test section consists of three basin-type insulators, two coaxial sections, and a high-voltage bushing. It is 5 m long, and the pressure on the outer shell is the same as that of an actual 110 kV GIS in operation. This ensures that the measurement results obtained on the test section accurately reflect the discharge characteristics of internal faults within the GIS during operation. To facilitate in-depth research on various measurement system schemes, we designed flat plate electrodes. During the manufacturing process of the test section, three flat plate electrodes (Figure 1) were installed inside the GIS, near the inner wall of the GIS outer shell. [img=350,185]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gddl/2001-5/44-1.jpg[/img] The establishment of the GIS test section provided the research work with realistic and basic conditions. 2.2 Design and Determination of the Measurement System Internal discharge of GIS is accompanied by the generation of sound, light, and electricity, as well as chemical changes in SF6 gas and the propagation of mechanical vibrations in the outer shell. Scientists at home and abroad have studied methods for detecting internal faults in GIS based on the discharge characteristics of GIS. It has been reported that the main detection methods include optical methods, chemical methods, acoustic methods, and electrical methods. However, these methods are not very successful in detecting faults in operating GIS. We have studied, analyzed, and compared the various detection methods mentioned above, and believe that the electrical method can measure the internal partial discharge of GIS more accurately, but it is quite difficult. We have determined the electrical method as the research direction. The GIS fault detection device measurement system consists of a signal detector system, a signal processing system, and a waveform and spectrum analysis system. The first two systems are crucial, as they affect the sensitivity and accuracy of the measurement. The design and research of the signal processing system underwent repeated improvements, ultimately achieving satisfactory results. Through the design, development, and experimental verification of four signal detector systems based on different principles, the planar electrode (also known as the inner antenna) and the loop outer antenna (also known as the outer antenna) were deemed to be the most effective, especially the outer antenna, which is convenient to use and suitable for fault detection in operating GIS systems. [b]3 Simulation Tests of Several GIS Faults[/b] All simulation tests were conducted on a 110 kV GIS test section filled with SF6 gas, and the verification results were consistent with actual operating conditions. Simulation tests were performed on the following faults, and waveform diagrams and spectrum analysis were recorded: a) Metal particle fault simulation test; b) Large metal object fault simulation test; c) Insulator surface creepage discharge fault simulation test; d) GIS inner conductor metal protrusion fault simulation test. Through simulation tests of the above types of faults, various fault waveforms and much valuable data were obtained. It can be concluded that the farther away from the fault point, the weaker the discharge signal strength, thus enabling accurate determination of the fault location. The performance and effectiveness of the four developed signal detector systems were verified. Both the inner and outer antennas could detect partial discharge signals, with the inner antenna exhibiting higher sensitivity, followed by the outer antenna. However, the outer antenna signal detector system is simple in structure and easy to use; simply placing the antenna near the operating GIS basin insulator is sufficient to obtain a signal, requiring no modifications to the GIS, no power outage, and no impact on the normal operation of the GIS. Therefore, the outer antenna signal detector system is highly suitable for fault detection in operating GIS. [b]4 Fault Detection in Operating GIS[/b] The successful simulation tests of several faults in the GIS test section provided technical support and laid a solid technical foundation for on-site testing, promoting the availability of on-site testing conditions. According to statistics, there are approximately 55 110-500 kV GIS substations in Guangdong Province. Starting in the second half of 1998, we conducted on-site testing and surveys. To date, we have inspected 45 GIS substations. Except for one insulator with partial discharge found in the 500 kV GIS outgoing line bay of the pumped storage power plant, and a severe partial discharge above the surge arrester in the 110 kV GIS Zhujiu C line bay of the Zhuhai substation, the other substations are operating well, and no abnormalities have been found. The defects in the 110 kV GIS Zhujiu C line bay of the Zhuhai substation are briefly described below: The 110 kV GIS of the Zhuhai substation is a product of cooperation between Xi'an High Voltage Switchgear Factory and Mitsubishi of Japan, and was put into operation in 1996. The Zhujiu C line bay was only put into operation in early 1999. On July 27, 1999, a live-line test was conducted on the 110 kV GIS at the station. Severe gas cell discharge was found between the surge arrester and the voltage transformer on the Zhujiu C line (see Figures 2 and 3). The discharge signal was very strong, with an amplitude exceeding 50 V. However, the strength of a typical GIS internal insulation fault signal measured with an external antenna is only in the millivolt range, indicating a significant fault with an imminent risk of insulation breakdown. The initial assessment was that the fault point was above the U-phase of the surge arrester, and it was recommended to immediately shut off power and have the manufacturer handle the issue. In early August, the manufacturer sent personnel to handle the problem. Upon disassembly, the fault point (Figure 4) was found to be precisely at the conductor insertion contact above the U-phase of the surge arrester, perfectly matching the detected fault location. [img=250,304]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gddl/2001-5/45-1.jpg[/img][img=259,168]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gddl/2001-5/45-2.jpg[/img][img=300,235]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gddl/2001-5/45-3.jpg[/img][b]5 Conclusion[/b] The GIS fault detection device has a simple structure and is easy to use. During detection, no movement of any GIS component is required, no power outage is needed, and the normal operation of the GIS is not affected. It has high testing sensitivity and accurate fault location. The actual application of this device in 45 GIS stations during power grid operation has yielded excellent results. The application of this device is of great significance to the safe operation of GIS and has broad application prospects. [b]References[/b] ［1］Qiu Yuchang. GIS insulation fault detection and diagnosis technology［J］. High Voltage Apparatus, 1986(3). ［2］Qiu Yuchang. Online monitoring of GIS insulation using ultra-high frequency method［J］. High Voltage Apparatus, 1997(4).