1. Overview In recent years, with the development of urban power grid construction, the number of GIS substations has been increasing. Currently, there are nearly 40 GIS substations in Henan Province, and this number is expected to continue to increase in the next few years. Due to the high operating voltage and extremely limited internal space of GIS equipment, the operating field strength is very high. On the other hand, the insulation margin in GIS equipment is relatively small. Under strictly controlled environmental conditions, the breakdown strength of SF6 gas in GIS equipment can be expected to reach a fairly high level, but in reality, it usually only reaches half of the expected value, or even lower. Once a defect occurs inside the GIS equipment, equipment failure is very likely to occur. With the increase in the number of GIS substations, the probability of GIS equipment failure is also increasing. Because GIS equipment was previously considered maintenance-free, and the number of GIS substations is relatively small, the means and measures for monitoring equipment failures are relatively limited. Therefore, strengthening and improving the handover testing and operation monitoring of GIS equipment is of great significance to ensuring the safe operation of GIS equipment. Studies have shown that insulation failures are the most common internal failures of GIS equipment. In 2001, all three GIS equipment failures in Henan Province were insulation failures. Similar situations have been observed in other provinces in China. Partial discharge in GIS equipment is often a precursor and manifestation of insulation faults. It is generally believed that discharge in GIS equipment decomposes SF6 gas, severely affecting the electric field distribution, leading to electric field distortion, corrosion of insulating materials, and ultimately insulation breakdown. Practice has proven that partial discharge detection can effectively prevent GIS accidents. Currently, relevant departments in Henan Province are researching and formulating specific implementation plans for partial discharge measurement of GIS equipment, intending to include partial discharge measurement in the handover testing and operational monitoring projects of GIS equipment. [b]2 Partial Discharge Detection Methods for GIS Equipment[/b] Partial discharge in GIS equipment is both a precursor to internal insulation faults and a typical manifestation of insulation faults. 2.1 Common Internal Defects in GIS Equipment a. Defects within the solid insulating materials of GIS equipment, such as air gaps remaining inside basin-type insulators or at the interface with conductors during the manufacturing process. b. Residual free conductive particles within GIS equipment, such as metal scraps or metal particles. This is a relatively common defect, generally caused by manufacturing, installation, etc. c. Defects such as burrs and sharp corners on the conductor surface in GIS equipment are prone to corona discharge. While these defects generally do not cause insulation breakdown under stable operating voltages, they can lead to breakdown under impulse voltages. d. Poor conductor contact within GIS equipment. These defects often trigger partial discharge in GIS equipment. 2.2 Characteristics of Partial Discharge Caused by Internal Defects in GIS Equipment a. Corona discharge occurring around the conductor: Because molecules in the gas move freely, the corona discharge process in GIS equipment is similar to corona discharge in air. b. Air gap discharge inside insulators in GIS equipment is basically the same in both positive and negative half-cycles of the power frequency, meaning the discharge fingerprints in the positive and negative half-cycles are basically symmetrical. Discharge pulses generally appear in the rising part of the absolute value of the test voltage amplitude. The discharge frequency depends on the applied voltage. Only when the discharge is strong will it extend to the falling part of the voltage absolute value phase, and the magnitude of each discharge is not equal. Insulator defects may not appear at the factory, but may cause damage during transportation and installation. Some defects may initially be harmless, but may move slightly under mechanical vibration and electrostatic forces, creating potential hazards. c. Defects on the insulator surface (such as contamination) contribute to the increase of surface charge, which may lead to surface discharge, causing insulation degradation and even breakdown. d. The phase distribution of discharges from free conductive particles and metallic protrusions on solid conductors differs significantly. This characteristic can often be used to distinguish the type of defect. Free conductive particles in GIS equipment have the ability to accumulate charge. Under AC voltage, electrostatic force can cause conductive particles to jump within the GIS cylinder, such as through vertical rotation or dancing motions. This movement and the occurrence of discharges are largely random, and the process depends on the applied voltage and the characteristics of the particles. If a jumping particle approaches or moves into a high-field region within the GIS equipment, the accompanying partial discharge may form a conductive path, causing insulation breakdown. Relatively speaking, the various effects produced by residual metal debris or particles within the GIS equipment are the most severe; therefore, discharges from metal particles pose a relatively greater threat to GIS equipment.