[b]1 Application and Advantages of GIS Equipment[/b] SF6 gas-insulated fully enclosed switchgear (GIS) features small size and excellent technical performance. Currently, in Guangzhou, in addition to substations such as Tianhe, Luming, and Wuxianmen, GIS equipment has recently been adopted at the Zhujiang New Town 220 kV Tancun substation, the old city 10 kV step-up substation, the 220 kV Liwan substation, and the Fangcun 220 kV Huadi substation. To reduce building area and control building height, meet urban planning requirements, and harmonize with the surrounding environment for urban beautification, GIS equipment is being considered for 110 kV terminal substations in Guangzhou's urban area, such as the planned Baogang, Jiaochang, Ersha, Xichang, Yushu, Huasui, and Zengjiao 110 kV substations. With the increasing use of GIS equipment, experience should be continuously summarized, and the design should fully consider the convenience of construction, operation, and maintenance to further optimize and improve the design. The main advantages of GIS are: a) Small footprint: A 220 kV GIS device typically occupies only 37% of the area of ​​a conventional device; a 110 kV GIS device occupies approximately 46%, aligning with my country's basic policy of saving land and reducing expensive upfront costs such as land acquisition, demolition, and compensation. b) Because the components of GIS equipment are fully enclosed, they are unaffected by pollution, salt spray, humidity, or other environmental factors. The conductive parts of the GIS equipment are shielded by the outer casing, ensuring good grounding. Radiation from the conductors, electric field interference, and circuit breaker noise are all shielded by the casing. Furthermore, the GIS equipment is securely installed on pre-embedded foundation components, resulting in a low center of gravity, high strength, and excellent seismic resistance, making it particularly suitable for use in urban centers or residential areas. Compared to conventional equipment, GIS is more likely to meet urban environmental protection requirements. c) SF6 gas, as an insulating medium, is non-flammable, has good fire resistance, and possesses excellent insulation and arc-extinguishing properties. It is safe and reliable in operation, requires less maintenance, and has a long maintenance cycle, making it suitable for unmanned substation operation, achieving the goal of reducing manpower and increasing efficiency. d) Short construction period. The components of GIS equipment are highly versatile, using a modular structure and assembled in a single transport unit, which is then transported to the construction site and fixed in place. This reduces on-site installation workload by approximately 80% compared to conventional equipment. [b]2 Selection of Main Wiring for GIS Equipment[/b] The selection of the main wiring for GIS equipment should adhere to the design principles of substation electrical main wiring—reliability, flexibility, and economy. Given the characteristics of GIS equipment—few failures, long maintenance cycles, and high operational reliability—its main wiring can be simplified. For example, 110 kV and 220 kV distribution devices generally do not require bypass busbars, but GIS equipment has a larger power outage range than conventional equipment when a fault occurs. When a partial fault occurs in GIS equipment, the SF6 gas in the faulty gas chamber must be completely extracted during maintenance of the faulty component. Since the distance between the conductive contacts of GIS equipment is designed for SF6 gas at a certain pressure, it is much smaller than that under air insulation. Therefore, the busbar of this gas chamber must be de-energized for maintenance. This requires substation operation, maintenance, and repair personnel to be familiar with the working principles and structure of GIS (Gas Insulated Switchgear). Otherwise, misoperation or human-caused internal short circuits can easily occur. The two 110 kV GIS internal short circuit accidents at Wuxianmen illustrate the importance of paying special attention to this point during substation operation and maintenance. Therefore, the main wiring of GIS equipment should not be overly simple. Both 110 kV and 220 kV busbars should adopt segmented wiring to avoid localized faults causing a complete busbar outage and expanding the fault range. Currently, most 110 kV and 220 kV GIS equipment in large hub substations uses single-busbar segmented or double-busbar segmented wiring. Single-busbar segmented wiring is simple, economical, and convenient, suitable for substations with four 110 kV or 220 kV feeders. Double-busbar segmented wiring allows for rotating busbar maintenance, flexible scheduling, convenient expansion, and easy testing, suitable for substations with six or more 110 kV or 220 kV feeders. For a typical 220 kV terminal substation, the 220 kV side can use a line-transformer group connection method, while the 110 kV side uses a single busbar segmented connection method. The 110 kV GIS equipment in the 110 kV terminal substation still considers using a line-transformer group connection method, and the 10 kV section of the terminal substation uses a single busbar segmented connection, with each section serving as a backup. 3. Issues to be noted in the design and operation of substation GIS 3.1 Overall Layout When arranging the GIS room indoors, the width of the GIS room is determined by the width of the GIS equipment and the width of the passageway. The width of the working passageway should consider that when using cables as incoming and outgoing lines, since the standard values ​​for AC withstand voltage tests of GIS equipment and high-voltage cables are different, temporary SF6 air bushings need to be installed for on-site withstand voltage tests. Attention must be paid to the safe distance between live parts and walls. The width and turning radius of the mobile SF6 gas recovery device, as well as the width required for maintenance and handling of GIS components, must also be considered. The height of the GIS room must consider the feasibility of equipment transportation, installation, overhaul, and testing. During maintenance, circuit breakers and other components should be able to be lifted as a whole, and attention should be paid to the safe distance between live parts and the roof during on-site withstand voltage tests. When GIS is installed outdoors, the width of the passageway and the height of the overhead jumpers should allow for operations such as vehicle lifting, and generally, the distance from the outer edge of the GIS should not be less than 3.5 m. The overall layout of the GIS must also consider factors such as power cable laying, overhead line routing, and the arrangement of control and relay protection panels. 3.2 Ventilation of the GIS Room Relevant standards stipulate that the volume fraction of SF6 gas in the GIS room should be ≤1000 mL/m³, and the oxygen content in the air should not be less than 18%. Therefore, ventilation equipment must be installed in the GIS room, with a ventilation volume 3 to 5 times the volume of the GIS room. This is to ensure ventilation before personnel enter the GIS room under normal conditions, and to reliably expel SF6 gas in the event of a GIS casing rupture. According to the requirements for ventilation, indoor ventilation outlets are usually arranged in the upper part of the room. However, since SF6 gas is about 5 times heavier than air, it tends to accumulate in cable trenches or low-lying areas near the floor. Therefore, the outlets of ventilation equipment should be located in the lower part of the GIS room to ensure rapid and reliable exhaust of SF6 gas. 3.3 Civil Engineering Design of GIS Equipment GIS equipment consists of rigid components connected by bolts. To prevent SF6 gas leakage, the vertical error of the busbar flange connection should not exceed 0.5 mm, and the horizontal error between the pre-embedded channel steel in the foundation should not exceed 2 mm. For ease of construction and installation, ground anchors should be pre-embedded at both ends and appropriate locations within the GIS room to facilitate the placement of the GIS equipment. The interior decoration of the GIS room is primarily for moisture and dust prevention. During on-site installation and maintenance of GIS, the dust content in the air should generally not exceed 0.1 mg/m³, and the relative humidity should not exceed 70%. Therefore, the GIS room should not use easily dusty cement floors or lime walls, and unnecessary doors and windows should be minimized. 3.4 Faults in GIS Equipment GIS is a high-voltage electrical equipment characterized by high reliability, low maintenance workload, and long repair cycles, with a failure rate only 20%–40% of conventional equipment. However, GIS also has inherent drawbacks. Factors such as SF6 gas leakage, external moisture infiltration, the presence of conductive impurities, and insulator aging can all lead to internal flashover faults in GIS. The fully sealed structure of GIS makes fault location and repair relatively difficult, resulting in complex repair work. The average power outage repair time after an accident is longer than that of conventional equipment, and the outage area is larger, often involving non-faulty components. Internal flashover faults in GIS equipment typically occur within the first year of operation after installation or major overhaul. According to statistics, the failure rate in the first year is 0.53 times/interval, decreasing to 0.06 times/interval in the second year, and then stabilizing. Based on operational experience, the failure rates of disconnecting switches and basin-type insulators are the highest, at 30% and 26.6% respectively; the busbar failure rate is 15%; the voltage transformer failure rate is 11.66%; the circuit breaker failure rate is 10%; and the failure rate of other components is 6.74%. Therefore, during the first year of operation, operators should strengthen daily inspections, especially the inspection of disconnecting switches. During inspections, they should pay attention to changes in SF6 gas pressure, abnormal sounds (changes in sound quality, differences in duration), overheating, abnormal odors, rust, etc. If there are any abnormalities in the GIS, the suspected equipment must be tested in a timely manner. [b]4 Conclusion[/b] With the development of cities, the application of GIS in substations will become more and more widespread. Therefore, continuous summarization from all aspects such as design, installation, operation, maintenance, and repair is necessary to enable GIS equipment to play a greater role. [b]References[/b] ［1］ Li Ming, Huang Weishu. Operation of SF6 Gas and SF6 Gas Insulated Substations［M］. Beijing: Water Resources and Electric Power Press, 1993. ［2］ Luo Xuechen. SF6 Gas Insulated Fully Enclosed Switchgear (GIS)［M］. Beijing: China Electric Power Press, 1999.