Gas Insulated Switchgear (GIS) refers to a metal-enclosed switchgear that uses gas instead of air at atmospheric pressure as an insulating medium. It is a high-voltage power distribution system composed of components such as circuit breakers, busbars, isolating switches, voltage transformers, current transformers, surge arresters, and enclosures. This system is known as a Gas Insulated Substation. GIS utilizes sulfur hexafluoride (SF6) gas, which has excellent insulation and arc-quenching properties, as the dielectric and arc-quenching medium. All high-voltage electrical components are sealed within a grounded metal cylinder, offering advantages over traditional open-type systems, including compact space requirements, complete component sealing free from environmental interference, high operational reliability, convenient operation, long maintenance intervals, low maintenance workload, rapid installation, and minimal electromagnetic interference.
Over the past 30 years, GIS technology has advanced significantly and is now widely applied in power systems around the world. As global power systems evolve and operational reliability demands increase, GIS technology will continue to develop and become the dominant solution for high-voltage electrical equipment in this century.
Installation of GIS
Several technical aspects must be considered during GIS installation and testing. To ensure smooth implementation, designers should pay close attention to two key factors during the construction design phase, as neglecting them can lead to significant challenges during installation.
The first factor is the lifting method for GIS. Most indoor GIS installations use electric single girder bridge cranes, with two lifting speeds. The lower speed is primarily used for precise positioning. Projects like the Gongbo Gorge 330kV GIS and the Cotton Beach 220kV GIS have successfully adopted this method, proving its effectiveness.
The second factor is the pre-buried method for the GIS foundation. Manufacturers usually provide load conditions, retention holes, and pre-burial requirements, but the actual pre-burial method is determined by the designer based on this information. Channel steel and bolts are commonly used, though they offer limited adjustability. If bolts encounter reinforcement bars, adjustments may be necessary, increasing complexity. In contrast, pre-buried channel steel avoids these issues, making it more widely used in practice.
During GIS installation, the designer’s presence on-site is often required. At this stage, three key elements must be understood: cleanliness, tightness, and vacuum. These are critical to ensuring the quality of GIS after commissioning.
Ensuring cleanliness is one of the most important tasks in GIS final assembly and on-site installation. Due to less-than-ideal site conditions, dust control is essential. Water should be sprinkled on the site before installation, and the area should remain undisturbed for 48 hours. Aluminum tubes may have burrs or chips that can cause discharges during pressure tests, so special attention must be given to cleaning conductors. Manufacturers should also implement additional cleaning methods, such as vibration techniques, to remove residual particles. Some domestic GIS products still have debris when shipped, and poor site management increases contamination risks. Strict adherence to cleaning procedures is crucial to avoid costly rework and delays, as seen in the Wanjiazhai GIS case where internal debris caused multiple discharges during testing.
Sealing is vital for GIS insulation, as SF6 gas leakage can result in catastrophic failures. Sealing checks should be conducted throughout the manufacturing and installation process. The primary factor affecting the seal is the welding quality of the tank, followed by the manufacture and installation of sealing rings.
Vacuum requirements are another critical factor in the final assembly and installation process. A vacuum of 133 Pa is typically required before filling with SF6 gas, followed by a 30-minute vacuum period. Moisture control is essential, as excess moisture can lead to condensation on insulators, forming harmful compounds like HF. IEC standards recommend that new SF6 gas should not exceed -5°C at rated density to prevent moisture-related issues.
Testing of GIS
GIS testing includes type testing, factory testing, and field testing. Type testing verifies product correctness and performance, while factory testing checks for defects. Field testing ensures that no damage occurred during packaging, transportation, or installation, making it a critical step before commissioning.
Field test results show that common issues include loose insulation parts, scratched surfaces, misaligned electrodes, and foreign particles entering the system. These factors can cause insulation failure and are classified into active (free particles) and fixed (installation-related) defects. Statistics indicate that two-thirds of SF6 equipment insulation failures occur in units not tested on-site. Operational experience shows that up to 67% of accidents happen within the first four months of operation, highlighting the importance of on-site testing before commissioning.
Grounding Issues in GIS
There are two main grounding methods for GIS: single-point and multi-point grounding. Single-point grounding involves insulating one end of each segment and grounding the other at a single point. While it reduces casing temperature and losses, it can lead to higher grounding voltages and stronger magnetic fields, reducing reliability. Multi-point grounding, on the other hand, connects the casing to ground at multiple points, improving safety and reducing magnetic leakage. Though it increases casing loss, this is negligible in most projects, such as the Guangzhou Pumped Storage Power Station, where losses are only 2.43–3.79W/(m·ph).
Improvements in GIS Design
Despite extensive engineering experience, some design standardization gaps remain. For example, expansion joints require clear technical specifications, especially for imported equipment. Standards should include quantitative calculations to ensure safe and reliable operation. Additionally, grounding wire materials and dimensions need clearer guidelines. While copper offers better conductivity and corrosion resistance, steel is more cost-effective. However, proper connections between copper and steel are essential to prevent corrosion. Current Chinese regulations do not fully align with manufacturer requirements, leading to potential mismatches. Addressing these issues through standardized guidelines will improve design quality and reduce operational risks.
By addressing these challenges and refining design standards, the reliability and efficiency of GIS systems can be significantly enhanced, ensuring safe and long-term operation in modern power systems.
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