Gas Insulated Switchgear (GIS) refers to a metal-enclosed system that uses gas, typically sulfur hexafluoride (SF6), instead of air at atmospheric pressure as an insulating medium. It consists of various high-voltage components such as circuit breakers, busbars, isolating switches, voltage and current transformers, surge arresters, and a casing. This type of system is known as a gas-insulated substation. The use of SF6 gas offers excellent insulation and arc-quenching properties, and all the high-voltage elements are sealed within a grounded metal cylinder. Compared to traditional open-type systems, GIS provides several advantages, including reduced floor space, complete sealing against environmental interference, higher operational reliability, easier operation, longer maintenance intervals, faster installation, lower operating costs, and no electromagnetic interference.
Over the past three decades, GIS technology has advanced significantly and is now widely used in power systems around the world. As global power systems continue to evolve and the demand for reliable operations increases, GIS is expected to become the dominant solution for high-voltage electrical equipment in this century.
Installation of GIS
During the installation of GIS, there are several technical aspects that must be carefully considered. To ensure a smooth installation process, designers need to pay attention to two key factors during the construction design phase. Otherwise, it can lead to numerous challenges during actual installation.
The first factor is the method of lifting GIS equipment. Most indoor installations currently use electric single-girder bridge cranes. These cranes have two speeds, with the slower speed used for fine adjustments when positioning the equipment. For example, projects like the Gongbo Gorge 330kV GIS and the Cotton Beach 220kV GIS have successfully used this approach, proving its effectiveness.
The second important aspect is the pre-burial method of the GIS foundation. While manufacturers usually provide details on load conditions, retention holes, and pre-burial requirements, the specific method is determined by the designer based on the manufacturer's information. Channel steel and bolts are commonly used, but they offer limited adjustability. If bolts encounter reinforcement bars, repositioning may be necessary, which can complicate the installation. In contrast, pre-buried channel steel avoids these issues, making it a more practical choice in many cases.
During the installation, the presence of the designer on-site is crucial. They must understand the three main elements of GIS installation: cleanliness, tightness, and vacuum. These factors are essential for ensuring the quality and long-term performance of the system after operation.
Ensuring cleanliness is one of the most critical tasks in GIS installation. Dust and foreign particles can cause serious issues during pressure tests. Therefore, the installation site should be cleaned thoroughly before starting. Water should be sprinkled to reduce dust, and the area should remain undisturbed for 48 hours. Aluminum conductors, which are often used as electrodes, can leave burrs or chips that act as discharge sources. Special cleaning procedures should be implemented to remove these particles. Additionally, some domestic GIS products may still have debris inside, so strict control and careful handling are required.
Sealing is another crucial factor in GIS performance. Any leakage of SF6 gas can lead to serious failures. The sealing process depends on the welding quality of the tank and the proper installation of sealing rings. Vacuum requirements are also important, as they help reduce moisture content in the system. Maintaining a vacuum of 133 Pa before filling with SF6 is standard practice, followed by continued evacuation for 30 minutes. Moisture can condense on insulators, leading to chemical reactions that degrade insulation materials. Therefore, maintaining low dew point levels is essential.
Testing of GIS
GIS testing includes type testing, factory testing, and field testing. Type testing verifies the product’s correctness and performance, while factory testing checks for any manufacturing defects. Field testing ensures that the equipment remains intact during transportation and installation, and is a vital step before commissioning. Many field test results have shown that common issues include loose parts, scratches on conductive surfaces, cracked insulators, misalignment, and foreign particles entering the system.
According to statistics, about two-thirds of insulation failures in SF6 equipment occur in units that were not tested on-site. Operational data from Canadian utilities show that even after installation, failures can occur in the first few months, accounting for a significant portion of total incidents. Therefore, it is crucial to perform insulation tests before putting GIS into service.
Grounding of GIS
There are two main grounding methods for GIS: single-point grounding and multi-point grounding. Single-point grounding involves grounding one end of each segment, reducing current flow through the housing and minimizing temperature rise. However, this method can result in higher grounding voltages and stronger magnetic fields, which can affect reliability. Multi-point grounding, on the other hand, allows for better distribution of currents, reducing external magnetic leakage and improving safety. While it may increase housing losses, the impact is generally negligible in most projects.
Improvements in GIS Design
Despite the widespread use of GIS, there are still areas in design standards that require refinement. One key issue is the specification of expansion joints, especially for imported systems. Expansion joints are designed to accommodate thermal expansion, foundation movement, and seismic activity. Standards should include quantitative requirements to ensure compatibility between manufacturers and users.
Another area of concern is the material and size of grounding wires. While copper is preferred for its conductivity and corrosion resistance, steel is more commonly used due to cost considerations. However, special connections are needed to prevent galvanic corrosion. Manufacturers often calculate grounding wire dimensions based on thermal stability, but differences in allowable temperature rise can lead to discrepancies in design.
In conclusion, addressing these design challenges is essential to ensure the quality and reliability of GIS systems. By developing clear standards and learning from past experiences, engineers can improve the performance and safety of future installations.
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