Introduction
The modern power grid is currently confronted with the dual challenges of enhancing reliability and reducing carbon emissions. Urban substations have extremely limited space resources, while offshore platforms, new energy stations, and industrial users have rigid demands for the compactness and high reliability of switchgear. These practical pressures have prompted engineers to break away from a single technical approach and systematically evaluate the applicability of various insulation switchgear solutions.
Gas Insulated Switchgear (GIS) and Solid Insulated Switchgear (SIS) represent two different insulation philosophies. GIS uses a sealed gas insulation system, historically based on SF6, while SIS uses solid dielectric materials such as epoxy resin to surround and insulate energized components.
The selection of GIS and SIS requires a comprehensive consideration of multiple factors such as voltage level, installation conditions, operation and maintenance strategies, environmental compliance requirements, and the total life cycle cost. Based on objective engineering comparison analysis, this article aims to provide clear technical applicability guidance for utility engineers, EPC contractors, and procurement decision-makers, clearly defining the advantages and disadvantages, as well as the boundary conditions, of each solution in different scenarios.
Understanding Gas Insulated Switchgear (GIS)
Gas-insulated switchgear (GIS) is a type of metal-enclosed switchgear where the live conductors and the grounded enclosure are completely enclosed within an insulating gas medium. A typical GIS unit consists of circuit breakers, disconnectors, grounding switches, busbars, and cable connection modules. Each functional component is installed in an independent airtight compartment.
The core working principle lies in the utilization of the excellent insulation strength of high-pressure gas, which significantly shortens the insulation distance between phases and to ground, thereby achieving a more compact overall layout than traditional air-insulated solutions while ensuring the rated electrical performance.
With its compact structure, GIS is widely applicable to power engineering scenarios where space is limited or environmental conditions are harsh. Taking underground substations in densely populated cities as an example, due to the rigid constraints of the existing building layout and underground civil engineering conditions, there is often no room for expansion in the installation space. In such cases, compact switchgear is not an optional upgrade option but a necessary condition for meeting the feasibility of the project.
Offshore wind power platforms represent another typical application scenario. Such facilities not only have extremely limited space for equipment installation, but also face severe challenges such as poor accessibility for maintenance, salt fog corrosion, and high humidity environments. The fully sealed metal enclosure structure of GIS can effectively resist external environmental erosion, ensuring the operational reliability of equipment in long-term high-salt and high-humidity conditions. Therefore, it has become the preferred solution for the collection system of offshore wind power.
SF₆ has long dominated the GIS system due to its excellent insulation strength and breaking capacity. However, its extremely high global warming potential (GWP) is forcing the industry to re-examine the gas usage strategy, accelerating the research and application of leak control, recycling and reuse, as well as environmentally friendly alternative technologies.
What Is Solid Insulated Switchgear (SIS)?
Solid Insulating Switchgear (SIS) replaces the traditional gas or air as the main insulating medium with solid dielectric materials. Its typical structure involves the overall encapsulation of conductors and switch components with epoxy resin and composite insulating materials. Through the high dielectric strength of the solid medium, electrical isolation between the charged body and the grounded enclosure is achieved, ensuring compact insulation performance without the need for pressurized gas.
The structural design of SIS has abandoned the traditional airtight compartment architecture. Each conductive component is tightly encapsulated by solid insulating media, forming a highly integrated solid-state insulation system. This technical approach fundamentally eliminates the management processes of SF₆ gas during the entire life cycle of equipment manufacturing, operation, and decommissioning, including procurement, testing, gas replenishment, and disposal. It significantly reduces the carbon footprint and operational complexity of switchgear.
One of the core advantages of SIS lies in its inherently SF₆-free insulation system, which completely eliminates the need for leak monitoring, gas recovery device configuration, and SF₆-specific handling procedures during operation. However, the performance of the solid insulation solution is highly dependent on the precise control of the manufacturing process – insulation reliability is directly influenced by the consistency of material batches, the accuracy of molding processes, and the design of electric field matching at the solid-solid/ solid-gas interfaces. This places a higher threshold on the production process capabilities of manufacturers.
The core concerns of the SIS project are: partial discharge, resin quality, thermal performance, mechanical strength and long-term insulation reliability.
Technical Comparison: GIS vs. SIS
The following table compares key differences between GIS and SIS technologies.
| Category | GIS | SIS |
| Insulation medium | Pressurized insulating gas, commonly SF6 or alternative gases | Solid dielectric materials such as epoxy resin or composite insulation |
| Environmental consideration | Requires gas monitoring, leakage control, and gas management procedures | No SF6 gas leakage monitoring required during operation |
| Physical footprint | Extremely compact, especially for high-voltage installations | Compact medium-voltage design with dimensions depending on configuration |
| Maintenance focus | Gas density checks, leakage inspection, breaker and mechanism maintenance | Insulation condition, mechanical checks, and condition monitoring |
| Voltage range | Medium voltage to ultra-high voltage applications | Primarily medium-voltage applications |
| Typical applications | Urban substations, underground facilities, offshore platforms | Indoor MV systems, renewable projects, environmentally sensitive installations |
The selection of GIS and SIS should not be based solely on individual technical indicators, but rather should be comprehensively evaluated based on the overall demand system of the project. When high voltage levels, compact footprint and sealing protection constitute rigid constraints, GIS holds an advantage due to its mature gas-insulation system; while when the absence of SF₆ operation, low carbon footprint and solid sealing insulation technology become the core design goals, SIS demonstrates its value through its inherent environmental-friendly characteristics.
Environmental Regulations and the SF6-Free Transition
SF₆, due to its extremely high global warming potential (GWP), has become the central focus of environmental issues in the switchgear industry. Regional and global regulations are continuously exerting pressure, urging manufacturers and utility companies to reduce the usage of SF₆, enhance leak monitoring, and carefully evaluate various alternative technical solutions.
The currently feasible alternative paths can be mainly classified into two categories: the first is the environmentally friendly GIS that uses low GWP insulating gases (such as fluorinated acrylonitrile mixtures, synthetic air, etc.); the second is the solid insulation switchgear (SIS) that completely eliminates gas media.
It is important to note that the transition to a non-SF₆ solution does not imply a complete replacement of all in-service GIS systems. Engineers still need to conduct a comprehensive assessment of key dimensions such as electrical performance, operational reliability, voltage level compatibility, installation conditions constraints, and life cycle economics based on the actual project situation before making a rational decision.
How to Choose the Right Insulated Switchgear for Your Project
The selection of insulation switchgear must be based on the engineering assessment of the system. The procurement decision should focus on the overall performance and total cost of ownership (TCO) of the equipment throughout its operational cycle, rather than relying solely on the price tag as the sole criterion.
- Confirm voltage level and fault duty requirements. Determine whether the project requires GIS high-voltage capability or fits SIS medium-voltage applications.
- Evaluate environmental conditions including temperature, humidity, pollution, chemical exposure, and installation location.
- Review compliance requirements related to SF6 restrictions, carbon reduction targets, and local regulations.
- Perform lifecycle cost analysis covering initial investment, maintenance requirements, environmental management, and long-term operation.
- Assess supplier engineering capability, testing resources, customization ability, and technical support.
Important buyer evaluation factors include:
- Rated voltage and current requirements
- Short-circuit withstand capability
- Installation footprint limitations
- Environmental conditions
- Monitoring and diagnostic requirements
- Future expansion planning
Future Development of Insulated Switchgear Technology
The technological evolution of insulating switchgear will continue to be driven by multiple factors such as increasingly stringent environmental regulations, the construction of smart grids, and the growing demand for compact power infrastructure. Under this trend, GIS will maintain its irreplaceable position in large-capacity power transmission and scenarios with extremely limited space by virtue of its high voltage level and extremely compact layout capabilities. At the same time, SIS is expected to gain more widespread attention in the medium-voltage distribution field, especially in projects where the absence of SF₆ operation and gas-free management are the core design goals, where its technical advantages will become increasingly prominent.
Technologies such as digital online monitoring, health status assessment, and intelligent adaptive control will increasingly be deeply integrated into the integrated solutions of GIS and SIS. In the future, switchgear will no longer merely be the execution unit for power on-off, but will also become intelligent terminal nodes with perception, diagnosis, and collaborative capabilities.
Final Thoughts
GIS and SIS represent two distinct technological approaches to addressing the insulation requirements of modern power grids. GIS maintains its irreplaceable advantages in high-voltage scenarios and those with extremely limited space, while SIS provides a reliable and SF₆-free insulation alternative for numerous medium-voltage applications. The final selection must be based on a systematic engineering trade-off considering electrical parameters, environmental conditions, operation and maintenance strategies, and the total life cycle cost – no insulation technology can be suitable for all project scenarios.
KENJING offers medium-voltage switchgear solutions that cover two major technical routes: gas-insulated and solid-insulated. The product line includes GIS-type and SIS-type ring-main units (RMU), and customized insulation accessories can also be provided. The company relies on the project-based consulting model to provide users with full-process technical support from equipment selection consultation to system configuration design, assisting engineers in building a switchgear architecture that meets the project requirements.
FAQs
What is the main difference between GIS and SIS?
GIS uses gas insulation inside sealed compartments, while SIS uses solid dielectric materials to insulate energized components.
Are solid insulated switchgears completely SF6-free?
Yes, SIS designs based on solid insulation and vacuum interruption do not require SF6 gas insulation.
Which insulation type is more cost-effective?
The answer depends on the project. Initial equipment cost, maintenance requirements, environmental compliance, and lifecycle costs should all be considered.
