Comparison of SF6 Gas-Insulated Ring Main Units and Solid-Insulated Ring Main Units

Introduction​
Currently, SF6 gas-insulated ring main units (hereinafter referred to as "SF6 RMUs") dominate the market. However, SF6 gas is internationally recognized as one of the primary greenhouse gases. To achieve environmental protection and emissions reduction, its use must be reduced and restricted. The emergence of solid-insulated ring main units (RMUs) has addressed the issues associated with SF6 RMUs while incorporating numerous new features.

​1 Ring Mains Power Supply and Ring Main Units (RMUs)​​
The "urbanization" process places ever-increasing demands on power distribution reliability. More users require dual (or multiple) power source supplies. Employing a "radial power supply" system can lead to difficulties in cable installation, troubleshooting challenges, and inconvenience during grid upgrades and expansions. Conversely, a "ring mains power supply" can conveniently provide dual (or more) power sources for critical loads, simplifies distribution lines, facilitates cable routing, reduces switchgear requirements, lowers failure rates, and makes fault point identification easier.

​1.1 Ring Mains Power Supply​
Ring mains power supply refers to a system where two (or more) outgoing lines from different substations or different busbars within the same substation are interconnected to form a loop for power supply. Its advantages include: each distribution branch can draw power from the main feeder on its left side or the main feeder on its right side. This means if a fault occurs on either main feeder, power can continue to be supplied from the other side. Although it is single-circuit power supply in essence, each distribution branch effectively gains the benefits similar to dual-circuit supply, significantly improving reliability. Regulations in China stipulate that the main ring mains connection in cities follows the "N-1 Security Criterion". This means that if there are N loads on the line, when any one load experiences a fault, the system can accept the transferred load, ensuring the remaining "N-1" loads continue to receive safe power supply without causing outages or load shedding.

​1.2 Ring Mains Connection Methods​

• ​Standard Ring Connection:​​ Supplied by a single source, forming a ring through the cables themselves, ensuring reliable power supply to all other loads when a section of cable fails.
• ​Ring Connection from Different Busbars:​​ This connection has two power sources, typically operated in open-loop, offering higher supply reliability and greater operational flexibility.
• ​Single Ring Connection:​​ Power sources are taken from different substations or two busbar sections. When any cable section in the network is under maintenance, it does not cause any load outage.
• ​Double Ring Connection:​​ Each load can receive power from an independent ring network, providing very high reliability.
• ​Dual Source Double "T" Connection:​​ Two cable circuits are connected from different busbar sections. Each load can draw power from both cables. This method essentially achieves no outage for dual-source users and is particularly suitable for certain critical users.

​1.3 Ring Main Units (RMUs) and Their Features​
RMUs refer to switchgear cabinets used for ring mains power supply. Cabinet types include load switches, circuit breakers, load switch + fuse combinations, combination apparatus, bus couplers, metering units, voltage transformers (VTs), etc., or any combination or extension thereof.

RMUs feature a compact structure, small footprint, low cost, easy installation, and short commissioning times, meeting the requirement for "equipment miniaturization". They are widely used in residential complexes, public buildings, small & medium enterprise substations, secondary switching stations, compact substations, and cable junction boxes.

​1.4 RMU Types​

• ​Air-Insulated RMUs:​​ Use air as the insulation medium. They have a large footprint and volume, and are susceptible to environmental influences.
• ​SF6 RMUs:​​ Use SF6 gas as the insulation medium. The main switch is enclosed within a sealed metal shell filled with SF6 gas, with the operating mechanism located outside the shell. Due to the sealed enclosure, they are unaffected by the external environment. Their volume is significantly smaller than standard air-insulated RMUs, making them the most commonly used type currently.
• ​Solid-Insulated RMUs:​​ Use solid insulation materials as the main insulation medium. The switch and all live parts are encased or potted with insulating materials like epoxy resin. Because the safe phase-to-phase and phase-to-ground insulation distances inside the switch are reduced, their size and volume are similar to SF6 RMUs. They produce no SF6 emissions and achieve true maintenance-free operation.

​2 Usage Limitations of SF6 RMUs​
SF6 is a major contributor to atmospheric greenhouse effects. However, SF6 possesses ideal electrical properties (excellent insulation, arc-quenching, and cooling performance), strong electronegativity, good thermal conductivity and stability, is reusable, insensitive to ambient conditions (humidity, pollution, high altitude), and enables compact cabinet designs. Consequently, it is widely used as an insulation and arc-quenching medium in electrical equipment. SF6 consumption is highest in the power industry; statistics indicate that 80% of the SF6 gas produced annually is used in electrical equipment.

The United Nations Intergovernmental Panel on Climate Change (IPCC) and the US Environmental Protection Agency (EPA) both classify SF6 as an extremely harmful and impactful greenhouse gas. The EU F-Gas Regulation (2006) stipulates: except for power switchgear where no viable alternative exists, the use of SF6 is prohibited in most fields.

Furthermore, SF6 RMUs are complex to use and require substantial investment, necessitating many auxiliary devices:

• ​Purchase SF6 Leakage Monitoring Equipment:​​ Used for SF6 gas leak detection, monitoring SF6 concentration and oxygen content, detecting trace moisture, etc.
• ​Equip SF6 Recovery Units:​​ By-products like SF4 are generated within the gas compartment during the SF6 arc-quenching process. Therefore, at end-of-life, not only must the remaining SF6 gas be recovered, but the residual toxic by-products require special treatment.
• ​Configure SF6 Gas Purification Equipment:​​ To purify and recycle the SF6 gas.
• ​Install Ventilation Equipment in Substations.​​

When using SF6 RMUs, it is imperative to:

• ​Minimize SF6 Leakage:​​ SF6 RMUs use pressurized sealed cavities, but gas leakage is unavoidable. Performing switching operations when SF6 pressure is low results in low reliability, directly threatening operator safety and reducing equipment lifespan.
• ​Before workers enter the substation, forced ventilation must be performed first, and they must wear special protective gear.​​
• ​Operations and procedures are complex, requiring repeated training for relevant personnel.​​

​3 Characteristics and Applications of Solid-Insulated RMUs​
The potential environmental threat of SF6 RMUs limits their further development. Finding alternatives to SF6 has been a subject of research worldwide. Solid-insulated RMUs were first developed and introduced by Eaton Corporation (USA) in the late 1990s. During operation, they do not generate any toxic or harmful gases, have no environmental impact, offer higher reliability, and achieve true maintenance-free operation.

Solid-insulated RMUs refer to systems where primary conductive circuits—such as the vacuum interrupter, disconnector, earthing switch, main busbar, branch busbar—are individually or in combination enclosed with solid insulation materials like epoxy resin. They are encapsulated within fully insulated, sealed functional modules that can be further combined or expanded. Exterior surfaces of the modules accessible to personnel are coated with a conductive or semi-conductive shielding layer and can be directly and reliably earthed.

​3.1 Characteristics of Solid-Insulated RMUs​

• ​Eco-Design:​​ Does not use SF6 as insulation or switching medium. Instead, vacuum is used as the arc-quenching medium for switches, and environmentally friendly materials with no environmental impact (and which are recyclable) are used as the main insulation medium. Furthermore, the number of components is minimized to ensure low energy consumption during operation and fewer potential failure points.
• ​Truly Maintenance-Free:​​ Solid-insulated RMUs eliminate the SF6 pressure vessel. The internal insulation and arc-quenching of the switch body utilize a vacuum medium; external insulation uses solid dielectric materials like insulating cylinders. The insulating cylinder employs solid-casting technology, integrating the vacuum interrupter, main conductive circuit, and insulating supports into a single unit, sealed within a metal enclosure, unaffected by the external environment. Due to the fully insulated and sealed overall structure, and the absence of issues like SF6 leak detection, refilling, and waste disposal, true maintenance-free operation is achieved.
• ​High Cost-Effectiveness:​​ Although the initial investment for solid-insulated RMUs is slightly higher than for SF6 RMUs, the total life-cycle cost is significantly lower, as shown in Table 1. User considerations are increasingly comprehensive, encompassing not only the initial purchase price but also overall life-cycle costs, including safety risks, grid quality, cost control, and sustainability. The costs required for maintaining, refilling gas, handling leaks, and final recovery of SF6 RMUs during their lifespan are almost equivalent to the purchase cost. In contrast, solid-insulated RMUs involve one initial investment with essentially no subsequent costs. Therefore, from a long-term perspective, the economic efficiency of solid-insulated RMUs is far superior to that of SF6 RMUs.

​Table 1: Life-Cycle Cost Comparison between SF6 RMUs and Solid-Insulated RMUs​

• ​Compact Structure:​​ Designed to be as compact as possible while ensuring cabinet safety and ease of operation. Their footprint and volume are even smaller than SF6 RMUs, helping users save space and providing direct economic benefits.
• ​Internal Arc-Resistant Design, Safer and More Reliable:​​ For primary and secondary switchgear, significant damage due to internal arcing occurs at least once annually. Most solid-insulated RMUs incorporate internal arc-resistant design. When an internal arc occurs, its impact on the RMU is minimized as much as possible, ensuring safer and more reliable equipment operation.
• ​Visualized Isolation Gap:​​ Features a visual observation window for easily checking the contact status of the internal triple-position disconnector, providing visible isolation on-site and enhancing operator safety.
• ​Smart Capabilities:​​ Easier to implement distribution automation compared to SF6 RMUs. After installing a Distribution Terminal Unit (DTU) and communication devices, functions like status data acquisition and monitoring, "Four-Remote" functions (remote signaling, remote measurement, remote control, remote regulation), communication, self-diagnosis, and logging/reporting can be readily achieved.

​3.2 Application Status​
Currently, the widespread adoption of solid-insulated RMUs is constrained by their relatively higher price and complex manufacturing processes. Their process requirements exceed those of SF6 gas-insulated RMUs. If process techniques are inadequate, insulation risks, failure probabilities, and hazards may be higher than SF6 RMUs, necessitating strict quality control of raw materials and craftsmanship. Additionally, the wiring flexibility of solid-insulated RMUs can be limited, particularly for functional units like PT (VT) cabinets and metering cabinets, offering fewer connection options and limiting user choice, which also somewhat restricts the application and development of solid-insulated RMUs.

With continuous optimization of production structures and increasing standardization in product manufacturing, the product quality of solid-insulated RMUs is becoming more stable, and prices are gradually decreasing. Some countries offer incentives of 5%~10% for products that do not use SF6, to reduce its usage and emissions. This means users don't solely consider purchase costs in decision-making. We can also learn from international practices: prioritize the use of solid-insulated RMUs in environmentally sensitive projects and new projects (e.g., residential communities, public buildings, municipal construction), while gradually phasing out SF6 RMUs. Phase out and replace aging or operational SF6 RMUs according to their manufacturer-promised lifespan and provide subsidies to users adopting eco-friendly solid-insulated RMUs to support such products. As user environmental awareness grows and life-cycle cost considerations increase, the prospects for solid-insulated RMUs are broad.

​4 Conclusion​
Solid-insulated RMUs are technically equivalent to SF6 RMUs and possess some features that SF6 RMUs lack, such as no harmful gas emissions, true maintenance-free operation, and lower total life-cycle cost. They are increasingly garnering user attention and preference.