Solid-Insulated Ring Main Units: Current Applications and Future Trends in Medium-Voltage Networks

Current Application Status of Solid-Insulated Ring Main Units in Medium-Voltage Distribution Networks

(1) Solid-insulated ring main units (RMUs) have been widely applied in urban residential areas and other medium-voltage distribution applications. The core components of medium-voltage RMUs mainly include the load switch and the fuse. These units offer advantages such as a simple structure, compact size, and low cost, while effectively improving power supply parameters and enhancing operational safety. Current development shows that the rated current of medium-voltage RMUs can reach up to 1250A, typically 630A. Classified by insulation type, they are mainly divided into air-insulated and SF₆ gas-insulated types, primarily used for switching load currents, interrupting short-circuit currents, and providing control and protection functions.

(2) Vacuum load-switch-based RMUs can form a clear and reliable isolation gap. Common load switches used in air-insulated RMUs include gas-generating, compressed-air, vacuum, and SF₆ types; in contrast, gas-insulated RMUs primarily use SF₆ load switches. Three-position load switches are commonly used in RMUs, enabling load interruption, reliable grounding, and circuit isolation. Among these, gas-generating, compressed-air, and SF₆ load switches can achieve three-position operation.

(3) Practical application schemes for RMUs have become increasingly refined. Due to their compact size and structure, RMUs typically employ simple load switches combined with high-voltage fuses. Under normal conditions, load switches handle load current operations, while fuses rapidly interrupt short-circuit currents. Their combined operation can effectively replace circuit breakers within certain capacity limits. With the advancement of information technology and distribution automation, circuit breakers have become increasingly miniaturized and are now widely used in RMUs. High-performance RMUs should support normal operation, operational maintenance, and main circuit voltage testing.

Development Trends and Technical Characteristics of Solid-Insulated Ring Main Units

(1) Development Trends. The use of SF₆ gas-insulated electrical equipment is expected to gradually decrease. Although SF₆ has been widely used in medium-voltage RMUs due to its excellent overall performance, growing environmental awareness has highlighted its potential harm to the ecosystem and human health. Consequently, the high-voltage electrical equipment manufacturing industry is moving toward reducing SF₆ usage. Both domestic and international manufacturers have anticipated this trend and are actively promoting the research, development, and application of solid-insulated RMUs.

(2) Effective Application of Encapsulated Pole Technology. Solid insulation typically uses epoxy resin as the main insulating material and vacuum as the arc-quenching medium. By actuating the operating mechanism, functions such as load current switching are achieved, effectively controlling the power distribution system and ensuring equipment and personnel safety. The use of solid insulation significantly reduces the required phase-to-phase and phase-to-ground insulation distances within switchgear, shortening air insulation gaps from 125mm to just a few millimeters. Without SF₆ gas, the equipment is more compact compared to traditional C-GIS. Additionally, the simplified operating mechanism reduces the number of components, greatly enhancing mechanical reliability.

(3) Technical Characteristics. The main competitors of solid-insulated RMUs are air-insulated RMUs with SF₆ switches and SF₆ gas-insulated RMUs. Excluding environmental considerations, the advantages of solid-insulated RMUs are evident: first, structural simplification—by eliminating pressurized gas chambers, pressure gauges, and filling valves, reliability is improved, maintenance costs are reduced, and the switch's rated operating conditions are optimized; second, the main switch is equipped with an isolation gap, providing a clearly visible open state that fully meets power grid safety operation requirements; third, strong adaptability, enabling stable operation in harsh environments such as extreme cold and high temperatures, indicating low environmental sensitivity. External insulation primarily uses epoxy resin sleeves or insulating tubes, effectively avoiding SF₆ liquefaction at low temperatures and expansion at high temperatures.

Conclusion

In summary, solid-insulated RMU technology in China has matured, effectively overcoming the limitations of air-insulated and gas-insulated RMUs, and is suitable for special environments such as high altitudes and heavily polluted areas. Its widespread application will strongly promote the development of the power grid system and contribute to sustainable socioeconomic development.