The 33rd China International Exhibition on Electric Power Equipment and Technology
Shanghai International Energy Storage Technology Application Expo / Hydrogen Energy Expo
A Ring Main Unit (RMU) is a factory-assembled, sealed, medium-voltage (typically 11–36 kV) switchgear unit used at distribution network connection points — particularly in urban cable ring main networks — to connect consumers, substations, and renewable energy sources to the distribution system. A standard RMU contains two ring main switches (for the incoming and outgoing ring cable connections) and one or more protection devices (circuit breaker or fuse) for the consumer feeder, all housed in a compact, sealed enclosure filled with SF? or clean insulating gas. RMUs are designed for minimal maintenance, long service life (30+ years), and small footprint, making them ideal for installation in cable distribution boxes, building substations, and underground vaults. Smart RMUs incorporate sensors, communication interfaces, and remote operation capability for integration with distribution automation systems.
5 Key Questions About Ring Main Unit (RMU)
A ring main network is a distribution topology in which substations and consumers are connected in a loop (ring), with the ring normally operated open at one point. This topology provides two supply paths to every consumer — if a fault occurs on one section, the ring can be reconfigured to restore supply from the other direction. Ring main networks offer higher reliability than radial networks (where a single fault causes a complete outage) while using less cable than fully meshed networks. Urban medium-voltage cable networks in China are predominantly ring main topology, with RMUs at each connection point enabling automated fault isolation and supply restoration.
Traditional RMUs use SF₆ (sulphur hexafluoride) gas as the insulation and arc-quenching medium, offering excellent electrical performance in a compact sealed enclosure. However, SF₆ is a potent greenhouse gas (GWP 23,500), and its use is being restricted in Europe and increasingly scrutinised globally. Clean air RMUs use dry air, nitrogen, or fluoronitrile-based gas mixtures as alternatives, eliminating SF₆ while maintaining comparable electrical performance. Major manufacturers including ABB (AirPlus), Schneider (RM AirSeT), and Siemens (8DJH) have launched SF₆-free RMU products. Chinese manufacturers are developing domestic clean air alternatives in response to international market requirements.
Modern smart RMUs incorporate: integrated current and voltage sensors for real-time measurement without external instrument transformers; fault passage indicators (FPIs) that detect and signal the passage of fault current; remote control capability via communication interfaces (IEC 61850, DNP3, Modbus) for integration with distribution automation systems; local HMI displays showing operational status; self-powered operation using current transformers to eliminate the need for auxiliary power supplies; and condition monitoring sensors measuring SF₆ gas pressure, temperature, and partial discharge. These features enable automated fault location, isolation, and restoration (FLISR) without operator intervention.
RMUs are designed for installation in distribution substations, cable distribution boxes, building electrical rooms, and underground vaults. Key installation requirements include: adequate ventilation or cooling to manage heat dissipation; cable entry provisions matching the installed cable system; earthing connections to the substation earth grid; auxiliary power supply for motorised switches and communication equipment (or self-powered operation); and access for maintenance and inspection. In China, RMU installation must comply with DL/T 5103 (Design Code for 35 kV and Below Distribution Substations) and relevant local utility standards.
China's RMU market is one of the world's largest, driven by ongoing urban distribution network modernisation, new urban development, and renewable energy connection projects. The market is transitioning from simple manual RMUs to smart automated units with remote control and monitoring capability, driven by State Grid and Southern Grid's distribution automation investment programmes. Domestic manufacturers including CHINT, Schneider China, and ABB China dominate the market, with increasing competition from specialised domestic companies. The shift to SF₆-free insulation is beginning to influence Chinese procurement specifications, particularly for export-oriented projects.
Key Takeaways
Power cables are the arteries of electrical infrastructure, carrying power from generation to consumption. Underground cable networks are expanding rapidly in China's cities as overhead lines are removed for aesthetic and safety reasons. EP Shanghai connects cable manufacturers with utility buyers, construction companies, and project developers.
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