Current Status and Improvement Measures for SF₆ Ring Main Units
1 Current Status of SF₆ Ring Main Units
1.1 Structure of Ring Main Units
Ring main units (RMUs) consist of a gas chamber, operating mechanism compartment, cable compartment, and busbar connection compartment (present in some units).
The gas chamber is positioned at the rear or in locations less prone to harm personnel during internal arcing. The operating mechanism, cable, and busbar connection compartments are at the front, facilitating switch operation, cable installation, and space-efficient wall mounting, significantly reducing maintenance and operational space requirements.
1.2 Gas Chamber and Key Internal Components
Gas chambers are primarily made of epoxy resin or stainless steel. Epoxy-resin cast chambers offer stable airtightness during production and achieve IP67 protection, whereas stainless steel chambers exhibit inconsistent airtightness due to welding variations across manufacturers or batches.
Load switch disconnectors employ diverse designs: RM6 and M24 RMUs use double-break disconnectors, reducing arc extinguishing demands per break and enhancing voltage withstand capability. GA, GE, and GAE RMUs feature single-break disconnectors with larger contact gaps (50mm) and two specialized rivets on moving contacts to prevent welding during arc suppression or short-circuit conditions, ensuring stable contact resistance throughout their service life.
Busbar designs vary by cabinet structure and connection methods, but managing electric/magnetic field effects on feeder units remains critical.
1.3 Product Lineup
Manufacturers combine functional units to fulfill diverse user needs: dual-power transfer cabinets, busbar section cabinets, cable incoming cabinets, etc., serving both outdoor ring networks and indoor substations.
2 Existing Issues
(1) High cost of imported SF₆ RMUs limits user acceptance.
(2) Compact design reduces busbar spacing and contact gaps versus air-insulated switches. Standby feeder circuits may retain voltage due to fields from live busbars; grounding is essential. Most users leave switches open during standby cable installation, and manuals often overlook this risk, increasing accident potential.
(3) Poor environmental adaptability in insulated/sealed RMUs necessitates heaters to prevent moisture/condensation.
(4) Overly complex mechanisms; simplification is vital for reliability (e.g., switch-fuse units).
(5) Cumbersome installation/reconfiguration: Complex cabinet/cable setup and retrofitting raise error risks for utility/user technicians.
(6) Persistent use of two-position switches.
(7) High cost for voltage signal/control power extraction from incoming cabinets.
3 Improvement Measures
3.1 Localizing Imported Products
3.1.1 Full Localization
Some suppliers manufacture entire cabinets domestically (gas chambers to sheet metal) using local processes/parts.
3.1.2 Partial Localization
Most importers source gas chambers/core parts abroad but localize sheet metal/accessories. This maintains quality (imported chambers, HRC fuses, voltage indicators, self-powered protection devices) while cutting costs via local auxiliary parts. Schneider and F&G lead in cost-performance balance.
3.2 Enhanced Technical Support
Strong technical support builds user trust. Expertise should cover power systems/engineering beyond product knowledge. Effective solutions (e.g., custom development, troubleshooting) directly benefit users economically.
