To improve the reliability of urban distribution networks, a ring-connected power supply system can be used. In a ring-connected power supply system, each end-user transformer receives two power supplies from a power supply point (ring network node), one for backup and one for use. This power supply method is called ring network power supply.
The Ring Main Unit (RMU) is a key component of ring network power supply. It is a high-voltage switchgear used in ring network power supply systems. Typically a load switchgear, a load switch-fuse combination switchgear, or a small-capacity vacuum circuit breaker, it performs control and protection functions, switching load current, interrupting short-circuit current, and transformer no-load current. It can also switch charging current in overhead lines and cables. RMUs offer advantages such as simple structure, compact size, low cost, and safe power supply.
RMUs are primarily used in ring network power supply or dual-power radial power supply systems to control, distribute, and switch branch lines. They are also commonly used for control and protection on the high-voltage side of transformers in prefabricated substations (box-type substations). They can also be used in terminal power supply systems to prevent short-circuit faults at feeder terminals from escalating. If a RMU with a small-capacity vacuum circuit breaker is used, the circuit breaker provides protection. Since the ring main unit is used at the terminal of the urban power grid, the power distribution method is flexible and scalable, and the price is reasonable. It is particularly suitable for use in residential areas, high-rise buildings, public distribution stations and box-type substations.
Ring main units are categorized into the following types based on the insulation medium:
1. Air-Insulated Ring Main Units
Ring main units primarily insulated with air are equipped with gas-generating, compressed air, or vacuum load switches. These load switches are available in side-mounted or front-mounted configurations, and the operating mechanisms are all front-mounted. This type of unit is relatively large, with a typical width of 800mm for 12kV and 1700mm for 40.5kV. If a vacuum load switch is used as the main switch, since the vacuum interrupter chamber used can only interrupt, not isolate, an isolating switch must be added before the load switch during design to provide a complete isolation port.
2. Air and SF₆ Gas Mixed Insulation Ring Main Units
This type of ring main unit generally uses an SF₆ load switch as the main switch. It is enclosed in a fully sealed epoxy resin or stainless steel housing filled with low-pressure SF₆ gas as the insulating medium (SF₆ gas also serves as the arc extinguishing medium). The remaining primary high-voltage live components, such as the primary busbar, primary connecting cables, voltage/current transformers, and lightning arresters, are installed in the open air. SF₆ load switches offer excellent electrical performance, are significantly smaller than comparable air-insulated ring main units, can easily accommodate three-position switches, and offer comprehensive functionality, ease of assembly, and ease of network automation. They have seen widespread adoption in recent years.
3. Gas-insulated Ring Main Unit
This type of ring main unit encloses all high-voltage live conductors (such as the three-position load switch and connecting busbars) in a stainless steel casing filled with low-pressure SF₆ gas, using SF₆ gas as both insulation and arc extinguishing medium. The stainless steel casing provides excellent strength, corrosion resistance, and heat dissipation, while also providing reliable grounding, ensuring maximum personal safety. To facilitate maintenance and replacement, the operating mechanism and high-voltage current-limiting fuses are enclosed in the open air. This type of ring main unit also utilizes prefabricated silicone rubber-insulated cable terminals for cable connection, reducing the size of the ring main unit while also facilitating installation and ensuring safe operation and maintenance.
This type of product offers the following features: ① All high-voltage live components (except fuses) are sealed in a low-pressure SF₆ gas tank, shielded from environmental influences; ② A modular design allows for various main wiring configurations using different modules, and the combination can be modified to create a multi-circuit switchgear system as needed; ③ An interface insulation structure facilitates the connection of high-voltage live components and cabinet expansion; ④ The fuse trip mechanism is located at the front of the switchgear for easy replacement; ⑤ Fully shielded cable inlets and outlets; ⑥ High-voltage monitoring components and integrated digital relays can be used; ⑦ High-voltage metering cabinets can be used.
4. Solid-Insulated Ring Main Unit
This type of ring main unit completely eliminates the use of pressure vessels. The internal insulation and arc extinguishing of the switchgear are achieved using a vacuum dielectric, while the external insulation utilizes insulating tubes to solidify the switch components. Enclosed busbars connect the various circuits. The insulation sleeve utilizes sealed pole technology, organically integrating the vacuum interrupter, main conductive circuit, and insulation support into a fully insulated, sealed, and maintenance-free structure. The insulation sleeve is made of epoxy resin, and the busbars are encapsulated with silicone rubber. This structure effectively mitigates corrosion of the power supply by isolating it from air, moisture, dust, and hot and cold air sources.
This product has the following features: ① All high-voltage live components are enclosed by the epoxy resin sleeve and silicone rubber-encapsulated busbar, achieving a fully insulated and sealed structure; ② The switchgear is equipped with isolation features, and the switch operating position is directly observable through an inspection window on the panel; ③ The vacuum switch's external insulation utilizes sealed poles, making it suitable for outdoor operation; ④ The modular unit design facilitates replacement and maintenance; ⑤ Phase-separated insulation provides excellent interphase insulation performance; ⑥ The enclosure has no removable fasteners, providing excellent theft protection; ⑦ Plug-in cable terminals are used for incoming and outgoing wiring, eliminating exposed live parts.
