Ring Main Unit (RMU): Function and Selection

• Ring Supply Topology: Unlike traditional radial systems where a single cable fault results in a total blackout for all downstream loads, the RMU enables a "hand-in-hand" (loop) structure. If one segment fails, the faulty section is isolated, and power is restored from the alternative feeder.
• Sectionalizing and Fault Isolation: It provides the ability to divide a distribution line into smaller segments. In the event of a cable fault, the switchgear quickly isolates the damaged section, allowing the non-faulty parts of the loop to remain energized.
• Load Protection: RMUs are equipped with protection units (LBS + Fuse or Circuit Breakers) to safeguard transformers from overcurrent and short-circuit faults, preventing local failures from cascading into the main grid.
• Compact Footprint and Cost-Efficiency: Compared to traditional air-insulated substations, RMUs are remarkably compact. This makes them ideal for urban environments, residential areas, and underground installations where space is at a premium.

II. Selection Logic for RMUs

Selecting the right RMU requires a deep evaluation of insulation medium, protection coordination, and environmental resilience.

1. Insulation Medium: The Technology Choice

• Air Insulated Switchgear (AIS): Traditional and easy to maintain, but larger in size and susceptible to environmental factors like humidity and pollution.
• SF6 Gas Insulated Switchgear (GIS): Utilizes SF6 gas for superior dielectric strength. It is extremely compact and maintenance-free. However, due to SF6 being a potent greenhouse gas, it is facing increasing regulatory scrutiny globally.
• Solid Insulated Switchgear (SIS) — The Future Trend: ROCKWILL excels here by utilizing APG (Automatic Pressure Gelation) technology. High-voltage components are encapsulated in solid epoxy resin. This eliminates toxic gases, is environmentally friendly, and offers the best performance in harsh conditions (high altitude, extreme cold, or high humidity).

2. Protection Coordination: V-Unit vs. C-Unit

• LBS + Fuse Combination (V-Unit):
  • Application: Ideal for protecting distribution transformers up to 1250kVA.
  • Advantage: Cost-effective. Fuses provide extremely fast interruption (current-limiting) for short-circuit faults.
• Circuit Breaker (C-Unit):
  • Application: Required for large transformers (>1250kVA), critical infrastructure, or feeders requiring complex relay protection (e.g., zero-sequence protection).
  • Advantage: Capable of multiple operations and integration with auto-reclosing schemes.

3. Environmental and Functional Requirements

• IP Ratings: Outdoor installations require at least IP65 protection and anti-corrosion coatings to withstand weathering.
• Altitude Derating: For installations above 2000m, dielectric strength decreases. Enhanced external insulation or SIS/GIS units must be selected.
• Smart Grid Automation: Modern RMUs must be "Automation-Ready," integrating DTUs (Data Terminal Units) for "Three-Remote" functions (Tele-signalling, Tele-metering, Tele-control), facilitating remote dispatch and self-healing grid capabilities.

III. Expert Perspective

When making a final decision, I strongly recommend focusing on Life Cycle Cost (LCC) rather than just the initial CAPEX.

• Eco-Friendly Compliance: Transitioning to Solid Insulated RMUs or Eco-gas (Dry Air/N2) Insulated units mitigates future regulatory risks associated with SF6 bans.
• Maintenance-Free Design: Fully sealed SIS or GIS units may have a higher initial cost but significantly reduce operational expenses (OPEX) over their 30-year lifespan by eliminating routine internal maintenance and environmental degradation.

If you have a specific project—such as a coastal wind farm or a high-altitude mining site—please provide the parameters, and I can perform a detailed technical breakdown for your application.

IV.ROCKWILL RMU Products

ROCKWILL offers a complete RMU product line covering 12kV–40.5kV:

ROCKWILL RMU Key Advantages

1. Fully Sealed, Maintenance-Free Under All Conditions

• Stainless-steel gas tank with full welded sealing, leak rate < 0.1%/year
• IP54+ protection rating, suited for outdoor, underground, and humid environments

2. Complete Safety Interlocks

• Five-prevention interlocking: prevent mis-switching, pulling a disconnector under load, earthing while live, closing with earthing closed, and entering a live compartment

3. Modular Design

• Standard units freely combinable across 12/15/24/35kV
• Pre-wired distribution automation interfaces (motorized mechanism + RS485/optical fiber)

4. Globally Proven

• From the Philippines and Venezuela to European projects, RCS outdoor RMUs run stably for years in tropical and coastal salt-spray environments

Whether it's an urban distribution network retrofit, an industrial park switchgear room, or a renewable energy grid connection project, ROCKWILL RMUs provide a customized solution to match your circuit count, insulation needs, and automation requirements.

Contact us:

• Web: www.rockwill.com / www.cnrockwill.com

Edited From：Dyson