Study on the Expansion Issues of Fully Insulated SF₆ Ring Main Units

1. Characteristics of Fully Insulated SF₆ Switchgear
1.1 Overview

Fully insulated SF₆ switchgear comprises functional units such as load switches, load switch-fuse combinations, disconnectors-circuit breakers, etc., all sealed within stainless steel gas boxes filled with low-pressure SF₆ gas. This gas serves both as the arc-quenching and insulating medium. The switchgear employs electric or manual spring-operated mechanisms. Each cabinet is an independent gas box, allowing for expansion through busbar connectors in either direction. Suitable for medium voltage distribution systems, these units are widely used in substations and switching stations to fulfill various power distribution tasks.

1.2 Special Components of Fully Insulated SF₆ Switchgear

Key components include:

• Sealed Gas Box: Inside the sealed gas box are switchgear and busbars, filled with SF₆ gas at a rated pressure of 0.03 MPa. Advanced laser welding and simultaneous vacuum helium leak detection ensure superior sealing. No re-gassing or replacement is needed during its operational life, making it maintenance-free. Depending on expandability, gas boxes can be either shared or standalone; only standalone boxes support expansion.
• Pressure Relief Device: Located at the bottom of the gas box, the pressure relief channel features an explosion-proof membrane. In case of internal arc faults, rapid gas expansion forces the membrane open, releasing pressure and guiding SF₆ gas into trenches to ensure safety for operators and other equipment.
• Cabinet Frame: The frame (excluding the gas box) serves as the mounting base for all components and supports the gas box. It typically consists of three main compartments: operating mechanism room, cable room, and pressure relief channel.

1.3 Main Advantages of Fully Insulated SF₆ Switchgear

• Fully Sealed and Insulated: All high-voltage live parts are sealed inside the gas box, minimizing environmental impacts. Ideal for humid or polluted environments, especially suitable for regions like the Pearl River Delta.
• Compact Design: Utilizing three-position load switches reduces component count. Conductive parts use SF₆ insulation, resulting in a more compact structure compared to air-insulated SF₆ semi-insulated cabinets.
• Altitude Independent: Internal elements are housed in pressurized enclosures, ensuring consistent performance regardless of altitude.
• Expandability: With each cabinet being an independent gas box and equipped with reserved expansion interfaces, expansion is possible using busbar connectors. These connectors consist of three silicone rubber adapters installed horizontally in the inner cone bushings of adjacent cabinets, connecting their busbars without compromising the seal integrity.

2. Analysis of Expansion Solutions for Fully Insulated SF₆ Switchgear

How to add new cabinets to existing fully insulated SF₆ ring main units (RMUs) has become a challenge in recent years due to the lack of standardized interface specifications among different manufacturers. Below is an analysis based on a specific project:

Case Study X: A residential area's substation currently houses three expandable fully insulated SF₆ RMUs from HD Switchgear Company, including one incoming line unit and two outgoing line units. The project requires adding another outgoing line unit and extending a ZRC-YJV22-3×120 cable to a new outdoor switchgear serving new users. Several solutions were considered:

• Complete Replacement: Replacing all three existing RMUs could potentially damage the connected cables if not carefully removed and reinstalled, leading to significant additional costs. Given that the current units have been in operation for less than two years, this option is wasteful.
• Adding a New Cabinet: As switchgear is supplied by the power bureau, procuring a non-HD product might lead to compatibility issues due to varying busbar interface standards. Procurement through specialized bidding processes would also delay project timelines.
• Using Semi-Insulated SF₆ Cabinets: Introducing a semi-insulated cabinet alongside a busbar riser cabinet to bridge the gap between fully and semi-insulated units poses challenges due to the absence of standardized connection components, requiring close cooperation from the original manufacturer.
• Installing Additional Semi-Insulated Units: Adding a semi-insulated outgoing line unit and a busbar riser cabinet next to the existing incoming line unit allows for re-routing cables via the riser cabinet, effectively expanding capacity without disrupting existing installations. This solution was ultimately chosen for its practicality and effectiveness.

The fourth method was adopted, successfully addressing the expansion needs, and the project is now completed and operating smoothly.

3. Conclusion

In conclusion, fully insulated SF₆ switchgear offers clear technical advantages but faces challenges due to the lack of unified interface standards, complicating direct integration between products from different manufacturers. While the fourth method solved the immediate problem, future projects should prioritize using products from the original manufacturer, confirming their expandability, particularly avoiding shared-box structures which generally cannot be expanded. This approach ensures better compatibility and feasibility for system upgrades.