A Review of Several Technical Issues in the R&D of Solid-Insulated Ring Main Units

1.Current Application Status of Solid-Insulated Ring Main Units in Medium-Voltage Distribution Networks

1.1 Widespread Application in Urban Residential Areas

The core components of medium-voltage ring main units (RMUs) primarily consist of load switches and fuses. These units offer several advantages, including a simple structure, compact size, and relatively low cost. Additionally, they effectively improve power quality and enhance the reliability and safety of power supply. Observations indicate that the maximum rated current for medium-voltage RMUs is 1250A, with a typical value of 630A. Based on insulation media, RMUs are generally classified into two types: air-insulated and SF₆ gas-insulated. They are primarily used for switching load currents and interrupting short-circuit currents, and also provide certain control and protection functions.

1.2 Vacuum Load-Switch RMUs Provide Reliable Isolation Gaps

Load switches used in air-insulated RMUs include four types: gas-generating, compressed-air, vacuum, and SF₆. Among these, SF₆ load switches serve as the primary switches in air-insulated RMUs. The load switches in sealed enclosures typically have three operational positions: load interruption, reliable grounding, and circuit isolation.

1.3 Enhanced Practical Application Level

Generally, RMUs have a compact design. Given this characteristic, most load switches (typically high-voltage switches) feature a relatively simple structure and are equipped with high-voltage fuses. Normal operation of RMUs primarily relies on load switches, while short-circuit currents are effectively interrupted by fuses. The effective combination of these two components maximizes the replacement of circuit breakers, though such replacement is limited to specific capacity ranges.

2.Development Trends and Technical Characteristics of Solid-Insulated Ring Main Units

2.1 Effective Application of Encapsulated Pole Technology

Epoxy resin is commonly used as the main insulating material in solid insulation, with vacuum serving as the arc-quenching medium. To effectively achieve functions such as load and current isolation, the operating mechanism must be properly actuated to enable effective control of the power distribution system, further protecting equipment and personnel safety. Solid insulation significantly reduces the phase-to-ground insulation distance within switchgear, shortening air insulation gaps from 125mm to just a few millimeters. Without SF₆ gas, the overall volume is reduced compared to traditional C-GIS. Additionally, the use of a relatively simple operating mechanism effectively reduces the number of components, enhancing equipment reliability.

2.2 Technical Characteristics of Solid-Insulated RMUs

Compared to air-insulated RMUs with SF₆ switches and SF₆ gas-insulated RMUs, solid-insulated RMUs offer several advantages: First, the housing structure is simpler. By eliminating pressure vessels, pressure gauges, filling valves, and other gas-related components, the reliability of RMUs is greatly improved, maintenance costs are reduced, and the operational condition of the switch is effectively enhanced. Second, the main switch is equipped with an isolation component, providing a clearly visible break. Solid insulation primarily uses epoxy resin sleeves and insulating tubes. These materials effectively mitigate SF₆ liquefaction in cold regions and reduce adverse effects from thermal expansion in high-temperature areas, demonstrating broad applicability in extreme environments.

3.Issues Requiring Attention in Solid-Insulated Ring Main Units

3.1 Insulation and Surface Coating Technology

According to the mechanism of partial discharge formation, internal discharges in solid insulation are primarily caused by the presence of voids. Current sealing techniques typically involve preheating metal molds, placing preheated components into the mold, slowly injecting under vacuum, and curing. This process is not only inefficient but also costly. Incomplete bubble removal can lead to numerous voids in the solid insulation, potentially causing partial discharge during operation, leading to long-term cracking of insulation components and affecting the safe and reliable operation of equipment.

Therefore, advanced, high-quality, and efficient epoxy resin encapsulation technology must be introduced. Additionally, since surface shielding directly affects equipment performance, the issue of surface shielding—i.e., surface shielding technology—must be addressed during design and production. The purpose of metallizing the surface of insulation materials is to provide reliable grounding for solid insulation components, minimize the impact of environmental factors, and enhance product performance. Currently, a significant portion of solid-insulated RMUs on the market lack proper surface treatment, resulting in high partial discharge levels in supplied products.

3.2 Lack of Operation and Maintenance Experience

Solid-insulated RMUs are a relatively new technology. Due to the lack of scientific and reasonable trial operation arrangements by power grids, the current total operational volume is small, operational time is short, and provincial grid connections are incomplete. For example, in high-temperature, high-humidity, coastal, high-rainfall, and large diurnal temperature variation regions, practical issues need to be addressed. Comprehensive work experience must be accumulated nationwide to conduct targeted and forward-looking improvements to solid-insulated RMUs, continuously enhancing product safety and stability.

3.3 Development of Solid-Insulated RMU Equipment

In recent years, with the rapid development of China's distribution network, the technical level of the distribution system has significantly improved. SF₆ gas has been widely used in medium-voltage RMUs. From China's current development status, SF₆ gas offers the best overall performance. However, with the increasing awareness of environmental protection and the importance of natural conservation, the extensive use of SF₆ gas in RMUs poses certain environmental hazards and potential risks to human health. Therefore, reducing the use of SF₆ gas RMUs and promoting solid-insulated RMUs will be a primary goal for the development of high-voltage switchgear. Domestic manufacturers have anticipated this trend and have gradually begun developing and producing solid-insulated RMU products.

3.4 Optimization of Insulation Module Structure Design

The structural design of insulation modules must, while meeting functional, inspection, and installation requirements, aim to reduce material consumption and prevent residual stress. The presence of residual stress can cause internal and external cracks in solid insulation components, leading to partial discharge during operation and potential insulation breakdown. Therefore, in-depth research on the overall structure, thickness, and transitions of insulation modules is necessary, with consideration given to heat dissipation concepts.

3.5 Research and Design of Shielding Layers

The primary purpose of external design is to plan metal shielding grounding: first, in the event of insulation failure, to confine short-circuit faults to phase-to-ground only, reducing arc energy and fault risk; second, to maintain insulation performance in any environment without requiring cleaning of the module's outer surface, ensuring unchanged electric field distribution even with metallic foreign objects inside the cabinet.

4.Development Directions for Solid-Insulated Ring Main Unit Technology

4.1 Development of New High-Performance Insulating Materials

Although epoxy resin performs well as a solid insulation material, the casting process in vacuum ceramic arc-quenching chambers can damage raw materials. Therefore, developing advanced epoxy resins that surpass current material characteristics is essential. In 2014, the journal Science published an article on the discovery of a new recyclable thermosetting polymer, marking a significant advancement in the recycling of "polysalicylate" thermosetting plastics. This polymer exhibits high recyclability, addressing the issue that thermosetting plastics were previously non-recyclable. In the next decade, new products will increasingly utilize recyclable thermosetting materials.

4.2 Research on Internal Component Interface Connection Methods

Since solid-insulated RMUs are manufactured by encapsulating main circuit conductors and mounting parts within solid insulation, the inflexibility of embedded components necessitates standardized connections between different cast elements. The overall configuration of the RMU is linear, and connecting conductors must be cast with epoxy resin, requiring a certain degree of freedom during product rationalization. To address this, an interface technology using silicone rubber material for gate-to-conductor connections is proposed. This method enables standardization, allowing components to be interconnected and reasonably configured, reducing the design complexity of the overall insulated enclosure.

4.3 Market Promotion Prospects for Solid-Insulated RMUs

Solid-insulated RMUs feature a simple structure, compact size, non-toxicity, pollution-free operation, and environmental friendliness, aligning with the global green development concept. However, for a product to be successful, manufacturers must profit from sales, making market application a key focus. Understanding and meeting user needs is critical to the successful introduction of new products. The application prospects of solid-insulated products are analyzed based on user requirements.

4.3.1 Products Meeting Consumer Needs

During use, users first require that product technical parameters comply with relevant standards (technical specifications, national and industry standards, etc.) and obtain certification from the national power industry. As no specific standards currently exist for solid-insulated products, manufacturers must assist in establishing national or industry standards. It is reported that by December 31, 2002, claims for approximately 15 million tons of solid-insulated products had been filed.

4.3.2 High User Requirements for Distribution Network Products

Medium-voltage distribution equipment is a fundamental component widely used in power grids. With the continuous expansion of grid infrastructure, demand for medium-voltage equipment as a bridge between main networks and end-users is growing. Users require products with long service life, strong environmental adaptability, compliance with environmental standards, compact size, easy and quick installation, economic convenience, and a reliable operational history.

Solid-insulated RMUs have compact dimensions, with single load switch cabinet units measuring 860mm (depth) × 420mm (width) × 1200mm (height) or 850mm (depth) × 500mm (width) × 1600mm (height). Their size falls between that of air-insulated switchgear. Based on the above technical performance, solid-insulated RMUs provide enhanced personal safety. However, due to their relatively short operational history, only a limited number of units are currently in service across several Chinese factories. Given the limited operational data, it is currently impossible to evaluate the aging of epoxy insulation or determine inspection cycles.

Some domestic equipment manufacturers guarantee a product lifespan of over 20 years through advanced production facilities (workshops, molds, and automated laser welding). This represents a competitive advantage. Economically, current production of solid-insulated enclosures requires significant investment in specialized equipment, molds, and materials, resulting in higher market prices. Additionally, as power systems primarily use equipment bidding with a focus on low prices, SF₆ RMU modules have become increasingly cost-competitive, making it difficult for solid-insulated RMUs to gain market dominance in the short term.

4.3.3 Product Environmental Requirements

In most regions, users require normal operating environments not exceeding 40°C and not below -10°C. However, the increasing frequency of natural disasters globally has raised user demands. For example, RMUs installed in harsh environments must be fully enclosed and maintenance-free, ensuring complete insulation. Circuit breaker equipment must exhibit strong environmental adaptability, dust and moisture resistance, short-term immersion protection, and enhanced vibration resistance.

Casting internal components of circuit breakers with epoxy resin significantly increases the dielectric strength of insulating materials, preventing degradation due to vibration during transportation and use. Compared to gas-insulated RMU products, some manufacturers have conducted tests in Qinghai and coastal humid regions, as well as at altitudes around 4000m, achieving IP68 protection ratings. Therefore, solid-insulated RMU products better meet environmental requirements.

5.Conclusion

Based on the current development status of solid-insulated RMUs in China, the insulation technology is relatively mature, effectively addressing the shortcomings of air-insulated and gas-insulated RMUs, and suitable for special environments such as high-altitude regions. The adoption of solid-insulated RMUs can effectively promote the development of China's power grid and contribute to sustainable socioeconomic development. With growing attention to environmental protection, it can be confidently stated that solid-insulated RMUs have broad application prospects.