The Development of Solid-Insulated Ring Main Units

Development Status at Home and Abroad

Toshiba Corporation of Japan developed high-performance epoxy resin materials and casting technology in 1999, and subsequently launched a 24 kV solid-insulated ring main unit (RMU) in 2002. The product line has since been expanded, and the company is now advancing toward higher voltage levels of 72 kV and 84 kV. Holec, originally a European pioneer with advanced design concepts and environmentally friendly manufacturing processes that produce no pollution, was later acquired by Eaton.

Holec's solid-insulated RMUs were among the first introduced to China, and many domestic manufacturers' self-developed solid-insulated RMUs bear clear influences from Holec's designs. Although China started later in this field, its development has been rapid. Representative companies such as Beijing Shuangjie, Shenyang Haocheng, and Beihai Galaxy have developed products that have passed type tests, achieved mass production capabilities, and are being increasingly promoted and deployed.

Key Technologies and Development Trends

The breakthrough and advancement of solid insulation technology are fundamental to the successful promotion and application of solid-insulated switchgear. Numerous manufacturers worldwide, including Toshiba and Hitachi, have invested significant human, material, and financial resources into solid insulation technology, achieving notable technical progress. Based on the integration of global research outcomes, the key technical challenges and development trends are as follows:

• Development of new high-performance epoxy resins. Using high-performance epoxy resins to directly encapsulate vacuum interrupters facilitates heat conduction and eliminates the need for silicone rubber buffers.
• Insulation design to ensure the required withstand voltage and partial discharge levels.
• Research and development of epoxy resin casting processes to address issues such as partial discharge and cracking in solid insulation components.
• Research and development of surface shielding layers for solid insulation components.
• Stability analysis of epoxy resins. Using accelerated aging tests to study the normal service life of epoxy resins and analyze the trends and rates of performance changes, such as partial discharge, during the service life.
• Intelligent design. Employing advanced sensing and measurement technologies to achieve qualitative and quantitative online monitoring of characteristic parameters such as partial discharge levels.

Existing Problems and Limitations

Solid-insulated RMUs have higher technical and process requirements than SF₆ gas-insulated RMUs. If the technology is immature or the processes inadequate, the risks of insulation failures, operational faults, and potential hazards are greater than with SF₆ gas-insulated units. Therefore, solid-insulated RMUs demand higher standards in technology, manufacturing processes, and raw material quality. Despite growing user acceptance in recent years, several issues remain from the perspective of long-term industrial development and equipment reliability:

(1) Partial Discharge Issues

Unlike gas insulation, where gas leakage can be monitored and discharges may self-recover, solid insulation, once damaged by discharge, cannot recover. Discharges tend to grow over the product's lifetime, potentially leading to insulation breakdown and phase-to-phase short circuits.

(2) Insulation Component Cracking

Early solid-insulated RMUs, both domestically and internationally, have begun to exhibit cracking in insulation components due to long-term power frequency vibration, operational vibration, mechanical impacts, thermal cycling, and environmental temperature fluctuations, leading to increased accident rates.

(3) Safety and Reliability of Isolation Function

The safety and reliability of the isolation function in solid-insulated RMUs are critical. Currently, traditional three-position disconnect switches are primarily used, fully encapsulated within the solid insulation. The insulation performance of the isolation break depends on both the air gap between moving and stationary contacts and the surface creepage distance of the insulating component. Surface flashover along the insulating component increases the risk of break failure and potential personnel hazards. Additionally, environmental factors and material aging can increase surface leakage currents, significantly reducing insulation performance and threatening safe and reliable operation.

(4) Insulation Material Selection and Development

The quality and performance of the primary insulation materials directly affect the reliability and stability of the entire unit. Given the extensive use of insulation materials, considerations for recycling, separating, treating, and reusing scrap materials and components are essential to minimize resource waste.

(5) Encapsulation Process Issues

Product design should facilitate ease of manufacturing and assembly, while the manufacturing and assembly processes should aim for minimal or no environmental pollution and optimal use of energy and resources. For encapsulated products, the formulation of the encapsulation process and selection of encapsulation equipment are particularly critical.

Key Technology Analysis

(1) High-Quality, High-Efficiency Encapsulation Technology

Based on the mechanism of partial discharge, internal discharges in solid insulation components are primarily caused by voids (bubbles) within the material. Conventional encapsulation involves placing preheated components into a preheated metal mold, evacuating the mold cavity, slowly injecting heated, curable epoxy resin, and curing. This method is inefficient, costly, and often fails to completely eliminate bubbles, leading to numerous voids. These voids can cause partial discharge after commissioning, eventually resulting in insulation breakdown and compromising safe and reliable operation. Therefore, adopting advanced, high-quality, and efficient epoxy resin encapsulation technology is essential.

(2) Optimization of Insulation Module Structure Design

Insulation module design must meet functional, inspection, and installation requirements while also ensuring aesthetic appeal, reduced material consumption, and avoidance of residual stress. Residual stress can cause internal and external cracks in insulation components, which may lead to partial discharge and eventual insulation breakdown during operation. Thus, in-depth research on the overall layout, thickness, and transitions of insulation modules is necessary, along with consideration of heat dissipation design.

(3) Optimization of Electric Field Design

Corona discharge occurs when the electric field strength near a conductor's surface reaches the breakdown strength of the surrounding gas, typically in highly non-uniform fields. Sharp edges or points on high-voltage electrodes may concentrate the electric field, causing corona discharge. As a form of partial discharge, corona can progress to insulation breakdown over time, affecting safe and reliable operation. Therefore, designing conductive components to ensure a sufficiently weak and uniform electric field is a key technology. Effective methods include using simulation software for electric field calculations, optimizing the distribution of electric fields, and refining insulation and electrode shapes. Shielding rings or similar measures to reduce electric field strength may also be necessary.

(4) Research and Design of Shielding Layers

The primary purposes of applying a grounded metal shielding layer on the outer surface of insulation modules are: first, to confine short-circuit faults to phase-to-ground only in the event of insulation failure, reducing internal arcing energy and fault risk; second, to maintain insulation performance in any environment without requiring surface cleaning, achieving maintenance-free operation, and ensuring unchanged electric field distribution even if metallic foreign objects enter the enclosure.

(5) Research and Analysis of Epoxy Resin Stability

As a polymer material, epoxy resin can degrade (age) during processing, application, and storage, affecting its performance and service life. The most common aging factors are heat and ultraviolet radiation. In switchgear, continuous heat generation during operation inevitably accelerates the aging of epoxy resin. Therefore, using simulated aging tests to statistically analyze the performance of solid insulation components made from different materials and at various aging stages is essential to establish critical relationships.

Conclusion

Solid insulation technology has gained recognition from users and the market and is being increasingly promoted and deployed. This requires equipment manufacturers to produce products that meet the demands of power supply reliability and stability. Significant research has been conducted on encapsulation processes and surface shielding layer design for solid-insulated RMUs, yielding tangible results. However, these efforts are still insufficient. Greater emphasis must be placed on research into new encapsulation materials, prevention of insulation component cracking, and innovative component structural designs. In summary, further technical research, accumulation, and breakthroughs are needed for solid-insulated RMUs.