Design of Solid Insulators for Solid-Insulated Ring Main Units

Selection of Vacuum Interrupter Mounting Method in Solid Insulation Design

The key issue in designing solid insulation components is whether to use direct encapsulation or subsequent potting for the vacuum interrupter. If direct encapsulation is adopted, there may be a certain scrap rate due to APG process issues or vacuum interrupter quality problems. Additionally, direct encapsulation results in poorer heat dissipation for the main conductive circuit and demands higher material performance, making mass production difficult since different customers may select vacuum interrupters from various manufacturers.

If subsequent installation and potting are used, external insulation can still be ensured, and the cost of the vacuum interrupter is lower, as specialized encapsulated pole-type interrupters are not required. During potting, a buffer layer around the interrupter is unnecessary—surface treatment suffices. This process has been maturely applied in outdoor vacuum circuit breakers for many years. Moreover, when the product heats up, the silicone rubber surrounding the interrupter has greater flexibility, providing better stress relief.

Selection of Glass Transition Temperature in Solid Insulation Design

Generally, the higher the glass transition temperature, the more brittle the material and the more prone it is to cracking. If glass transition temperature is selected solely based on thermal resistance, only a few materials can achieve both high glass transition temperature and excellent crack resistance. However, such materials are very expensive, significantly increasing production costs. If a new product's price is much higher than existing ones, customer acceptance will be greatly reduced.

Therefore, the selection of glass transition temperature can refer to that used in SF₆ gas-insulated switchgear insulation components, such as SF₆ enclosures, where upper and lower contacts are also embedded in resin. The materials used typically have a glass transition temperature around 100°C, and these products have been in service for many years with very few incidents caused by overheating, indicating the rationality of this choice. From the perspective of switchgear, temperature rise control is also essential—considering adequate current-carrying capacity of the main circuit, control of material conductivity, plating quality, and assembly precision, while also controlling and reducing ambient temperature through structural design. Material specifications should be comprehensively evaluated, combined with operational experience from similar products.

Design of Outlet Bushings in Solid Insulation Components

In the design of outlet bushings for solid insulation components, inlet bushings are generally straight-through type, while outlet bushings sometimes adopt a bent design. Bent bushings are more difficult to manufacture, with main challenges including:

• Alignment between conductor and mold, where deformation may occur during conductor pre-treatment;
• Cracking after product molding, as the conductor is at high temperature during molding, and improper process control may lead to cracks after cooling. Additionally, during design, consideration should be given to whether discharge to the installation nut might occur during subsequent installation.

Design of Conductive Components and Connection of Conductive Circuits in Solid Insulation

When designing the main conductive components, smooth transitions should be achieved wherever possible under the premise of meeting current capacity requirements—preferably rounded rather than angular. Welding should be used for connections instead of bolted joints to minimize corona discharge and prevent cracking. For movable connections, a knife-switch type connection is preferred, which reduces cost compared to plug-in types, lowers requirements for conductor dimensions and positional accuracy, and allows easier adjustment of loop resistance.

Based on overall loop resistance requirements, it is advisable to specify the loop resistance of conductive parts embedded in resin, particularly for welded conductors, to avoid product scrapping due to excessive resistance caused by poor welding quality. By optimizing conductor shape design, electric field strength to ground (surface grounding layer) can be reduced, adhesion with resin improved, and overall mechanical strength of the insulating component enhanced.

Design of Surface Grounding Layer in Solid Insulation Components

Surface grounding layer treatments include externally coating with conductive silicone rubber, applying conductive adhesive (or paint), or metal spraying. Regardless of the method used, the core objective is to control partial discharge. Without effective control, partial discharge can easily lead to breakdown, which is also related to the design of resin layer thickness. Compared to other shielded insulation components, the structure of solid insulation differs significantly—other components typically feature a concentric cylindrical electric field between high-voltage and ground ends, whether the high-voltage end is a shielding mesh or a circular conductor.

In solid insulation, however, the high-voltage section includes both circular and flat surfaces, while the ground end is flat, necessitating careful consideration of how these structural differences affect performance. From a technical standpoint, two key requirements for the grounding layer are continuity and partial discharge level. During transportation, installation, especially on-site work, any impact damage or peeling may cause partial discharge at the grounding layer's edge, posing new challenges for subsequent operation and protection.

From a heat dissipation perspective, metal spraying offers the best performance due to superior thermal conductivity, significantly enhancing stability against various aging factors, particularly thermal cycling. Protection of the grounding layer must be considered during insulation component manufacturing, and product protection during grounding layer processing is also essential.

Assembly Design of Solid Insulation Body and Bushings

Most designs separate the main body from inlet and outlet bushings, including the connection between fuse insulating tubes and bushings, which are in hard contact during installation. Dimensional control is important, but process control during assembly is equally critical. If contact gaps exist, or if dust or moisture (from environmental condensation) is introduced during assembly, flashover discharge to the mounting nut may occur. Additionally, ring main units have compact structures, so layout must consider ease of installation for inlet/outlet insulation and cables, especially cable terminations, which already demand high installation quality. Inconvenient installation may easily lead to quality issues and cause insulation breakdown.

Conclusion

Solid-insulated ring main units have significant market potential. Research on their core component—the solid insulation element—holds broad prospects. As solid insulation design continues to improve, the technology for solid-insulated ring main units will achieve further advancement.