Design Solution of 24kV Dry Air Insulated Ring Main Unit

The combination of ​Solid Insulation Assist + Dry Air Insulation​ represents the development direction for 24kV RMUs. By balancing insulation requirements with compactness and employing solid auxiliary insulation, insulation tests can be passed without significantly increasing phase-to-phase and phase-to-ground dimensions. Encapsulating the pole column solidifies the insulation for the vacuum interrupter and its connecting conductors.

Maintaining the ​24kV outgoing busbar phase spacing at 110mm, the electric field intensity and non-uniformity coefficient can be reduced by encapsulating the busbar surface. ​Table 4​ calculates the electric field under different phase spacings and busbar insulation thicknesses. It shows that appropriately increasing the phase spacing to ​130mm​ and applying a ​5mm epoxy encapsulation​ to the round bar busbar results in an electric field strength of ​2298 kV/m. This maintains a certain margin below the maximum withstand strength of dry air (3000 kV/m).

​Table 4: Electric Field Conditions under Different Phase Spacing and Busbar Insulation Thicknesses​

Due to the ​low insulation strength of dry air, solid insulation alone cannot solve the withstand voltage issue for the isolating gap. A ​dual-isolation-break configuration​ effectively distributes voltage across two gas gaps. Grading rings (field shields) are designed at concentrated field areas like the isolation and earthing stationary contacts to reduce field strength and minimize air gap dimensions. As shown in ​Figure 3, a reinforced nylon main shaft rotates the dual-break mechanism to achieve operational, isolation, and grounding states. Grading rings at the stationary contacts, with a ​60mm diameter​ and epoxy encapsulation, allow a ​100mm clearance​ to withstand a ​150kV lightning impulse withstand voltage.

Other approaches, such as ​longitudinal phase-segregated layouts, using high-strength single-phase alloy tanks, or moderately increasing gas pressure, can also meet 24kV withstand requirements. However, RMUs require ​low cost, and excessively high costs are unacceptable to users. Through optimized design, such as moderately increasing RMU width, the goal of ​low cost and miniaturization​ can be achieved for 24kV eco-friendly gas-insulated RMUs.

​1. Arrangement of Earth Switches in Eco-Gas RMUs​
Two main circuit methods can implement grounding functions:

• Outgoing side earth switch (lower earth switch)
• Busbar side earth switch (upper earth switch), optionally E0 rated, requiring main switch coordination for earthing operations.

​State Grid's "12kV RMU (Cabinet) Standardized Design Scheme" 2022 Edition​ specifies that all three-position switches (isolate, connect, earth) should utilize the busbar side arrangement, termed the "Busbar Side Combined Function Earth Switch".

Power safety regulations mandate that ​no circuit breaker (CB) or fuse can exist between the earth conductor/earth switch and the equipment under maintenance. If a CB exists between the earth switch and the equipment due to design constraints, measures must ensure the CB cannot open after both the earth switch and CB are closed. Therefore:

• A ​Line Side Earth Switch, located downstream of the CB, connects directly to the earthed outgoing cable, naturally meeting the regulation as no CB exists between it and the equipment.
• A ​Busbar Side Earth Switch, located upstream of the CB, has the vacuum CB between it and the earthed outgoing cable, violating the direct connection requirement. After closing the earth switch and CB, ​measures preventing CB opening​ must be implemented. Examples include disconnecting the CB trip circuit via a blocking plate or using mechanical interlocks to prevent accidental CB opening and consequent loss of earthing.

The National Grid standard also mandates ​mechanical and electrical interlocks​ to ​prevent manual or electrical opening of the CB​ when the combined function earth switch is using the CB (closed) to ground the cable side.

The primary reason for choosing the Busbar Side Isolating-Earthing Three-Position Switch in the National Grid standard is ​earthing/grounding making capacity:

• ​SF6 RMUs: SF6 has ~3x the insulation strength of air and ~100x greater arc-quenching capability due to superior cooling, ensuring adequate earth switch making capacity.
• ​Eco-Gas RMUs: Eco-gases lack inherent arc-quenching ability and have poorer insulation. Achieving the required making capacity thus demands ​very high closing speeds. However, standard RMU operating mechanisms lack the energy for such speeds. Using a line-side earth switch necessitates expensive higher-speed mechanisms, robust arc-resistant contacts, and force analysis, increasing cost and complexity. ​Busbar Side Earth Switches, while requiring CB interlocking solutions, offer stronger making capacity and can ensure earthing reliability.

​Analysis of SF6 vs. Eco-Gas Technology and Products​ indicates that ​12kV Eco-Gas RMUs​ can meet insulation and temperature rise requirements with minimal size increase, representing a mature technical solution.

Conversely, ​24kV Eco-Gas Insulated products​ are still limited. The key challenge is the significantly higher voltage level leading to much larger dimensions and higher costs, impeding development. Balancing factors like ​insulating gas type, filling pressure, gas tank volume, and auxiliary insulation cost​ is crucial to designing ​low-cost, compact RMUs. Successfully replacing SF6 will not only capture the domestic market but also enable ​global outreach, promoting China's low-carbon, eco-friendly products worldwide.