24kV Maintenance-Free Eco Gas Insulated RMU with Smart Monitoring – Urban Grid Ready

Compared with 12kV, 24kV can supply more electrical energy, reduce line losses, and is widely used in overseas markets.

SF₆ is a greenhouse gas whose ozone layer depletion potential is over 20,000 times that of CO₂. Its use must be restricted; therefore, medium-voltage switchgear must not use SF₆ as an insulating gas.

For switchgear, environmentally friendly gases refer to those that do not contain SF₆ as insulating or arc-extinguishing media. Examples include naturally occurring gases (such as nitrogen and carbon dioxide), gas mixtures, and synthetic gases.

The key challenge for environmentally friendly gas-insulated switchgear lies in meeting insulation requirements. While 12kV environmentally friendly gas-insulated ring main units (RMUs) are quite mature, 24kV models have relatively few developers. This is because domestic demand for 24kV equipment is low and its insulation design is more complex—only a handful of complete set manufacturers with export needs are developing such products.

In essence, 24kV switchgear design can be simplified through the following approaches:

• Solid composite insulation: This ensures the busbar meets voltage withstand requirements. Increasing the isolation gap or expanding the gas tank size can also satisfy voltage withstand standards.

• Increased gas pressure: Raising the relative pressure from 0.04MPa to 0.14MPa addresses both insulation and gap voltage withstand requirements, with the only additional step being the replacement of the arc-extinguishing chamber with a 24kV-rated one.

Alternatively, C4/C5 synthetic gas mixed with CO₂ can be used, as its insulation strength is similar to that of SF₆. Minor enhancements to the insulation system of SF₆-based RMUs can make them meet 24kV voltage withstand requirements. However, C4/C5 is also a greenhouse gas—though its global warming potential (GWP) is only 1/20 that of SF₆. Additionally, it decomposes into toxic gases after arc extinction, which is not conducive to sustainable development.

The clearance between live parts of the switch is determined by the impulse withstand voltage:

• For 24kV equipment, the impulse withstand voltage is 125kV, corresponding to an air clearance of 220mm (or 95mm if 3M heat-shrinkable sleeves and BPTM round busbars are used).

• For 12kV equipment, the impulse withstand voltage is 75kV, with an air clearance of 120mm (or 55mm with the same 3M sleeves and BPTM busbars).

For side-mounted switch units in RMUs, the clearance requirements for composite insulation can be fully met.

 The earliest 24kV solid-insulated ring main units include Eaton's SVS and Xirui's products. Due to the fact that the switches designed by Xirui for overseas markets are two-position—meaning the switch is either in the closed position or the grounded position—this design failed to meet China's requirement for three-position operation with step-by-step control, so an isolation position had to be added between the two positions.

How to achieve product miniaturization, cost-effectiveness, and environmental adaptability determines the development direction of 24kV eco-friendly gas-insulated ring main units. Solid composite insulation has high cost and still finds it difficult to solve the voltage withstand problem of isolation breaks. Meanwhile, because alternative gases such as dry air and nitrogen have insufficient insulation strength, the break distance and ground distance need to be similar to those required for natural air, i.e., ≥220mm. This makes such rotary three-position switches require a large size, while linear-motion switches face certain difficulties in either increasing the height dimension or the width dimension. The adoption of double-break isolation and grounding switches can solve the problem of oversized isolation switches.

To provide gas filling pressure, the problem of enclosure strength needs to be solved. The use of an aluminum alloy cylindrical structure enables dimension optimization, uniform electric field, and good heat dissipation. The internal busbars are arranged in a delta (triangular) configuration, and the three-position switch and vacuum interrupter are installed vertically, which maximizes the use of spatial dimensions and achieves small size and high power capacity.