Epoxy Resin Wall Bushing for Indoor 24 kV SF₆ Gas Insulated Switchgear

Introduction

In indoor medium-voltage SF₆ gas-insulated switchgear, the wall bushing plays a critical role as the interface between the sealed gas compartment and external electrical connections. For 24 kV GIS systems, epoxy resin wall bushings have become the preferred solution due to their compact design, high dielectric strength, and excellent compatibility with SF₆ insulation systems.

Unlike air-insulated bushings, epoxy resin wall bushings are designed to operate partly inside an SF₆ environment and partly in air, while maintaining strict insulation coordination, gas tightness, and mechanical stability. Their design directly affects dielectric reliability, gas containment, and long-term service life of the GIS.

Function of a Wall Bushing in SF₆ GIS

A wall bushing in SF₆ GIS performs several essential functions simultaneously:

• Transfers electrical current through the grounded enclosure wall

• Maintains insulation between live conductor and grounded tank

• Separates SF₆ gas environment from external air

• Preserves gas pressure and leak tightness

• Withstands mechanical stress from conductor connection

Failure of a wall bushing can lead to gas leakage, partial discharge, or flashover, making its design and quality critical.

Why Epoxy Resin Is Used for GIS Wall Bushings

Epoxy resin is widely adopted for GIS wall bushings because of its electrical, mechanical, and manufacturing advantages.

Key benefits include:

• High dielectric strength

• Uniform insulation without internal voids

• Excellent compatibility with SF₆ gas

• Strong mechanical rigidity

• Compact structure for space-saving GIS layouts

• Stable performance under temperature variation

Compared with porcelain or composite alternatives, epoxy resin offers better dimensional accuracy and integration with metal enclosures.

Typical Structure of a 24 kV Epoxy Resin Wall Bushing

A standard epoxy resin wall bushing for indoor SF₆ GIS consists of:

• Central copper or aluminum conductor

• Epoxy resin molded insulation body

• Integrated metal flange for enclosure mounting

• Gas sealing O-rings or gaskets

• External air-side insulation profile

The epoxy body is usually vacuum-cast to eliminate air bubbles and ensure partial discharge-free performance.

Electrical Ratings and Insulation Levels

For 24 kV indoor SF₆ GIS, wall bushings typically comply with the following insulation levels:

These values align with IEC 62271-1 and IEC 62271-200 requirements.

Insulation Coordination in SF₆ GIS Applications

Gas-Side Insulation

Inside the GIS tank, SF₆ gas provides very high dielectric strength. This allows:

• Short internal clearances

• Compact conductor spacing

• Reduced bushing length on gas side

However, surface quality and conductor geometry must prevent electric field concentration.

Air-Side Insulation

On the external side, insulation relies on epoxy resin and air. Key considerations include:

• Clearance distance

• Creepage distance

• Pollution level (usually light for indoor use)

Even for indoor installations, conservative creepage design improves long-term reliability.

Creepage and Clearance Design for Indoor Use

Typical design values for indoor 24 kV epoxy wall bushings:

Although SF₆ GIS is sealed, the air-side surface remains exposed, so creepage design cannot be neglected.

Gas Sealing and Leak Tightness

Gas tightness is one of the most critical functions of a GIS wall bushing.

Key design features include:

• Precision-machined metal flange

• Double O-ring sealing system

• SF₆-compatible elastomer materials

• Controlled surface roughness

Typical leak rate requirements are ≤ 0.5% per year, in line with IEC standards.

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Partial Discharge Performance

High-quality epoxy resin wall bushings are designed to be partial discharge-free under operating voltage.

Typical PD limits:

• PD inception voltage ≥ 1.2 × rated voltage

• PD magnitude ≤ 5 pC at test voltage

Low PD performance is essential to avoid long-term insulation degradation inside sealed GIS compartments.

Mechanical Strength and Installation Considerations

Epoxy resin wall bushings must withstand:

• Tightening torque during installation

• Thermal expansion of conductor

• Mechanical vibration

Correct installation practices include:

• Even torque application

• Clean flange and gasket surfaces

• Correct alignment of conductor

Improper installation is a common cause of premature failure.

Indoor GIS Environmental Advantages

Because the bushing is used in indoor SF₆ GIS, environmental stress is relatively low:

• No UV exposure

• Minimal pollution

• Stable temperature

This allows epoxy resin to achieve long service life, often exceeding 30 years.

Standards and Compliance

Epoxy resin wall bushings for 24 kV SF₆ GIS typically comply with:

• IEC 62271-1 – Common specifications

• IEC 62271-200 – Metal-enclosed switchgear

• IEC 60071 – Insulation coordination

• IEC 60270 – Partial discharge measurement

Compliance ensures interchangeability and utility acceptance.

Common Failure Causes and Prevention

Typical Failure Causes

• Poor epoxy casting quality

• Inadequate sealing design

• Surface contamination during installation

• Incorrect torque application

Preventive Measures

• Use PD-tested bushings

• Follow manufacturer installation procedures

• Perform gas leak testing

• Maintain clean working conditions

Advantages Over Air-Insulated Wall Bushings

Compared with air-insulated designs, epoxy resin wall bushings offer:

• Smaller size

• Better insulation stability

• Higher mechanical integration

• Reduced maintenance

• Improved gas sealing

These advantages align perfectly with the compact philosophy of SF₆ GIS.

Application Areas

Epoxy resin wall bushings for 24 kV SF₆ GIS are commonly used in:

• Utility distribution substations

• Industrial power systems

• Compact indoor substations

• Ring main units with GIS design

They are essential components wherever compact, sealed MV switchgear is required.

Conclusion

The epoxy resin wall bushing is a critical interface component in indoor 24 kV SF₆ gas-insulated switchgear. Its performance directly affects insulation integrity, gas containment, and long-term reliability of the entire GIS system.

By combining high dielectric strength, excellent gas sealing, compact design, and reliable mechanical performance, epoxy resin wall bushings meet the demanding requirements of modern medium-voltage GIS installations. Proper design, manufacturing quality, and installation practices ensure safe operation and extended service life in indoor environments.