Epoxy Resin Bushing
Epoxy Resin Bushing High Insulation Solid Cast Transformer Bushing
Epoxy Resin Bushing: Definition, Types, Specifications, Advantages & Selection Guide
Introduction
An Epoxy Resin Bushing, also known as solid cast epoxy bushing, is a high-performance insulating component manufactured by vacuum casting or APG (Automatic Pressure Gelation) technology with epoxy resin as the main dielectric material. It enables live conductors to penetrate the grounded enclosure of dry-type transformers, oil-immersed transformers, medium voltage switchgears and other power equipment, providing reliable electrical isolation and mechanical connection between internal windings and external circuits.
Unlike traditional porcelain bushings, epoxy resin bushings adopt an integrated solid casting structure. The copper conductor is embedded tightly within the epoxy matrix without internal cavities. This design effectively eliminates partial discharge risks caused by trapped air gaps. With lightweight construction, excellent impact resistance and flexible mold forming capability, epoxy resin bushings have become the preferred insulation solution for compact, indoor and mobile power equipment, and are widely adopted in IEC standard markets as well as ANSI export transformer projects.
Epoxy resin bushings cover low voltage, medium voltage ranges from 1.2kV up to 40.5kV. Cycloaliphatic epoxy formulations are selected for outdoor environments to enhance UV resistance, while standard bisphenol-A epoxy works stably for indoor applications. This comprehensive guide covers working principles, classification, technical specification table, core advantages, typical application scenarios, selection criteria and routine maintenance guidance for epoxy resin bushings.
Working Principle & Main Construction
A standard epoxy resin bushing consists of three core integrated parts formed via one-time casting:
- Central Copper Conductor: Transmits continuous load current between internal equipment windings and external cable terminals. Terminal options include spade terminals, threaded studs and straight draw-lead connectors.
- Solid Epoxy Insulation Body: The main dielectric barrier separating live conductor from grounded metal housing. Shed structures can be molded on the outer surface to extend creepage distance for pollution-resistant operation.
- Mounting Flange: Integrally cast epoxy or metal flange for installation on transformer tank, cabinet wall or enclosure. Matching oil-resistant gaskets can be equipped for oil-filled transformer applications.
Main Applicable Industry Standards
- IEC 60137: Insulated bushings for alternating voltages above 1000 V
- IEC 60076-11: Requirements for dry-type transformers
- IEEE C57.19.00 / IEEE C57.19.01: For ANSI version epoxy resin bushings for North American transformers
- ISO 9001: Production quality control standards
Core Operation Mechanism
When power equipment operates, the bushing continuously carries rated load current. The solid epoxy insulation maintains stable electric field distribution around the conductor and prevents leakage current and flashover. The homogeneous solid structure avoids moisture penetration compared to hollow porcelain bushings. Customized creepage designs can be molded directly during casting to meet normal, medium and heavy pollution grade requirements without additional modification.
Main Types of Epoxy Resin Bushings
1. Low Voltage Epoxy Resin Bushing (1KV)
Commonly used on the secondary side of distribution transformers. Supports high continuous current up to 8000A, mainly with spade terminal connections. Compact structure, convenient for high-current wiring layout.Application: Secondary outlets of dry-type and oil immersed distribution transformers, low voltage busbar through-wall insulation.
2. High Voltage Epoxy Resin Bushing (35KV)
Equipped with integrated molded sheds to extend creepage distance. Widely used for primary side connections of medium voltage transformers. Optional straight or angled shape to adapt limited cabinet space.
Application: Medium voltage dry transformers, ring main units, compact substation transformer feeders.
Universal Technical Specification Table for Epoxy Resin Bushings
| Parameter Item | Low Voltage Epoxy Bushing | High Voltage Shedded Epoxy Bushing |
|---|---|---|
| Rated Voltage Class | 1kV | 35KV |
| Rated Continuous Current | 1000A-3150A | 315A |
| Raw Epoxy Material | Bisphenol-A epoxy | Bisphenol-A / Cycloaliphatic epoxy |
| Operating Environment | Indoor sheltered | Indoor / Semi-outdoor |
| Operating Temperature Range | -40°C ~ +125°C | -40°C ~ +125°C |
| Impact Resistance | Good | Good |
| UV Aging Resistance | Limited | Medium |
| Partial Discharge Level | ≤10pC | ≤10pC |
| Standard Terminal Type | Multi-hole spade lug | Eyebolt / Draw lead |
| Creepage Distance | Standard short creepage | Multi-shed extended creepage |
| Main Risk of Degradation | Overheating under overload | Dry band arcing in heavy pollution |
| Typical Equipment Match | Transformer secondary side outlets | Medium voltage dry transformers, RMU |
Core Advantages of Epoxy Resin Bushings
1. Integrated Solid Cast Structure Eliminates Internal Air Gaps
Vacuum casting technology ensures no voids inside insulation. Uniform electric field distribution greatly reduces partial discharge risk during long-term service, significantly improving operational reliability compared to hollow porcelain bushings.
2. Lightweight & High Impact Resistance
Epoxy resin material is far lighter than porcelain. It resists cracking and breakage during transportation, installation and accidental mechanical collision, lowering product rejection rate and on-site maintenance costs. It is ideal for mobile power equipment and projects with strict anti-vandalism requirements.
3. High Manufacturing Flexibility with Custom Shapes
Various outlines, flange dimensions, shed structures and terminal forms can be molded directly according to customer drawings. Straight, angled, compact or extended creepage designs are available, simplifying equipment mechanical layout for transformer and switchgear manufacturers.
4. Low Maintenance Requirements
Smooth dense epoxy surface is not easy to absorb dust. For indoor applications, periodic simple cleaning is sufficient. Cycloaliphatic outdoor types feature hydrophobic performance, reducing surface leakage current under humid and foggy weather.
5. Wide Compatibility with Multiple Power Equipment Types
Suitable for dry-type transformers and many indoor devices that cannot use oil-filled bushings. Special oil-grade epoxy versions can be matched with mineral oil and ester oil transformers, realizing unified bushing selection for multiple product lines.
6. Stable Thermal Cycling Performance
Qualified epoxy resin formulations withstand repeated temperature rise and fall cycles caused by transformer load fluctuation. Low thermal expansion mismatch between conductor and insulation reduces internal thermal stress and avoids insulation cracking.
Typical Application Scenarios
- Indoor dry-type distribution transformers and power transformers
- Pad-mounted transformers and pole-mounted transformers adopting ANSI or IEC standards
- High voltage switchgear, ring main units (RMU) and through-wall busbar insulation
- Wind power and solar photovoltaic step-up transformers
- Mobile power supply transformers, mining and traction electrical equipment
- Industrial dry-type transformers for chemical, metallurgy and manufacturing plants
- Retrofit replacement projects where fragile porcelain bushings need upgrading
- Battery energy storage system matching distribution transformers
Epoxy Resin Bushing Selection Guide
- Confirm Voltage Class and Required Pollution LevelMatch nominal system voltage, power frequency withstand voltage and lightning impulse voltage. Select cycloaliphatic epoxy for outdoor and coastal polluted areas; standard bisphenol-A epoxy is sufficient for clean indoor environments. Choose multi-shed models if extended creepage distance is required.
- Determine Rated Current and Terminal StyleCalculate maximum continuous load current. Select spade terminals for high-current secondary sides; eyebolt or threaded studs are preferred for medium voltage overhead cable connections.
- Select Epoxy Material Grade According to EnvironmentIndoor sheltered location: conventional bisphenol-A cast epoxy. Outdoor, direct sunlight, salt fog: cycloaliphatic epoxy resin bushing. Installation inside oil immersed transformer: select oil-compatible modified epoxy type.
- Verify Mounting Dimension and Installation SpaceCheck flange diameter, bolt circle layout, tank opening size and available installation space. Confirm whether straight or 90° angled structure is needed to avoid assembly interference.
- Check Mechanical Load LimitsEvaluate external cable pulling force and cantilever load. Ensure selected bushing meets the mechanical load test requirements specified by IEC or ANSI standards.
Routine Maintenance & Common Troubleshooting
Regular Maintenance Checklist
- Carry out visual inspection every 6–12 months. Check epoxy surface for cracks, tracking marks, discoloration and contamination accumulation.
- Inspect connection terminals for overheating, oxidation and loose bolts; retighten terminals according to specified torque values.
- Clean dust and contaminants on the bushing surface regularly to maintain effective creepage distance.
- For outdoor cycloaliphatic epoxy bushings, observe surface hydrophobic condition; pay attention to surface aging after long-term operation.
Check flange sealing gaskets regularly for aging and air/oil leakage.
Common Operational Problems
- Surface chalking and discoloration: Usually occurs on non-UV-stabilized epoxy exposed under long-term outdoor sunlight; replace with cycloaliphatic grade bushings.
- Tracking and carbonized surface traces: Caused by heavy pollution and insufficient cleaning, or insufficient creepage distance matching pollution grade.
- Terminal overheating: Loose connection bolts or insufficient contact area between conductor and external cable lugs.
- Epoxy body cracking: Excessive mechanical impact, overload thermal cycling stress or improper installation torque.
Safety Reminder: Do not apply strong corrosive chemical detergent to epoxy surface during cleaning. Avoid heavy impact on the insulation body during transportation and installation. Disconnect power supply before any inspection and maintenance work.
Conclusion
As a modern alternative to traditional porcelain bushings, epoxy resin bushings deliver excellent electrical performance, outstanding mechanical toughness and flexible structural customization. Indoor bisphenol-A types and outdoor cycloaliphatic variants fully cover low and medium voltage power equipment from dry-type transformers, switchgears to renewable energy transformers.
Reasonable material selection, standardized installation and regular inspection can maximize service life and reduce equipment failure risks. With the rapid growth of global dry-type transformer market and compact indoor power distribution equipment demand, epoxy resin bushings are continuously optimized with low partial discharge formulations and enhanced environmental resistance, becoming increasingly popular in both IEC international markets and North American ANSI export transformer projects.
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