Transformer bushing

Working Principle of Transformer Bushings Explained
Transformer bushings are critical components that connect the high-voltage (or low-voltage) windings of a transformer to the external power grid. They perform four core functions: electrical conduction, insulation, sealing, and mechanical support. Their working principle relies on multilayer insulation media synergy and electric field optimization to safely transmit current while isolating the internal oil/gas environment from external conditions. Below is a step-by-step breakdown of their operation:

I. Core Functions and Operating Scenarios

1. Current Conduction: Transmit high-voltage current from the transformer windings to the external grid via a conductive rod.
2. Insulation Barrier: Isolate the high-voltage conductor from the grounded transformer tank to prevent leakage or short circuits.
3. Environmental Sealing: Maintain the integrity of the transformer’s internal insulating oil (or gas) and block moisture or contaminants.
4. Mechanical Support: Withstand mechanical stresses from conductor tension, wind vibrations, and short-circuit forces.

II. Key Structural Components and Working Principles
1. Conductive System: Current Transmission Path
• Conductive Rod: A copper or aluminum rod running through the bushing’s core, directly carrying current.

• Terminals: Connect the top end to the grid and the bottom end to the transformer windings.

Principle: Current flows from the windings through the conductive rod to the external grid, requiring low resistance and high current capacity to avoid overheating.

2. Insulation System: Multilayer Dielectric Protection
• Primary Insulation:

• Oil-Immersed Bushings: Filled with transformer oil (or SF6 gas), leveraging high dielectric strength (oil: ≥20 kV/mm) to block discharges.

• Dry-Type Bushings: Use epoxy resin or silicone rubber for solid insulation.

• External Insulation:

• Porcelain or Composite Housing: Features shed (umbrella) designs to increase creepage distance, preventing surface flashover in rain or polluted environments.

Electric Field Control: Capacitive grading layers (in capacitor-type bushings) or shielding rings optimize electric field distribution, avoiding localized high field strength.

3. Sealing System: Isolation of Environments
• Flange Seals: Rubber gaskets or welded joints between metal flanges and porcelain/composite housings prevent oil/gas leaks.

• Oil/Gas Channels: Oil-immersed bushings connect to the transformer tank via valves to maintain internal pressure balance.

Dynamic Sealing: Elastic materials (e.g., fluorocarbon rubber) compensate for thermal expansion/contraction, ensuring long-term sealing.

4. Mechanical Support: Stress Resistance
• High-Strength Housing: Porcelain or composite materials with high bending strength (≥50 MPa for porcelain) and seismic resistance.

• Flange Anchoring: Bolted fixation to the transformer tank distributes mechanical loads from conductor tension and short-circuit forces.

III. Typical Workflow (Oil-Immersed Bushing Example)

1. Current Transmission: High-voltage current flows from windings → conductive rod → grid via terminals.

2. Insulation Protection:
   • Internal oil dissipates heat from the conductive rod.

   • External shed design blocks contamination-induced conductive paths.

3. Dynamic Balance:
   • Oil expansion due to temperature changes → returns to the tank via channels, avoiding pressure buildup.

   • Short-circuit forces → absorbed by mechanical structures to prevent rod displacement or fracture.

IV. Technical Challenges and Design Optimizations

1. Electric Field Concentration:
   • Solution: Capacitive grading screens (in capacitor-type bushings) enforce uniform voltage distribution.

2. Thermal Stress Failure:
   • Solution: Metal-ceramic composites with matched thermal expansion coefficients minimize seal cracking.

3. Pollution Flashover:
   • Solution: Hydrophobic coatings on silicone rubber sheds maintain high flashover voltage in polluted conditions.

V. Smart Bushings: Evolution in Operation
Modern smart bushings integrate sensors for real-time condition monitoring:
• Temperature Sensors: Embedded in conductive rods or flanges to detect hot spots.

• Partial Discharge Sensors: Monitor insulation degradation via micro-discharge signals.

• Oil Pressure Sensors: Track sealing integrity to prevent leaks or gas ingress.

Data Integration: Signals transmitted via fiber optics or wireless systems enable predictive maintenance and fault diagnosis.

Conclusion
Transformer bushings ensure safe power transmission by balancing electric, thermal, and mechanical stresses through multilayer insulation, precision sealing, and robust mechanical design. Advances in materials (e.g., nanocomposite insulation) and smart sensing are transforming bushings from passive components into active guardians of transformer reliability, ushering in an era of predictive and resilient grid operation.