Three-Phase Oil-Immersed Transformers: Core Components, Operation, And Maintenance

Three-Phase Oil-Immersed Transformers: Core Components, Operation, and Maintenance
Comprehensive Guide to Power Distribution Systems

Oil-immersed transformers are critical in power grids, ensuring efficient energy transmission. This guide breaks down their structure, functions, and maintenance practices for engineers and industry professionals.

Core Components: Functions of Key Parts

1. The "Heart" and "Blood Vessels"

Iron Core (Magnetic Circuit Skeleton):

• Function: Conducts magnetic fields while minimizing energy loss.
• Structure: Made of cold-rolled silicon steel laminations (0.35–0.5mm thick), coated with insulating paint or oxide layers to reduce eddy currents.
• Design: Consists of core columns (vertical sections housing coils) and yokes (horizontal sections forming a closed magnetic loop).

Windings (Coils):

• Primary (Input) vs. Secondary (Output): Transfer electrical energy from power sources to loads.
• Construction: Copper or aluminum wires insulated with high-strength enamel.
  • Low-voltage coils (fewer turns) sit close to the core.
  • High-voltage coils (more turns) wrap externally, creating oil channels for cooling.

2. Enclosure & Cooling System

Oil Tank:

• Role: Houses core/windings, filled with insulating mineral oil for dielectric protection and heat dissipation.
• Types of Oil:
  • No. 10 (-10°C pour point), No. 25 (-25°C), No. 45 (-45°C) based on regional climate.

Conservator Tank (Oil Pillow):

• Function: Compensates for oil expansion/contraction due to temperature changes.
• Features:
  • Internal diaphragm isolates oil from air.
  • Oil level gauge marks: +40°C (max load), +20°C (average), -30°C (min load).
  • Breather valve balances pressure during thermal expansion.

3. Safety & Control Components

Bushings:

• Role: Insulated connectors for high/low-voltage lines exiting the tank.

Explosion-Proof Pipe:

• Function: Releases sudden pressure surges (e.g., short circuits) to prevent tank rupture.

Gas Relay (Buchholz Relay):

• Monitoring: Detects internal faults via gas bubbles in oil.
  • Light Gas: Triggers alarm.
  • Heavy Gas: Initiates circuit breaker trip.

Temperature Sensors:

• Thresholds: Alarm at 80°C, automatic shutdown at 100°C.

Neutral Grounding Switch:

• Purpose: Protects insulation during power surges in 110kV+ systems.

Tap Changer:

• Types: On-load (live adjustment) and off-load (requires shutdown).
• Adjustment Rules: Incremental steps (1档 = 1 minute pause); emergency stop if multiple steps trigger.

Operation & Maintenance: Ensuring Safety and Reliability

1. Standard Operating Guidelines

• Temperature Limits:
  • Upper oil temperature ≤95°C (ideally ≤85°C).
  • Voltage deviation: ≤±5% of rated voltage (up to ±12% with manufacturer approval).
• Overload Management:
  • Normal Overload: Based on load history and cooling efficiency.
  • Emergency Overload: Follow manufacturer specifications.

2. Emergency Shutdown Scenarios

Immediate action required for:

1. Unusual internal noises (e.g., cracking sounds).
2. Persistent temperature rise despite normal cooling.
3. Oil leakage from conservator/explosion-proof pipe.
4. Oil level below minimum gauge mark.
5. Black carbon particles in oil or damaged bushings with arcing.

3. Troubleshooting Common Issues

• Overheating: Check load levels, cooling system functionality, and thermometer accuracy.
• Oil Problems:
  • Frozen oil: Monitor flow and temperature.
  • Rapid oil loss: Address leaks promptly (welding or seal replacement).
• Gas Relay Activation:
  • Non-Flammable Gas: Replenish oil.
  • Combustible Gas: Halt operations; inspect insulation failures or internal faults.

4. Routine Inspection Checklist

1. Listen for abnormal noises or discharge sounds.
2. Verify oil level and leak-free seals.
3. Monitor upper oil temperature (≤85°C).
4. Inspect bushings for cracks or arcing.
5. Check terminal connections for overheating.
6. Ensure explosion-proof membrane integrity.
7. Confirm gas relay oil level and diaphragm condition.
8. Test breather functionality and silica gel color (blue = dry; pink = saturated).
9. Validate cooling system efficiency (radiators/fans).
10. Inspect grounding connections for continuity.

Conclusion

Three-phase oil-immersed transformers are the backbone of modern power grids, combining robust design with precise engineering. Regular maintenance, adherence to operational limits, and prompt fault resolution ensure longevity and grid stability. For optimal performance, partner with certified manufacturers and adhere to international standards like IEC 60076.

For technical support or customized solutions, consult qualified electrical engineers.