Oil-Immersed Transformer: The Core Equipment in Power Systems

Introduction

Oil-immersed transformers, as indispensable key equipment in power systems, perform critical functions such as voltage transformation, power distribution, and electricity transmission. Since the invention of transformers in the late 19th century, the oil-immersed design has remained the preferred choice for medium and large transformers due to its excellent insulation and heat dissipation properties. This article provides a comprehensive overview of the structural principles, operational characteristics, application areas, and maintenance essentials of oil-immersed transformers.

1. Basic Structure and Working Principle of Oil-Immersed Transformers

1.1 Main Components

Oil-immersed transformers primarily consist of the following parts:

1. Core: Typically made of high-permeability silicon steel laminations, forming the main magnetic circuit.
2. Windings: Include high-voltage and low-voltage windings, wound with insulated copper or aluminum wire.
3. Insulating Oil: Usually mineral oil or synthetic ester oil, serving dual purposes of insulation and cooling.
4. Tank: A sealed container housing the transformer oil and active parts.
5. Cooling System: Includes radiators, oil pumps, fans, and other auxiliary cooling devices.
6. Protection Devices: Such as gas relays, pressure relief valves, and temperature gauges.
7. Voltage Regulation Device: On-load or off-load tap changers.

1.2 Working Principle

Oil-immersed transformers operate based on electromagnetic induction. When alternating current flows through the primary winding, it generates an alternating magnetic flux in the core, which induces an electromotive force in the secondary winding. By adjusting the turns ratio between the primary and secondary windings, voltage step-up or step-down is achieved.

2. Technical Features of Oil-Immersed Transformers

2.1 Insulation Performance Advantages

Transformer oil exhibits the following characteristics:

• High dielectric strength (typically >50 kV/2.5 mm)
• Good thermal stability
• Excellent arc-quenching properties
• Ability to penetrate fine gaps in windings, enhancing overall insulation strength

2.2 Heat Dissipation Advantages

The oil-immersed design enables efficient cooling through:

1. Natural convection of oil
2. Radiators to increase heat dissipation area
3. Forced oil circulation with air (OFAF) or water (OFWF) cooling when necessary

2.3 Other Technical Features

• Wide capacity range (from tens of kVA to hundreds of MVA)
• Broad voltage coverage (10 kV to 1000 kV)
• High operational reliability and long service life (typically 20-30 years)
• Relatively strong overload capacity

3. Application Areas of Oil-Immersed Transformers

3.1 Applications in Power Systems

1. Power Plants: As step-up transformers to elevate generator voltage to transmission levels.
2. Substations: As step-down transformers to reduce high voltage to distribution levels.
3. Transmission Networks: As interconnecting transformers linking grids of different voltage levels.

3.2 Industrial Applications

• Power distribution systems in large-scale industries like metallurgy and chemicals.
• Traction power supply systems for electrified railways.
• Power supply in special environments such as mines.

3.3 Special Applications

• HVDC converter transformers
• Rectifier transformers
• Special-purpose transformers like furnace transformers

4. Operation and Maintenance of Oil-Immersed Transformers

4.1 Routine Operation Monitoring

1. Oil Temperature Monitoring: Top oil temperature generally should not exceed 85°C.
2. Load Monitoring: Avoid prolonged overload operation.
3. Oil Level Checks: Maintain normal oil levels.
4. Abnormal Sound Monitoring: Watch for internal discharge or loose-part noises.

4.2 Regular Maintenance Items

1. Oil Quality Testing: Including breakdown voltage, moisture content, acid number, and dielectric loss.
2. Winding Tests: Insulation resistance, absorption ratio, and polarization index measurements.
3. Core Tests: Core grounding current measurements.
4. Cooling System Inspection: Radiator cleaning, fan, and oil pump operation checks.

4.3 Common Faults and Solutions

1. Insulation Aging: Monitor through dissolved gas analysis (DGA), replace oil or overhaul if necessary.
2. Winding Faults: Diagnose using DC resistance measurements and frequency response analysis.
3. Oil Leaks: Promptly replenish oil and locate leakage points.
4. Tap Changer Failures: Check contact erosion and replace if needed.

5. Development Trends of Oil-Immersed Transformers

5.1 Application of Eco-Friendly Transformer Oils

• Biodegradable synthetic ester oils
• High-fire-point oils (K-class oils)
• PCB-free environmentally friendly formulations

5.2 Intelligent Development

• Integrated online monitoring systems
• Fault early warning and diagnostic expert systems
• Application of digital twin technology

5.3 New Materials and Processes

• Amorphous alloy cores
• Development of new insulation materials
• Exploration of 3D printing in transformer manufacturing

Conclusion

Oil-immersed transformers have evolved over a century with mature technologies, yet there remains vast potential for innovation in materials, intelligence, and environmental friendliness. As power systems transition toward cleaner and smarter solutions, oil-immersed transformers will continue to play an irreplaceable role while adapting to new demands through technological advancements. Proper selection, operation, and maintenance of oil-immersed transformers are crucial for ensuring the safe and stable operation of power systems.