Oil-Immersed Transformer: The Core Equipment in Power Systems

I. Overview of Oil-Immersed Transformers

Oil-immersed transformers are the most widely used type of transformers in power systems. They utilize mineral oil as both an insulating and cooling medium, offering advantages such as simple structure, reliable operation, and easy maintenance. As a critical component in power transmission and distribution, oil-immersed transformers play an indispensable role in power plants, substations, and industrial power systems.

II. Basic Structure of Oil-Immersed Transformers

Oil-immersed transformers primarily consist of the following components:

1. Core: Typically made of high-permeability cold-rolled silicon steel laminations, forming the magnetic circuit of the transformer.
2. Windings: Composed of insulated copper or aluminum wires, divided into high-voltage and low-voltage windings, which serve as the electrical circuit of the transformer.
3. Tank: A sealed container made of welded steel plates that houses the transformer oil and internal components.
4. Transformer Oil: Functions as both an insulating and cooling medium, usually mineral oil or synthetic ester oil.
5. Cooling System: Depending on capacity, it can be natural cooling (ONAN), air cooling (ONAF), or forced oil circulation with air cooling (OFAF).
6. Protection Devices: Include a conservator (oil reservoir), breather, Buchholz relay, pressure relief valve, etc.

III. Working Principle of Oil-Immersed Transformers

Oil-immersed transformers operate based on the principle of 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 can be stepped up or down.

The transformer oil serves multiple purposes in this process:

• Provides insulation between windings, core, and tank
• Transfers heat generated by windings and core to the tank surface via convection
• Prevents oxidation and moisture ingress in internal components

IV. Technical Features of Oil-Immersed Transformers

1. Excellent Insulation Performance: Transformer oil has a much higher dielectric strength than air, allowing for a more compact design.
2. Effective Heat Dissipation: Oil has a high heat capacity, efficiently absorbing and transferring heat.
3. Strong Overload Capacity: Compared to dry-type transformers, oil-immersed transformers have better short-term overload capability.
4. Long Service Life: With proper maintenance, oil-immersed transformers can operate for over 30 years.
5. Higher Maintenance Requirements: Regular checks of oil level, oil quality, and oil sample analysis are necessary.

V. Applications of Oil-Immersed Transformers

Oil-immersed transformers are widely used in:

• Step-up and step-down substations in power systems
• Distribution systems in industrial plants
• Auxiliary and main transformers in power plants
• Urban and rural power distribution networks
• Special environments such as mines and offshore platforms (requiring specialized protection designs)

VI. Maintenance and Troubleshooting of Oil-Immersed Transformers

To ensure safe and stable operation, the following maintenance tasks should be performed:

1. Regular Inspections: Including oil level checks, temperature monitoring, and leakage inspections.
2. Oil Quality Testing: Periodic sampling to analyze breakdown voltage, moisture content, acidity, and dielectric loss.
3. Preventive Testing: Includes winding resistance measurement, insulation resistance testing, and turns ratio testing.

Common faults and troubleshooting methods:

• Abnormal Oil Level: Check for leaks or breather blockages
• Excessive Oil Temperature: Inspect load conditions and cooling system operation
• Buchholz Relay Activation: Analyze gas composition to determine internal fault type
• Insulation Deterioration: Detect early faults through dissolved gas analysis (DGA)

VII. Development Trends of Oil-Immersed Transformers

With technological advancements and increasing environmental requirements, oil-immersed transformers are evolving in the following directions:

1. High Efficiency and Energy Savings: Use of amorphous alloy cores and optimized electromagnetic design to reduce no-load and load losses.
2. Eco-Friendly Designs: Development of biodegradable vegetable-based insulating oils to replace mineral oils.
3. Smart Transformers: Integration of online monitoring systems for real-time tracking of oil temperature, oil level, and partial discharge.
4. Compact Designs: Structural optimization to reduce size, adapting to space constraints in urban substations.
5. Enhanced Reliability: Improved sealing technology to extend maintenance intervals and service life.

VIII. Conclusion

As the “heart” of power systems, the safe and stable operation of oil-immersed transformers directly impacts power supply reliability. With the application of new materials and technologies, oil-immersed transformers will continue to play a vital role in power transmission and distribution. Understanding their structure, working principles, and maintenance requirements is crucial for ensuring power system safety. In the future, smarter, greener, and more efficient oil-immersed transformers will provide strong support for energy transition and smart grid development.