Why are transformers submerged in oil?

In the vast and intricate network of the electrical grid, from power generation plants to local distribution substations, one will consistently find a critical piece of equipment: the Oil Immersed Transformer. A common question from those outside the field of electrical engineering is why these essential devices are filled with thousands of liters of mineral oil. The answer lies in a combination of fundamental physics and practical engineering, primarily centered on insulation and cooling.

The Primary Function: Electrical Insulation
At its core, a transformer contains a primary and secondary winding coiled around a laminated steel core. When in operation, these windings carry electricity at extremely high voltages, sometimes exceeding hundreds of thousands of volts. The electrical potential between these windings and the transformer's grounded tank is immense.

Air is a poor insulator at these voltage levels and distances. To prevent a catastrophic electrical arc or short circuit between components, a superior insulating medium is required. Transformer oil, a highly refined mineral oil, possesses excellent dielectric strength—significantly higher than air. By submerging the core and windings, the oil effectively prevents electrical breakdown, ensuring the internal components are safely insulated from each other and the transformer's enclosure.

The Critical Secondary Role: Heat Dissipation
Transformers are not 100% efficient. Energy is lost primarily as heat due to resistive losses in the windings (I2R losses) and magnetic losses in the core (eddy currents and hysteresis). This heat must be dissipated continuously to prevent overheating, which can degrade the insulation paper surrounding the windings and ultimately lead to transformer failure.

Oil Immersed Transformers leverage the oil's high thermal capacity and convective properties for cooling. As the oil in contact with the core and windings heats up, it becomes less dense and rises. Cooler, denser oil sinks to take its place. This natural convection cycle moves heat to the transformer's external radiator fins, where it is dissipated into the surrounding air. In larger transformers, this process is often assisted by pumps and fans to enhance circulation and cooling capacity.

Additional Protective and Diagnostic Benefits
The oil serves several other important functions:

Preservation of Core and Windings: The oil creates an environment that protects the internal paper and cellulose-based insulation from moisture and oxygen, which can cause corrosion and reduce insulating properties over time.

Arc Suppression: In the event of an internal electrical arc, the oil helps to quench the arc and prevent its sustained propagation, although such an event typically generates gases that indicate a serious fault.

Condition Monitoring: The oil itself acts as a diagnostic fluid. By regularly sampling and analyzing the oil, technicians can assess the health of the transformer. They can test for:

Dielectric Strength: To confirm its insulating capability remains high.

Water Content: As moisture reduces insulating properties.

Dissolved Gas Analysis (DGA): The presence and concentration of specific gases dissolved in the oil (like hydrogen, methane, and acetylene) are key indicators of developing faults, such as overheating, partial discharge, or arcing.

Considerations and Maintenance
While highly effective, the use of oil introduces specific considerations. The oil must be kept free of moisture and particulate contaminants to maintain its insulating properties. Furthermore, as a flammable substance, measures must be in place to contain the oil and manage fire risk, such as installing blast walls, fire suppression systems, or using less-flammable synthetic esters in high-risk areas.

Regular maintenance, including oil testing, filtration, and periodic inspection, is therefore mandatory to ensure the long-term reliability and safety of an Oil Immersed Transformer.

The practice of submerging transformers in oil is not a historical artifact but a continued, well-engineered solution to the twin challenges of insulation and thermal management. The Oil Immersed Transformer remains a cornerstone of modern electrical infrastructure due to its proven reliability, efficiency, and the valuable diagnostic data its oil provides, ensuring the stable and continuous transmission and distribution of electrical power.