Oil transformers use mineral oil as the heat dissipation and insulation medium. This is a widely used electrical equipment in the world. Most of the transformers in today's power transmission systems are oil-cooled. There are two main types of oil transformers: Open-type transformers (Conservator type transformers) and Sealed-type transformers (Hermetically sealed type transformers).
Cooling oil according to the principle of heat generated by the coil and magnetic core during operation will transfer to the oil. The temperature of the oil will spread out to the outside through the transformer's metal shell.
Oil-immersed transformers consist of the magnetic core, windings and bushings. The magnetic core provides a path for magnetic flow. Windings create a magnetic field and consist of a conductor coil, wrapped around the core and insulated with press board barriers and screens. The thickness of the winding insulation increases with voltage. Bushings connect transformer windings to a substation. A characteristic feature of an oil-immersed transformer is the oil conservator. It simplifies operation by monitoring the oil level in a tank and providing space for thermal expansion of the oil.
Moreover, the minimum net distance between the outer profile of the fuel oil-immersed transformer and the wall and door of the transformer room also has strict distance requirements. The various civil structures, fire barriers and storage tanks of oil-immersed power transformer substations are designed because of the flammability and liquidity of oil. In addition, "when the transformer room is located in the building, the door shall be a Class A fire door." In this way, the construction investment designed to prevent fire in each substation is naturally much larger.
Advantages of Oil-immersed transformers compared to Dry-type transformers
- The cost of procuring an oil transformer is usually 1.5 to 2 times lower than a dry transformer for the same capacity.
- Dry transformer' epoxy resin has poor heat dissipation performance than mineral oil.
- In general, the dry transformer's heat dissipation is inferior to that of the oil transformer, so the capacity of the dry transformer usually only meets the need for power distribution from the intermediate station to the end-consumption site. Also, on the grid system, oil transformers are more versatile (including transmission and distribution transformers).
- The dry-type transformer is usually suitable for voltages below 35 kV while oil-immersed are unlimited.
- The temperature monitoring of the dry transformer depends on the temperature sensor mounted inside the machine's body, so the obtained temperature is the temperature of a particular location, the temperature reflected by the screen is not the exact mean temperature. The transformer insulation oil conducts heat relatively evenly. The temperature reflected by the monitor can be considered the average temperature.
- Because of the monolithic casting structure of the dry transformer, compared with the ability to repair, recover, utilize, the oil transformer is more dominant. The insulating material of the machine dries out over time will age, and defects will accumulate. In the event of a failure, the dry transformer will be rejected entirely. The dry machine components, including copper (or aluminum) and the magnetic core, are nearly non-recyclable.