The biggest difference between oil-immersed and dry-type transformers is the presence or absence of "oil." Because oil is a liquid with fluidity, oil-immersed transformers always have a casing. Inside the casing is transformer oil, which immerses the transformer's coils, making the coils invisible from the outside. Dry-type transformers, on the other hand, do not have oil and therefore do not need a casing; the coils are directly visible. Another characteristic is that oil-immersed transformers have an oil conservator containing transformer oil, although some newer oil-immersed transformers do not have this conservator.
Both are power transformers, containing a core for the magnetic circuit and windings for the electrical circuit. The biggest difference lies in the "oil-type" versus "dry-type" designation. That is, they use different cooling media. Oil-immersed transformers use transformer oil (and other oils such as beta oil) as both cooling and insulation media, while dry-type transformers use air or other gases such as SF6 as cooling media. An oil-immersed transformer consists of a core and windings housed in a tank filled with transformer oil. Dry-type transformers typically encapsulate the core and windings with epoxy resin. A more commonly used type today is the non-encapsulated type, where the windings are wrapped with special insulating paper impregnated with special insulating varnish to prevent moisture absorption by the windings or core.
Oil-immersed transformers have external radiators, like heat sinks, designed for convenient heat dissipation, specifically for the flow and cooling of the internal insulating oil. Dry-type transformers lack this radiator; their cooling relies on fans beneath the transformer coils.
Due to fire safety requirements, oil-immersed transformers are generally installed in a separate transformer room or outdoors, while dry-type transformers are always installed indoors, typically in a distribution room.
In terms of production and usage, dry-type transformers currently only reach a voltage level of 35kV, and their capacity is smaller than that of oil-immersed transformers, around 2500kVA. Furthermore, the manufacturing process for dry-type transformers is more complex and costly than that of oil-immersed transformers of the same voltage level and capacity. Therefore, oil-immersed transformers are currently more widely used. However, due to their environmental friendliness, flame retardancy, and impact resistance, dry-type transformers are often used in high-requirement power supply and distribution locations, such as hotels, office buildings, and high-rise buildings.
It is evident that dry-type and oil-fired transformers each have their own advantages and disadvantages. Oil-fired transformers are inexpensive and easy to maintain, but they are flammable and explosive. Dry-type transformers, due to their good fire resistance, can be installed in load center areas to reduce voltage drop and power loss. However, dry-type transformers are expensive, bulky, have poor moisture and dust resistance, and are noisy.
Next, let's summarize the differences mentioned above into the following seven points:
1. Appearance: The packaging is different. Dry-type transformers allow direct viewing of the core and coils, while oil-immersed transformers only allow viewing of the outer casing.
2. Lead Type: Dry-type transformers mostly use silicone rubber bushings, while oil-immersed transformers mostly use porcelain bushings.
3. Capacity and Voltage: Dry-type transformers are generally suitable for power distribution, with capacities mostly below 2000kVA and voltages of 10kV and below, although some reach 35kV. Oil-immersed transformers, however, can achieve all required capacities and voltage levels. The ultra-high voltage 1000kV lines currently under construction in my country will definitely use oil-immersed transformers.
4. Insulation and Heat Dissipation: Dry-type transformers generally use resin insulation and rely on natural air cooling, with larger capacities cooled by fans. Oil-immersed transformers, on the other hand, rely on insulating oil for insulation. The circulation of the insulating oil inside the transformer transfers the heat generated by the coils to the transformer's heat sink (plates) for heat dissipation. 5. Applicable Locations: Dry-type transformers are mostly used in locations requiring fire and explosion protection, generally suitable for large buildings and high-rise buildings; while oil-immersed transformers, due to the potential for oil spillage or leakage in the event of an accident, causing a fire, are mostly used outdoors in locations with available space for an emergency oil tank.
6. Different Load Capacities: Dry-type transformers should generally operate at their rated capacity, while oil-immersed transformers have better overload capacity.
7. Different Costs: For transformers of the same capacity, the purchase price of a dry-type transformer is significantly higher than that of an oil-immersed transformer.
