Transformers can be classified based on various criteria such as cooling method, core material, application, voltage level, and phase. Each type has unique characteristics that make it suitable for specific applications.

　　Based on cooling method, transformers are divided into oil-immersed transformers and dry-type transformers. Oil-immersed transformers use mineral oil or synthetic oil as both an insulating and cooling medium. The oil circulates through the transformer, absorbing heat from the windings and core and dissipating it through radiators or cooling fans. They are known for their high efficiency, good insulation properties, and ability to handle high power ratings, making them suitable for outdoor installations, power transmission, and distribution systems. However, they require regular maintenance of the oil, are bulky, and pose a fire risk if the oil leaks and comes into contact with a spark or flame. Dry-type transformers, on the other hand, use air or gas for cooling. They are filled with air or a non-flammable gas such as nitrogen and rely on natural convection or forced air cooling (using fans). These transformers are more compact, require less maintenance, and are safer for indoor installations, especially in buildings where fire safety is a concern, such as hospitals, schools, and high-rise buildings. They have lower power ratings compared to oil-immersed transformers and are more sensitive to dust and moisture.

　　Based on core material, transformers are categorized into iron-core transformers and air-core transformers. Iron-core transformers use a core made of laminated silicon steel sheets, which have high magnetic permeability, reducing magnetic losses. They are widely used in most power and distribution applications due to their high efficiency and low cost. Air-core transformers, with a core made of air or non-magnetic materials, have higher magnetic reluctance and are used in high-frequency applications such as radio frequency (RF) circuits, where low magnetic losses and compact size are important.

　　Based on application, transformers include power transformers, distribution transformers, instrument transformers, and special-purpose transformers. Power transformers are used in power generation stations and transmission lines to step up or step down high voltages for efficient long-distance transmission. They have large ratings, typically above 50 MVA, and are designed for continuous operation at high voltages. Distribution transformers are used in distribution networks to step down the voltage from transmission levels to usable levels for residential, commercial, and industrial users. They have smaller ratings, usually below 25 MVA, and are installed in substations or on poles. Instrument transformers, such as current transformers (CTs) and voltage transformers (VTs), are used to measure high currents and voltages safely in electrical systems, providing isolated low-level signals for meters and protective relays. Special-purpose transformers include isolation transformers, which provide electrical isolation between input and output to enhance safety, and autotransformers, which have a single winding and are used for voltage adjustment with higher efficiency but lack isolation.

　　Based on voltage level, transformers are classified as low-voltage (below 1 kV), medium-voltage (1 kV to 33 kV), high-voltage (33 kV to 220 kV), and extra-high-voltage (above 220 kV). Low-voltage transformers are used in household appliances and small industrial equipment. Medium and high-voltage transformers are used in distribution and transmission systems, while extra-high-voltage transformers are employed in long-distance high-power transmission networks.

　　Based on phase, transformers can be single-phase or three-phase. Single-phase transformers are used in residential and small commercial applications where single-phase power is available. Three-phase transformers are used in industrial settings and large power systems, providing efficient power transmission and distribution for three-phase loads such as motors and machinery.

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