The main difference between cast resin and oil type transformers lies in their insulation and cooling methods:

Cast Resin Transformers: These transformers use solid epoxy resin to insulate the windings. The resin provides protection against moisture, dust, and environmental contaminants, making these transformers suitable for indoor or environmentally sensitive applications. They are typically air-cooled and offer high fire resistance, requiring less maintenance than oil-filled transformers. However, they may have lower cooling efficiency and are generally used in lower voltage applications.
Oil Type Transformers: These transformers use insulating oil to cool and insulate the internal components. The oil circulates within the transformer, dissipating heat and providing excellent electrical insulation. Oil type transformers are widely used in high-voltage and high-power applications due to their effective cooling properties. They require regular maintenance to monitor the oil quality and prevent leaks, and they carry a potential fire risk due to the oil.

The presence of hydrogen in transformer oil indicates potential internal issues, such as electrical discharges, arcing, or overheating. Hydrogen is one of the first gases to be produced when there is a fault in the transformer. Its detection through Dissolved Gas Analysis (DGA) can signal the need for further investigation to prevent possible transformer failure.