Oil immersed transformers are mainly composed of iron cores, windings, oil tanks, oil pillows, insulation sleeves, tap changers, and gas relays. They have the advantages of good insulation performance, good thermal conductivity, and low cost.

Oil immersed transformers are suitable for AC 50 (60) Hz, with a maximum rated capacity of 2500kVA for three-phase (833kVA for single-phase, which is generally not recommended). They can be used indoors (outdoors) and can be installed on poles when the capacity is 315kVA or below.

Oil immersed transformers can be divided into three-phase and single-phase. In three-phase power systems, three-phase transformers are generally used. When the capacity is too large and limited by transportation conditions, three single-phase transformers can also be used to form a transformer group in three-phase power systems.