1. Core
The core is the magnetic circuit part of the transformer. During operation, it generates heat due to hysteresis and eddy current losses. To reduce heat loss and decrease size and weight, the core is made of cold-rolled grain-oriented silicon steel sheets with a high permeability of less than 0.35mm. Depending on the arrangement of the windings within the core, there are core-type and shell-type transformers.
In large-capacity transformers, cooling oil channels are often provided in the core to ensure that the heat generated by core losses is effectively carried away by the circulating insulating oil, achieving good cooling.
2. Windings
Both the windings and the core are core components of the transformer. Because the windings themselves have resistance or contact resistance at the joints, heat is generated according to I²Rt. Therefore, the windings cannot carry currents higher than the rated current for extended periods. Furthermore, short-circuit currents will generate significant electromagnetic forces in the windings, damaging the transformer. The basic winding types are concentric and interleaved.
The main faults in transformer windings are inter-turn short circuits and short circuits to the casing. Inter-turn short circuits are primarily caused by insulation aging, transformer overload, or mechanical damage to the insulation during through-circuit short circuits. Inter-turn short circuits can also occur when the oil level drops, exposing the windings above the oil surface; additionally, during through-circuit short circuits, the overcurrent can deform the windings, causing mechanical damage to the insulation. In an inter-turn short circuit, the current in the short-circuited winding may exceed the rated value, but the overall winding current may not. In this situation, the gas protection system will trip, and in severe cases, the differential protection device will also trip. Short circuits to the casing are also caused by insulation aging, oil moisture, a drop in the oil level, or by lightning strikes and switching overvoltages. Furthermore, during through-circuit short circuits, the overcurrent can deform the windings, also causing a short circuit to the casing. In cases of casing short circuits, the gas protection device and the grounding protection device will generally trip.
3. Oil Tank
The transformer body (windings and core) of an oil-immersed transformer is housed in an oil tank filled with transformer oil. The oil tank is welded from steel plates. The oil tank of medium and small transformers consists of a shell and a cover. The transformer body is placed inside the shell, and the body can be lifted out for maintenance by opening the cover.
