The structure of transformers

The basic structural components of a transformer are the iron core and winding, which make up the body of the transformer. In order to improve the heat dissipation conditions, the bodies of large and medium capacity transformers are immersed in a closed oil tank filled with transformer oil, and the connections between each winding and the external circuit are led out through insulation sleeves.

In order to ensure the safe and reliable operation of the transformer, accessories such as an oil storage tank, gas relay, and safety airway are also installed.

（1） Iron core

The iron core serves as the magnetic circuit of a transformer; It also serves as the mechanical skeleton of the transformer. In order to improve the magnetic conductivity and reduce the loss caused by alternating magnetic flux in the iron core, the iron core of the transformer is stacked with electrical steel sheets with a thickness of 0.35-0.5mm. The two sides of the electrical steel sheet are coated with an insulation layer to provide insulation. Large capacity transformers often use cold-rolled electrical steel sheets with high magnetic permeability and low losses.

The iron core of power transformers generally adopts a core structure, which can be divided into two parts: the iron core column (the part with winding) and the iron yoke (the part connecting two iron core columns). The winding is mounted on the iron core column, and the iron yoke closes the magnetic circuit between the iron core columns. When the iron core column and iron yoke are combined to form the entire iron core, overlapping assembly is often used, so that the joints of each layer are not at the same location, which can reduce excitation current. However, the disadvantage is that the assembly is complex and time-consuming. In general transformers, the cross-section of the iron core column adopts a stepped shape with an outer circle. Square shape is only used when the transformer capacity is very small.

The exchange of magnetic flux in the iron core can cause eddy current loss and hysteresis loss, causing the iron core to heat up. In the iron core of large capacity transformers, oil passages are often installed. The iron core is immersed in transformer oil, and when the oil flows through the oil passage, it can take away the heat in the iron core.

（2） Winding

The winding is the circuit part of a transformer used to transmit electrical energy, generally divided into high-voltage winding and low-voltage winding. The winding connected to a higher voltage is called a high-voltage winding; The winding connected to a lower voltage is called a low-voltage winding. In terms of energy transformation and transmission, the winding connected to the power source that absorbs electrical energy from the power source is called the primary winding (also known as the primary winding or primary winding); The winding connected to the load and delivering electrical energy to the load is called the secondary winding (also known as the secondary winding or secondary winding). The winding is usually made of insulated copper wire. The high-voltage winding has multiple turns and a small cross-sectional area of the wire; The low voltage winding has fewer turns and a larger wire cross-section.

In order to ensure the safe and reliable operation and sufficient service life of transformers, there are certain requirements for the electrical performance, heat resistance, and mechanical strength of the windings. A winding is a combination of several coils connected according to a certain pattern. According to the different positions of the high-voltage winding and the low-voltage winding, the winding structure can be divided into two types: concentric and overlapping. Concentric winding refers to the installation of high-voltage winding and low-voltage winding concentrically on the iron core column.

For the convenience of insulation, the low-voltage winding is tightly against the iron core, while the high-voltage winding is wrapped outside the low-voltage winding, leaving an oil passage between the two windings. Oil passage one serves as the insulation gap between windings; The second is to act as a heat dissipation channel, allowing oil to flow through the cooling winding from the oil passage. In a single-phase transformer, the high and low voltage windings are divided into two parts, each set on two iron core columns, which can be connected in series or parallel; In three-phase transformers, all high and low voltage windings belonging to the same phase are assembled on the same iron core column. The structure of concentric windings is simple and easy to manufacture, and heart type transformers generally use this structure.

Overlapping winding is formed by dividing the high-voltage winding and low-voltage winding into several wire cakes and arranging them alternately along the iron core column. In order to facilitate insulation and heat dissipation, an oil passage is left between the high-voltage winding and the low-voltage winding, and the low-voltage winding is placed near the iron yoke at the top and bottom layers. The mechanical strength of the overlapping winding is high, and the lead wire is convenient. Shell type transformers generally use this structure.