Power transformer (transformer for short) is an electrical device used to change the size of AC voltage. It is based on the principle of electromagnetic induction, a registered AC voltage exchange into another level of AC voltage to meet the needs of different loads. Therefore, the transformer occupies a very important position in the power system and power supply system.

Generator output voltage, due to the limitations of the generator insulation level, usually 6.3KV, 10.5KV, the highest not more than 20KV. with such a low voltage for long-distance transmission is difficult. Because when the transmission of a certain power of electricity, the lower the voltage, the higher the current, the electricity is likely to be mostly consumed in the resistance of the transmission line. So can only use the step-up transformer will be the generator end voltage increased to tens of thousands of volts to hundreds of thousands of volts, in order to reduce the transmission current, reduce the transmission line energy loss without increasing the cross-section of the wire will be transmitted over long distances.

Transmission lines will be tens of thousands of volts or hundreds of thousands of volts of high voltage power transmission to the load area, it must be through the step-down transformer to reduce the high voltage to low voltage suitable for the use of power equipment. For this reason, in the power supply system, the need for step-down transformers, the transmission line transmission of high voltage into a variety of different levels of voltage to meet the needs of a variety of composite.