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As an important transmission and transformation equipment of the power system, the power transformer has a decisive influence on the safety of power supply. This is more prominent in high voltage and large capacity power systems. The transformer is like other high voltage electrical equipment, and its insulation is classified according to the working voltage. The current standard transmission voltages in China are 3, 6, 10, 35, 110, 22, 330 and 500kv.. Besides, there are 15, 20, 44, 60 and 154kv voltage levels at all levels, but they are no longer developing.

Most of the power transformers are non-immersed transformers. The insulation materials used are mainly insulation paper, insulation board and transformer oil, and a few other insulating materials.

Cable paper is the most basic insulation material for coil insulation, interlayer insulation and insulation, lead insulation and main insulation of current transformer. The paper which has been treated by acylation or cyanide can improve the heat resistance of the paper. Using this paper to make the turn insulation, compared with the ordinary cable paper, can allow the wire surface temperature to increase by 10-15 degrees, that is equivalent to the transformer capacity increased by 10-15%.

Wrinkled insulation paper has a good elongation. According to different requirements, elongation rate can be made into 15-17%, 50%, 70% and 100%. They are used for insulation of turn insulation, electrostatic lead and coil end.

As the main insulation coil insulation board, made of insulating tube, angle rings, end rings, oil stays, pads and core insulation insulation parts etc..

At present, the largest single capacity of the transformer in China is 360MVA, and the maximum voltage is 500KV. In foreign countries, a three-phase transformer with a capacity of 1000MVA has been produced with the highest transmission electric voltage of 1150KV. The experimental lines of 1500KV are also in operation.

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