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1 Overview Since the 20th century, China's coal mines have begun to use a large number of 127V lighting power supplies and their integrated protection devices. So far, the lighting equipment selected by coal mines in China is still dominated by traditional power sources. The power frequency transformers, combination switches and integrated protection devices are combined together and installed in an explosion-proof enclosure to supply power to underground coal mine equipment. As the scale of underground mining increases, the roadway and fully mechanized mining face lengthen, and the underground power supply voltage increases. In recent years, power supplies with large capacity of 6kVA, 8kVA or even 10kVA have appeared. However, most of these power supplies are mainly designed with power frequency transformers. They are large in size, high in quality, and low in efficiency. They cause relatively serious energy loss in the process of changing the voltage of 660V to 127V.
The use of power electronic transformers for voltage regulation has been rapidly developed in recent years. Its main performances include: no grid pollution; stable output voltage; adjustable power factor; high controllability; reliable circuit detection and protection. Therefore, the study of small-sized, small-quality, high-efficiency power electronic transformers to replace power frequency transformers has become a major research topic at this stage.

2 The design idea of ​​adopting power electronic transformer AC voltage regulating power electronic transformer is derived from AC/AC conversion circuit with high frequency link. The utility model relates to a conversion device which comprises a power electronic converter and realizes magnetic coupling by a high-frequency transformer. The outstanding feature is that real-time control of the voltage amplitude and phase of the AC side is realized by the converter, and the original secondary voltage of the transformer can be realized. Flexible adjustment of current and power. There are two main implementation schemes for power electronic transformers: one is AC/AC conversion without DC link in the transformation; the other is AC/DC/DC/AC conversion with DC link in the transformation. The main circuit structure diagram of the second scheme is shown in Figure 1.

Figure 1 Power electronic transformer main circuit structure diagram (please read the PDF document for details)