Research on key technologies for power electronic traction transformer

With the technological development of semiconductor and magnetic materials, the power electronic transformer has become a research hotspot in recent years. Compared with the traditional power frequency transformer, it has the advantages of high power density and high efficiency, full control of electric energy at the high-voltage side, and rapid protection, representing an important technical development trend of electric energy conversion in the context of carbon peaking and carbon neutrality. In the field of traction, the early researches on the power electronic traction transformer abroad mainly focused on the 15 kV/16.7 Hz traction network system, while the domestic traction network system works at 25 kV/50 Hz. The domestic application of the power electronic transformer faces the contradiction between the compact space requirements and a higher working voltage, bringing a series of difficulties such as topology selection of high-voltage main circuit, miniaturization of insulation system, insulation safety of cooling system. However, the reports presenting the development status on the 25 kV/50 Hz power electronic traction transformer are absent worldwide. Targeting the power electronic traction transformer system suitable for the China's traction network system, the key technologies such as main circuit topology and control, high-voltage insulation design, cooling design was studied. In the current study, the neutral point clamped (NPC) three-level high-voltage high-frequency DC-DC isolated main circuit based on 3 300 V Si-based semiconductor devices and bilateral synchronous modulation strategy were analyzed and applied to realize rapid switching of traction/braking energy; The design approach of insulation system based on composite materials was proposed to realize the high insulation and withstand voltage of the main circuit to the ground; The key technology of pump driven phase change evaporative cooling was proposed to realize the transformer-converter integrated cooling. In addition, the development and on-board test were completed for the single branch prototype and engineering machine of the power electronic transformer. The test results demonstrate the effectiveness of the design approach.