Diode-clamped three-level multi-module cascaded converter based power electronic traction transformer

A diode-clamped three-level multi-module based power electronic transformer is proposed in application of locomotive traction for single-phase 25kV/50Hz railway. Each module is composed of three-level H-bridge AC-DC rectifier and half-bridge bidirectional DC-DC converter. Compared to traditional two-level H-bridge structure, the three-level module can improve interface voltage and power scale. Meanwhile, the number of cascaded modules and medium frequency transformers is decreased by half. The AC-DC rectifier is controlled by direct current control loop with dc-link voltage equalization algorithm. A minimum RMS current control algorithm is adopted in DC-DC converter to improve system efficiency. Finally, experimental results on three-module cascaded prototype verified the performances of the developed topology and control under different operating conditions.