Improvement of Low-Switching-Frequency Rectifier With Hybrid Cascaded H-Bridge Multilevel Cell

Due to device losses and heat dissipation, single-phase rectifiers in high-speed railways are operated at low-switching frequencies. Harmonic resonance in the traction power supply substation is mostly caused by the low-frequency switching harmonics. In this article, a hybrid cascaded H-bridge multilevel rectifier is proposed to improve the overall performance by cascading a high-frequency switching cell. The added cell operates at full-range no-load with half the dc voltage of the low-frequency cell. The proposed asynchronous space vector pulsewidth modulation (SVPWM) provides equivalent switching frequencies in the low-voltage cell (LV <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\_$</tex-math></inline-formula> cell) while maintaining power frequency in the high-voltage cell. The limitation of no redundant vectors, the LV <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\_$</tex-math></inline-formula> cell's no-load operation, and the different voltage set values make voltage regulation challenging. An enhanced control strategy is proposed to maintain a stable voltage with wonderful dynamic performance. Simulation and experimental results show that better harmonic performance and lower power losses are achieved.