Modified Phase-Shifted PWM for Cascaded Half-Bridges of Photovoltaic Application

Photovoltaic power plants are generally far away from loads such as cities and industrial zones. Moreover, long-distance power transmission for renewable energy will become an inevitable trend in the future. Cascaded dc-dc converters can reach a high voltage to collect and transport photovoltaic energy, with advantages such as lower costs and larger capacities. However, the input power mismatch may cause a large amount of switching harmonic components, resulting in dc-link voltage fluctuations, which will not only increase the volume of filter components, but also cause unreliable circuit breaker actions. To solve the aforementioned problems, a hierarchical phase-shifted pulse width modulation (HPS-PWM) is proposed. By rationally grouping and layering the modules, low-order harmonics generated by the switching process of the dc-dc converter can be eliminated. The fluctuation of dc-link voltage can be suppressed by HPS-PWM, according to simulation results. When the number of modules is 27 and 81, the weighted total harmonic distortion (WTHD) of the HPS-PWM method can be reduced by more than 60% compared to the phase-shifted tracking pulse width modulation (PST-PWM) and fault tolerant adaptive phase-shifted pulse width modulation (FTA-PS-PWM). Finally, experiments verify the performance of HPS-PWM, agreeing with the simulation results.