Analysis and Suppression of Resonant Current Envelope Ripple of LLC Converter in Cascaded Modular PV Solid-State Transformer

Featured with the advantages of expandable modular structure, simple layout, and low dependence on copper and steel, three-phase solid-state transformer-based cascaded H-bridge (HB) topology has been replacing traditional inverters in high-voltage and high-power photovoltaic power generation applications. However, due to the double line-frequency power fluctuation on the dc side of HB converters, if the LLC topology is used as the front-stage isolated dc/dc converter, its resonant current will produce an envelope ripple with the same frequency. The envelope ripple will not only increase the current stress of switching devices and diodes in LLC converter but also increase the losses of switching devices and magnetic components. Aiming at this problem, this article analyzes the mechanism of enveloping ripple in detail by mean of the concept of calculus, expounds the influence of envelope ripple on the whole system, and then proposes a control method to suppress envelope ripple of resonant current. The validity of the proposed method is verified by simulation and experimental results.