An improved calculation based on time-domain method of circulating current for hybrid modular multilevel converter

The Hybrid Modular Multilevel Converter (HMMC) has become the hotspot because of its combination of the economy of the half-bridge sub-module (HBSM) and the full-bridge sub-module (FBSM), which has the capability of crossing the DC-side faults. During the operation of the HMMC, the circulating current between the arms directly affects its performance, which is one of the technical problems in the field of HMMC topology. This paper analyzes the topology and modulation scheme of the HMMC in detail, derives the mechanism of circulating current, and proposes an improved circulating current calculation method for the HMMC. This paper uses the time-domain method to derive the circulating current formula, taking into account the coupling between the various harmonic contents, so as to derive the HMMC circulating current expression containing the DC component, the double and quadruple frequency components easily and accurately. In order to verify the accuracy of the circulating current calculation, an HMMC model was built synchronously. Simulation research is also carried out in MATLAB/Simulink. And the design of the HMMC prototype is completed. The simulation results and experimental results verify the correctness of the improved calculation based on the time-domain method of circulating current.