Impact analysis of practical factors for an equivalent model of modular multilevel converter

There are some practical factors influencing the application of modular multilevel converter (MMC). This paper proposes a valid equivalent model of the half-bridge MMC topology using in high voltage level from a practical point of view. Compared with the existing MMC models, the novel model has three main advanced points including three practical factors. Firstly, it is based on an efficient time-varying Norton equivalence capable of distinguishing the different on-resistances of IGBT and its inverted parallel diode. This can reduce simulation computational time markedly without sacrificing any accuracy and present a closer imitation for real entity than previous models. Secondly, the mutual inductance among the six arm inductors of the converter is taken into account in the built model since they do exist in the actual situation. Thirdly, in this model, the capacitances of sub-module (SM) capacitors are described by normal distribution functions rather than numerical values, which can accurately simulate the uncertainty of capacitances because they are not exactly the values as their nameplates. Furthermore, this paper discusses the necessity of considering the above three aspects from some appealing external properties of MMC, such as the total harmonic distortion (THD) of output alternating voltage, the circulating current among the arms as well as the ripple of capacitor voltage in the SMs. The results can be used as the foundation for simplifying the simulation model by neglecting some factors. And according to the analysis, some properties can be well improved such as changing circuiting current by adjusting the arm inductors, namely modulating their mutual inductance by altering the winding direction or the distances among the six arm inductors and so on, which provides a reference for the practical design of MMC. In this paper, a single-ended inverter of MMC is built with PSCAD/EMTDC environment to present the meaningfulness and validity of the proposed modeling structure.