Short-Circuit Current Estimation of Modular Multilevel Converter Using Discrete-Time Modeling

Short-circuit faults are hazardous for high-power modular multilevel converters (MMCs). Hence, an accurate current calculation will be helpful for the design of the system protection. In this letter, a new method for the short-circuit current calculation based on discrete-time modeling is proposed. With commonly adopted nearest level modulation, the operating state during one period can be divided into several stages according to the number of submodules inserted into a bridge arm. Then, a piecewise analysis allows the construction of a new discrete-time model by recursion of the state variables stage-by-stage. From this model, the current evolution during a short-circuit fault can be accurately calculated when given an initial state. The effects of different fault occurrence times in one period are also calculated and discussed. Finally, PSCAD/EMTDC simulations and a circuit experiment with a single-phase five-level MMC verify the effectiveness of the proposed calculation method.