THD Minimization of Modular Multilevel Converter With Unequal DC Values

Different modulation techniques used to control multilevel converters can be classified based on the selected converter topology and optimization goals. Among all proposed modulation methods low switching frequency modulation techniques are very popular for multilevel converters yet non-real time low switching frequency methods cannot be applied to multilevel converters with unequal or varying DC values because these modulation techniques rely on look up tables and the size of look up tables will be huge in this case. This paper proposes a new modular multilevel converter (MMC) structure with unequal DC values. Some well-known low switching frequency modulation techniques and the commonly used PWM based methods are compared and using the new low switching frequency modulation technique called minimal total harmonic distortion (THD) modulation for MMC with unequal DC values is proposed. The PSCAD simulation results show that the new converter topology with unequal DC values has much lower THD compared to the typical MMC. Modulation algorithm is implemented in digital signal processor (DSP) and controller hardware in the loop (CHIL) implementation in RTDS verifies the real-time performance of the algorithm.