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Hybrid modulation method for low-switching frequency operation of modular multilevel converter

Advent of Modular multilevel converters (MMC) made the voltage sourced high voltage direct current (HVDC) transmission systems a reality mainly due to its modular structure, scalability and reliability. For these converters, minimization of sub-module (SM) capacitor voltage spread without increasing the switching frequency is the biggest challenge as it directly affects the efficiency and cost. Moreover, output voltage harmonics is another major concern because it influences filter design and electromagnetic interference (EMI) issues. Modulation techniques of MMC should address the above issues for HVDC application. A hybrid modulation method is proposed in this paper which unifies the advantages of conventional staircase modulation and pulse width modulation (PWM) methods. This simple method can optimize the capacitor voltage spread and output voltage harmonics at very low switching frequency (even less than 2 times fundamental frequency) without using any calculation intensive prediction or look-up tables. The modulation technique is simulated in MATLAB/SIMULINK platform for demonstrating its validity and the results are presented.