Novel Static Carrier Based Low Frequency Multilevel Modulations With Long Conduction Time : Analysis Of Capacitor Voltage Balancing

That work investigates the compatibility of voltage balancing algorithm and multilevel modulations of a three phase modular multilevel converter (MMC). A study of the "Reduced switching frequency" (RSF) voltage balancing algorithm with low frequency modulations is proposed. It is first shown that Nearest Level Modulation cannot be used in MMC with RSF algorithm. Two new hybrid modulations are then introduced, the Long Conduction time Pulse Width Modulation (LCPWM) and the Enhanced Long Conduction time Pulse Width Modulation (ELCPWM). They offer advantageous trade-off between switching frequency and voltage balancing accuracy with RSF algorithm. Based on innovative static carriers, they do not generate random narrow pulses, decreasing electrical stress on power transistors. The conduction times of the proposed modulations are deterministic and maximized. Their generation is highly simplified compared to triangular or saw-tooth carrier based modulations. An accurate simulation tool allows to compare various characteristics of proposed modulations to classical low frequency modulation and pulse width modulation on multi-megawatts three phase MMC.