Triphase cascaded converters with direct synchronous pulsewidth modulation

A novel method of direct synchronous pulsewidth modulation (PWM) is applied for control of modular multilevel converters consisting from three standard triphase inverter modules along with an 0.33 p.u. output transformer.The proposed method provides synchronisation of the voltage waveforms for both each module and the composed voltage at the output of the converter. Multilevel output voltage of the converter has quarter-wave symmetry during the whole range including the zone of overmodulation. Both continuous and discontinuous versions of synchronous PWM, based on a vector approach for determination of the pulse patterns, have been analysed and compared using simulations of the systems with low switching frequencies, which normally are used in high power systems.