Cascaded Converters with Synchronized Control in the Overmodulation Zone

A novel method of direct synchronous pulsewidth modulation (PWM) is disseminated for control in the overmodulation zone 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 each module and the composed voltage at the output of the converter. The four-level output voltage of the converter has quarter-wave symmetry during the zone of overmodulation. Basic versions of the discontinuous scheme of synchronous PWM, based on vector approach for determination of the pulse patterns, have been analysed and compared by simulations of the systems with low switching frequencies, which are used normally in high power systems