Analysis of standard and non-standard control modes for modular converters with synchronous modulation

The paper describes operation of cascaded (modular) converters with synchronous pulsewidth modulation (PWM), mainly for control regimes different from conventional volts/hertz control. Modular multilevel converter consists from three standard three-phase inverter modules along with an 0.33 p.u. output transformer. The proposed PWM method provides synchronisation of the voltage waveforms of each module and of the composed voltage at the output of the converter. The line-to-line output voltage of modular converter (with two, three or four levels in dependence of modulation index) has quarter-wave symmetry during the whole control range for both linear and non-linear control modes and regimes. Basic versions of continuous and discontinuous schemes 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