Power Quality Improvement for Chopper-Cell Based Modular Multilevel Converters

In this work, a pulse amplitude fundamental frequency modulation technique is proposed for 33-level three-phase modular multilevel converter (MMC) fed by dc voltage sources with unequal amplitudes. The key feature of this algorithm is its excellent harmonics performance with very low switching losses. This allows, reduction on filtering requirements and improvement of reliability of the converters. Two sets of switching angles and input dc voltages of bridge cells are obtained by defining two values of integer parameter d. The correctness of the computed angles is validated using 33-level three phase MMC. Then, the proposed method is compared to phase-shifted PWM (PS-PWM) using a 1 MW/5.5 kV system modelled in MATLAB-Simulink. The performance of the proposed modulation technique, in terms of output voltage waveform quality, are shown. It is shown that the proposed technique allows elimination of low order harmonics up to 67th for d=34 and the 65th for d=33 harmonic component, moreover, in comparison with PS-PWM, PAM exhibits superior harmonics performance with lower switching losses.