Harmonics elimination PWM based direct control for 23-level multilevel distribution STATCOM using differential evolution algorithm

Abstract

This work describes the application of a soft computing technique—known as the differential evolution (DE) to implement the harmonics elimination pulse-width modulation (HEPWM) for the direct control of distribution static synchronous compensator (D-STATCOM). The main contribution of the work is the application of DE to solve eleven HEPWM switching angles to synthesize a 23-level cascaded multilevel voltage source inverter (MVSI). The key feature of this algorithm is the wide modulation index (MI) range that can be produced (i.e. 5.40–8.15p.u) with very small step size (0.01), while maintaining the total harmonic distortion (THD) of the MVSI output voltage below 5%. Furthermore, the utilization of high levels MVSI allows for the output voltage of the D-STATCOM to be sufficiently high, thus avoiding the use of step-up transformer. The correctness of the computed HEPWM angles is validated using single phase 23-level cascaded MVSI experimental rig. Then, the proposed method is compared to the phase-shifted PWM (PS-PWM) using a ±6.5 MVAr/11kV D-STATCOM modelled in MATLAB-Simulink. For the same switching frequency, the HEPWM switching results in a superior harmonics spectra and lower switching losses. Furthermore, if the switching frequency is fixed to a particular value, the size of the series coupling inductor can be reduced to at least half. Dynamically, the steady state value of the reactive current (iCq) is reached in less than one mains cycle when a transition from the full inductive to full capacitive modes is imposed.