Direct control of D-STATCOM based on 23-level cascaded multilevel inverter using harmonics elimination pulse width modulation

Abstract

The distribution static synchronous compensator (D-STATCOM) is primarily used for solving power quality problems. Normally, the phase-shifted pulse width modulation (PS-PWM) switching is employed in conjunction with the direct control of the D-STATCOM. However, the PS-PWM exhibits high switching losses. To alleviate this problem, a direct control scheme for D-STATCOM based on the harmonic elimination PWM (HEPWM) switching is developed. Due to the difficulty in solving the equations for the HEPWM angles, no work is reported on the direct control for a multilevel voltage source inverter (MVSI) D-STATCOM with more than 15-levels. Thus, the main contribution of the work is the application of HEPWM for 23-level cascaded MVSI using a wide modulation index (MI) range (i.e. 5.40 – 8.15 p.u). The main motivation to utilize the high number of level is to allow for the output voltage of the D-STATCOM to be sufficiently high, thus avoiding the use of step-up transformer. Furthermore, the achieved MI keeps the total harmonic distortion of the MVSI output voltage below the IEEE 519 Standard (5%) over the entire operating range. The eleven HEPWM switching angles were computed using an optimization technique, known as the differential evolution. Since the angles were computed offline, they were retrieved from a look-up table whenever the output voltage of the MVSI was to be constructed. The HEPWM-based direct control was benchmarked against the popular PS-PWM using ± 6.5MVAr/11kV D-STATCOM modelled in MATLAB-Simulink and PLECS software. For the same switching frequency, the proposed HEPWM switching exhibited superior harmonic spectra, hence had lower losses. Furthermore, the size of the series coupling inductor can be reduced to at least half. Dynamically, the steady state value of the reactive current was reached in less than one mains cycle when a transition from the full inductive to full capacitive modes was imposed. In addition, the proposed D-STATCOM controller mitigated the swell and sag problems in less than one cycle.