Abstract
This research adopts a full-bridge inverter and division-summation (D-Σ) digital control to yield precise current command and achieve tight tracking and control characteristics. The inverter is associated with an LCL filter to reduce current ripple. The major contributions of this research can be summarized as follows. First, since the filter inductance varies with its current, current ripple will be different at different switching periods. Therefore, this research uses a variable switching scheme to limit the current ripple, and derives the related control laws for single-phase and three-phase configurations. Second, the inductor core is selected according to the specifications, and inductance varies from two to five times. It can reduce the volume of the inductor and cost. Third, LCL filter is easy to diverge and oscillate because it’s a third-order filter. Hence, let the resonant frequency be in a range between ten times the line frequency and one-tenth of the switching frequency to avoid the resonant problems by designing the filtering parameters of Li, Cs and Lg. Forth, since there typically exist grid voltage harmonics, the injected grid current will contain harmonic components due to the effect of the LCL-filter-capacitor. This thesis presents an extended application of the D-Σ digital control associated with a filter-capacitor-current compensation to reduce the injected grid-current harmonics. The control laws of the inverter with the D-Σ digital control and compensation approach are derived in detail, and the reduction of grid-current harmonics is analyzed. Finally, simulated and experimental results measured from a 5 kW single-phase bi-directional inverter have verified the feasible application of the D-Σ digital control and proposed compensation.