Direct Digital Control and Capacitor Current Compensation to Improve Grid-Current Distortion for Three-Phase Three-Wire LCL Grid-Connected Inverter Under Distorted Grid Voltages

Grid voltage is always distorted which results in grid-current distortion when there is no control at the capacitor current. This paper presents the grid-current improvement with direct digital control and capacitor voltage feedforward (CVF) for 3Φ3W LCL grid-connected inverter under distorted grid voltages. With the inverter-side current feedback, the simple direct digital control can cover inductance variation and determine the control laws to track inverter-side inductor current, achieving robustness. A simple hysteresis repetitive predictor with finite impulse response low-pass filter and compensator term is introduced to the CVF loop which can stabilize the system and compensate the distorted grid current with effective control. Therefore, the overall control system becomes simple, robust and effective. The virtual impedance method is adopted to confirm the stability when the filter inductances drop and control parameters vary. Simulated and experimental results from 12 kW are used to verify the theoretical analyses and discussions.