Control strategy reducing input impedance in two-stage three-phase 3L-NPC photovoltaic inverter

Grid connected photovoltaic inverters always adopt a cascade control system in interfacing photovoltaic cells for maximum power point tracking purposes. Unbalanced gird voltage generate a double frequency component in the dc-link voltage. Either selecting larger capacitance value or decreasing the crossover frequency of the dc-link voltage control decreases distortion of grid current references. Large electrolytic capacitors used in the dc-link lead to reliability reduction due to short life time. Decreasing the cross-over frequency improves the quality of grid current at the expense of poor response to input power transients. Based on a small signal liner model, this paper reveals that lower input impedance and smooth output admittance of the inverter adopts capacitor current control strategy both effectively improves response to input power transients and provides low gird current THD. Experiments are performed with 200μF of dc-link capacitance for a 3-KW cascade PV inverter.