Closed-form solution of time-varying model and its applications for output current harmonics in two-stage PV inverter

The single-phase photovoltaic (PV) inverter needs significant capacitance to buffer the double-line frequency power pulsation at ac port. The two-stage inverter allows the designer to choose the dc-link voltage and the capacitor size flexibly. With the reduced capacitance, the lifetime of the dc-link capacitor can be prolonged by replacing the electrolytic capacitors with film capacitors. However, the capacitance deduction results in high double-line frequency voltage ripple on dc-link, which increases a series of odd harmonics in the output current. This paper hence analyzes the harmonics caused by the voltage ripple in an inverter with feedback control. The inverter is modeled as a time-varying system by considering the dc-link voltage ripple. A closed-form solution is derived to calculate the amplitude of the ripple-caused harmonics. This analysis helps the designer to understand the effect of the dc-link voltage ripple on current harmonics, evaluate effectiveness of existing approaches, and stimulate new ideas and solutions. The study also derived the theoretical limit to select dc-link capacitance and sampling rate of current reference without violating the grid-tied regulations in power quality. The analysis is verified both by simulation and experimental evaluation.