DC Voltage Utilization Improvement to Enlarge Power Balance Constraint Range for Photovoltaic Cascaded Inverter

In the three-phase photovoltaic (PV) cascaded inverter, the output power of PV arrays is not equal due to the difference of solar radiation, temperature and other factors, which leads to the over modulation of PV cascaded inverter. In order to solve the above problems, the carrier phase-shifted PWM (CPS-PWM) control strategy based on third harmonic injection is proposed in this paper, taking a single-stage high-frequency-link isolated cascaded PV inverter topology with clamping circuits as an example. It can not only improve the utilization of DC voltage, but also enlarge the power balance constraint range between the units in the phase of cascaded PV inverter, so as to ensure that all units can avoid over modulation under some serious power imbalance cases, and will not increase the third harmonic component injection of grid current. Firstly, the topology and working principle of the proposed inverter based on CPS-PWM control strategy are introduced. Then, the improved CPS-PWM control strategy which can improve the DC voltage utilization of the PV cascaded inverter is analyzed, and the control strategy of intra-phase power balance is emphatically analyzed in the overall control strategy of PV cascaded inverter system, and it is discussed that the proposed control strategy can also enlarge the power balance constraint range between the units of the cascaded inverter. Finally, the effectiveness of the proposed PV cascaded inverter based on the improved CPS-PWM control strategy is verified by the simulation and experimental results.