Balanced Per-Phase sequential switching to suppress circulating current in grid connected modular solar inverters

Abstract

In solar inverter configurations, the central and string inverters are more popular in micro grid/ utility grid connected applications. Although string inverters exhibit superior performance in terms of improved efficiency and reliability compare to central inverters, but it suffer from the circulating currents if all string inverter modules connected to common DC bus. In this work a balanced per-phase sequential controller is proposed to suppress the circulating currents between the string inverter modules. The individual phases of the inverter modules connected at the point of common coupling switched 120°apart to derive the grid per phase voltage. For an instance to derive the R-phase grid voltage at 0°, the inverter-1 R-phase would be switched at 120°and inverter-2 R-phase at 240°connected together at point of common coupling with the grid. Similar phase sequential switching with other phases ensures not only intra phase balance but also inter phase balance at PCC. The intra phase balance at the PCC confirm the suppressed circulating currents between the phases and inverter modules. The proposed per phase sequential operation of the solar inverter modules exhibit an extra advantage of boosting output voltage compare to conventional parallel inverter topology. The modular grid inverter configuration along with the proposed closed loop control strategy is evaluated analytically and validated through MATLAB simulations and hardware result