Autonomous Control Schemes for Grid-Interfaced Series-Connected Low-Voltage Isolated PV Microinverter

This paper presents two autonomous control schemes for grid-interfaced, series-connected low-voltage microinverters. Low Voltage microinverter module contributes to a part of the grid AC voltage. Multiple such modules are connected in series to interface with the grid. This architecture eliminates the need for a high step-up stage in conventional microinverter, enhancing efficiency and reducing the cost per module. However, series-connected microinverters cannot use a simple current control for grid-interfaced operations. Thus, a proper control strategy needs to be implemented to achieve Maximum Power Point Tracking (MPPT). This paper provides a suitable autonomous control scheme for achieving MPPT in all series-connected low-voltage microinverter modules. In certain situations, the voltage variation between the connected modules becomes too wide due to specific units with very weak PV modules, requiring certain modules to operate with voltage beyond their permissible limits. For such situations, a second control scheme is suggested. In this scheme, all modules operate with equal output voltage, and power is shared equally between modules. This scheme can be useful in achieving individual module MPPT in case the panels are kept in close proximity and there is no irradiance or temperature variation. Both schemes are verified with simulation and experimental results.