Multilevel Power Point Tracking for Partial Power Processing Photovoltaic Converters

This paper presents a multilevel control and maximum power point tracking (MPPT) scheme for variable conversion ratio partial power processing photovoltaic (PV) dc-dc converters. A general system model is derived and linearized in state-space form for a switched-inductor (buck-boost dc-dc) converter with an arbitrary number of series-connected stages. The proposed control law is then used as the basis for a multilevel MPPT scheme that can optimize many series-connected PV cells, substrings, or modules simultaneously. The MPPT algorithm is shown to converge in a timeframe independent of the number of PV units and can work in concert with the central inverter MPPT algorithm without conflict. To validate the control and optimization scheme, we present a hardware prototype designed to fit in the junction box of conventional PV modules, operating at the submodule level. While the control law is developed specifically for the buck-boost topology, the MPPT algorithm is generalized and could be applied to many partial-power processing topologies provided there is capability to regulate the voltage differences or voltage ratios among adjacent PV stages.