A novel DC voltage regulation scheme for dual-inverter grid-connected photovoltaic plants

A multilevel power conversion scheme for three-phase grid-connected photovoltaic (PV) generation systems is considered in this paper. The scheme is based on two insulated strings of PV panels, each one feeding the dc bus of a standard two-level three-phase voltage source inverter (VSI). Inverters are connected to the grid by a three-phase transformer having open-end windings on inverters side. The resulting conversion structure performs as a multilevel power active filter, doubling the power capability of a single VSI with given voltage and current ratings. The multilevel voltage waveforms are generated by an improved space vector modulation, suitable for the implementation in industrial DSPs. A novel control scheme is proposed to regulate the two dc-link voltages of the multilevel converter according with the requirement of a MPPT algorithm. The whole generation system has been verified by experimental tests with reference to different operating conditions.