Control of a Three-Phase Multi-String Five-Level Inverter for Grid Integration of PV SystEms with Unbalanced DC-Link Voltages

A control scheme of a three-phase multi-level multi-string inverter for grid integration of photovoltaic (PV) power systems is presented. The three-phase inverter synthesizes a five level output voltage from six isolated PV sources using reduced number of power switches when compared to conventional multilevel inverter topologies. Only eighteen power semiconductor switches are required to synthesize the five level ac output, six of which are operated at line frequency. This reduces the switching losses and also the overall cost of the system. The inverter uses a new modulation scheme in its control for handling power imbalance between the PV sources in each phase and amongst the three phases. Some results obtained from the grid connected five level multi-string inverter are presented.