Harmonic elimination in a multilevel Current-Source Inverter-based grid-connected photovoltaic system

Multilevel inverters have received considerable attention for their capability to realize high power ratings and high-quality waveforms, at low switch stress and losses. Multilevel inverter based on Voltage-Source Inverter (VSI) topology is more thoroughly researched when compared to Current-Source Inverter (CSI)-based multilevel inverter. In this paper, a multilevel structure based on CSI topology is presented. Each inverter unit in the multilevel structure is equipped with its own Maximum Power Point Tracker (MPPT) and DC-side current controller. On the AC-side, a combined d_q-frame current controller is adopted. Operation of the multilevel structure is investigated under equal and unequal irradiation level conditions. From the simulation results, it is observed that, when PV arrays of different inverters in the multilevel structure are exposed to unequal insolation levels, unequal step sizes are generated in the multilevel currents on the AC-side. This, in turn, leads to the generation of low-order harmonics in the current that is injected into the grid. A modified control structure is implemented to eliminate the low-order harmonics, which is verified through simulation in PSCAD/EMTDC environment.