Three-Phase Quaternary Multilevel Inverter for Two-Stage Grid-Connected Solar PV Application

This paper focuses on the applicability and challenges in the implementation of Photovoltaic inverters (PV) in grid-connected mode. Three-phase cascaded H-bridge topology with three H-bridges per phase, and a voltage asymmetry ratio of 16:4:1 (Quaternary asymmetry) is investigated. The varying power distribution ratio over the whole range of modulation index of the asymmetric Multilevel inverter (MLI) is handled by implementing a two-stage configuration for PV power extraction. This helps to operate the PV inverters within a very small range of modulation index as required and doubles the inverter utilization, hence improving the efficiency. In the presented converter, the main bridges of each phase are fed with PV, and the auxiliaries are fed with DC sources. The dc-link capacitor's voltages of the main bridges are balanced by the control scheme. The reactive power support is also verified by a step change in the reactive current reference in the control scheme. The simulation is performed in MATLAB/Simulink for verification of the application.