Cascaded H-Bridge & neutral point clamped hybrid asymmetric multilevel inverter topology for grid interactive transformerless photovoltaic power plant

Hybrid multilevel inverter topology is proposed for high power transformer less grid interactive photovoltaic power plant. Cascaded H-Bridge & Neutral Point Clamp topologies are hybridized to form this topology. Trinary asymmetric ratio of dc bus voltage is adapted for increasing the number of levels. Only three basic switching stages (two single phase H bridge cell and an NPC structure with common source for all phases) are used in each phase to obtain 53 levels in line to line voltage using trinary voltage rato. NPC structure with a common DC bus for all phases is connected to main photovoltaic (PV) array. This is kept at highest voltage to optimize the DC bus capacitor rating, and to reduce dc bus voltage ripple to increase MPPT efficiency. Third harmonic voltage is injected in order to increase the number of voltage levels further, and to increase the DC bus utilization. Increasing the number of voltage levels gives more flexibility to reduce Total Harmonic Distortion (THD) of the system and in turn reduction in cost and size of the power filter. Switching loss is optimized by operating the inverter at fundamental frequency. Nearest voltage level control modulation technique is adapted. The converter is simulated in MATLAB/Simulink. The results are validated through prototype experimentation in the laboratory.