Modified five-level LLC resonant inverter for multiple PV string systems

Abstract

A modified multilevel LLC resonant inverter for multi-string photovoltaic (PV) applications. An efficient variable frequency and an integrated control method for the modulation index is proposed. To obtain the best efficiency, a steady output voltage gain, “ZVS, and a wide range of load variation, the designed LLC tank has been developed to run with a frequency range close to or at a resonant frequency. All the selected parameters for the analysis were emphasized in detail. Furthermore, the behaviour of the developed converter was investigated under varying load conditions at nominal input voltage, and under conditions of varying solar insolation and at full load. It was found that the control system responded to both changes successfully and maintained the value of the output voltage at its expected level by varying the switching frequency within a selected range, and produced $1000V$,$5\mathrm{kW}$ with $97.6\%$ efficiency. Also, the developed five-level converter differed in LLC resonant behaviour compared to its typical behaviour with conventional full/half-bridge converters, as the LLC RC operated below resonant frequency under all load and input conditions. The theoretical results were validated experimentally using a lab prototype and through simulation using MATLAB-SIMULINK. Selected findings were discussed to portray the efficacy of the designed converter.