Modelling and analysis of modular multilevel converter for solar photovoltaic applications to improve power quality

The design of control circuit for a solar fed cascaded multilevel inverter to reduce the number of semiconductor switches is presented in this study. The design includes `binary´, `trinary´ and `modified multilevel connection´ (MMC)-based topologies suitable for varying input sources from solar photovoltaic´s (PV). In binary mode, 2^Ns+1 - 1 output voltage levels are obtained where N_s is the number of individual inverters. This is achieved by digital logic functions which includes counters, flip-flops and logic gates. In trinary mode, 3^Ns levels are achieved by corresponding look-up table. MMC intends design in both control and power circuits to provide corresponding output voltage levels by appropriate switching sequences. Hence to obtain a 15-level inverter, the conventional method requires 28 switches and in binary mode 12 switches are needed. In trinary mode with the same 12 switches, 27 levels can be obtained whereas in MMC only 7 switches are employed to achieve 15 levels. The advantage of these three designs is in the reduction of total harmonic distortion by increasing the levels. Simulations are carried out in MATLAB/Simulink and comparisons were made. All the three topologies are experimentally investigated for a 3 kWp solar PV plant and power quality indices were measured.