Control of Modular Multilevel Converter-based Power Electronic Transformer for Grid Integration of Solar PV System

This paper presents a modular multilevel converter (MMC)-based power electronic transformer (PET) for interconnection of solar photovoltaic (PV) system and the grid. The system under study consists of a MMC-based PET and a three-phase modular multilevel inverter (MMI). The MMC-based PET extracts the maximum power from the PV system. The MMI delivers the solar PV generated power to the grid. The MMC-based PET also provides a galvanic isolation and replaces the primitive line frequency transformers. Various component of the control system includes perturbing and observation algorithm (P&O), phase shift control, model predictive control (MPC) and voltage balance algorithm. The P&O algorithm and phase shift control are employed to the MMC-based PET to extract maximum energy from the solar PV system and transfer it to the intermediate dc-link. The MPC scheme regulates the dc-link voltage of the MMI to inject the PV power into the grid. The voltage balance algorithm balances the sub-module capacitor voltage of MMCs. The system performance and effectiveness of the control algorithm is illustrated through computer simulations for different solar irradiance levels.