Design and implementation of a modular multilevel converter based wind energy conversion system

The modular multilevel converter (MMC) is an effective power electronic interface due to its modular structure for wind energy conversion system (WECS). Capacitor voltage balancing under nearest level modulation (NLM) method is a critical topic to hold the capacitor voltage of each submodule (SM) of MMC constant. In this thesis, new approaches are discovered by focusing on this topic. Accordingly, firstly, a detailed review on the circuit topology, control and applications of the MMC considering the recent improvements is presented. Secondly, a novel NLM method is proposed with increased output voltage quality for MMC. Thirdly, a generalized SM selection method using the formula of the number of states calculation incorporated into the capacitor voltage balancing scheme under NLM method is developed and sampling frequency analysis is performed. Fourthly, an efficient capacitor voltage balancing scheme under NLM method, DC-link voltage control scheme via hysteresis current control based PI controller and dq reference frame-based load voltage control technique at the point of common coupling are designed and developed for MMC based WECS. Simulation studies are carried out to verify the effectiveness of the proposed capacitor voltage balancing scheme and NLM method using various case studies. Ultimately, experimental studies are presented to demonstrate the applicability of the proposed capacitor voltage balancing scheme under NLM method for the MMC based WECS.