Modeling and control of a grid-connected small-scale windmill system using a pulse width modulated modular multilevel converter.

摘要

Topologies based on the modular multilevel converter (MMC) represent an attractive alternative against traditional two-level voltage source converter (VSC), as the interphase between renewable energy sources and the ac grid. In MMC topologies, total system losses are relatively low; thus the efficiency is consequently higher as compared to existing two-level VSCs. In this thesis, a pulse width modulated MMC-based back-to-back topology with a reduced number of levels is proposed for the grid-interconnection of a small-scale wind energy conversion system (SS-WECS). The SS-WECS is realized through the cascaded connection of a wind turbine, a permanent magnet synchronous generator (PMSG), two back-to-back connected MMCs interphased through a dc-link capacitor, a LC-filter and a coupling transformer. Mathematical modeling of the various components is developed throughout the thesis, along with a control strategy able to perform both maximum power point tracking and arbitrary reactive power injection. A pulse width modulation (PWM) technique is implemented in reduced level MMCs, to inject power into the grid with a low harmonic content. This allows size reduction of the LC-filter. The approach is validated via detailed PSCAD/EMTDC computer simulations using real wind speed data.