Predictive control of multilevel converters for megawatt wind energy conversion systems

This dissertation proposes two novel medium voltage (MV) multilevel converter configurations for use with permanent magnet synchronous generator (PMSG) based megawatt (MW) wind energy conversion systems (WECS). The classical control techniques, based on linear PI regulators and low band-width modulation, present several technical issues during lower switching frequency operation. To overcome these issues, a high performance finite control-set model predictive control (FCS-MPC) strategy is proposed to control the power converters employed in the MW-PMSG-WECS. The proposed three-level and four-level converters combine the advantages of proven wind turbine technologies, such as low-cost generator-side passive converters, and efficient gridside multilevel converters. The intermediate dc-dc multilevel converters ensure balancing of the capacitor voltages during all operating conditions. With this feature, the grid-side multilevel converters produce better grid current waveforms compared to the back-to-back connected converters. A generalized approach for the predictive control of an n-level diode-clamped converter was investigated. The FCS-MPC strategy for current control and decoupled active/reactive power regulation of grid-connected multilevel converters was also analyzed. The major WECS requirements such as maximum power point tracking, balancing of dc-link capacitor voltages, switching frequency minimization, common-mode voltage mitigation, regulation of net dc-bus voltage, and grid reactive power control have been modeled in terms of power converter switching states. These control objectives have been accomplished during each sampling interval by selecting the switching states which minimize the generator- and grid-side cost functions. Issues related to the weighting factors selection, control delay compensation, accurate extrapolation of references, control of variable switching frequency nature, prediction of variables over two samples with reduced computational burden, and robustness analysis, are also addressed in this dissertation. To keep the dc-bus voltage constant during low voltage ride-through operation, predictive control scheme is proposed for the power converters while storing surplus energy in the turbine-generator rotor inertia. The generation and exchange of reference control variables during symmetrical grid voltage dips is suggested to meet the grid code requirements. The proposed solution is efficient as no energy is dissipated in the dc-link crowbar. The simulation and experimental results validate the proposed MV converters and predictive control schemes.