Fixed Switching Frequency Direct Model Predictive Control for Grid-Connected Converters
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Model predictive control (MPC) has recently been gaining ground as a suitable control method for power electronic converters. It is formulated as an optimization problem in the time domain subject to certain constraints. Moreover, it is able to handle multiple-input multiple-output (MIMO) switched (non)linear systems and implement limitations in the form of hard and/or soft constraints. The important challenges in controlling grid-connected converters involve ensuring that the output harmonic spectra comply with specific grid codes and that fast responses are achieved during changes in power references. In grid-connected applications, most often the produced harmonic spectra need to be well-defined to meet the grid codes. A fixed switching frequency and a symmetrical switching pattern ensure discrete harmonic spectra, making the compliance with the grid codes easier. In addition, fast responses during transients can be achieved by eliminating the modulator, i.e., direct control. This thesis presents a direct MPC algorithm for a three-phase two-level grid-connected voltage source converter (VSC) with an LCL filter that can operate the converter at a fixed switching frequency despite the absence of a modulator. The performance of the proposed method is compared to open-loop carrier-based pulse width modulation (CB-PWM) and the IEEE519 grid code. Several refinements to the algorithm are presented which improve the performance of the system. Moreover, the algorithm is extended to emulate the 120 degree discontinuous PWM switching pattern. In steady-state operation, the method achieves similar total harmonic distortion (THD) levels to CB-PWM, and during transients faster responses are obtained due to the elimination of the modulation stage.