Fixed Switching Frequency Direct Model Predictive Control With Continuous and Discontinuous Modulation for Grid-Tied Converters With LCL Filters

Abstract

This article proposes two alternatives of a direct model predictive control (MPC) scheme for a three-phase two-level grid-connected converter with an $LCL$ filter. Although both approaches are implemented as direct control methods, i.e., they combine control and modulation in one computational stage, they operate the converter at a constant switching frequency and generate a discrete grid current harmonic spectrum. To achieve this, the first method allows for one switching transition per phase and sampling interval, implying that a fixed modulation cycle akin to pulsewidth modulation (PWM) results. Moreover, by appropriately designing the objective function of the optimization problem underlying MPC, grid current distortions similar to those of space vector modulation (SVM) are produced. As for the second approach, two phases are allowed to switch per sampling interval, emulating the behavior of discontinuous PWM. Consequently, due to the introduced formulations, harmonic limitations imposed by relevant grid codes can be met with the proposed methods. Furthermore, due to the multiple-input multiple-output (MIMO) nature of both approaches, all output variables of the system can be simultaneously controlled. Finally, the inherent full-state information of MPC renders an additional active damping loop unnecessary, further simplifying the controller design. The presented performance assessment highlights the potential benefits of both proposed MPC-based algorithms.