Gradient-Based Predictive Pulse Pattern Control for Grid-Connected Converters with LCL Filters

This paper presents a control algorithm for medium-voltage (MV) grid-connected converters with LCL filters. The controller is designed in the framework of model predictive control (MPC) to successfully address the multiple-input multiple-output characteristics of the system and provide active damping such that the apparent switching frequency of the converter can be equal to the filter resonance frequency. However, operation at such low switching frequencies—in the range of a few hundred hertz—requires filters with strong harmonic attenuation capabilities such that the low-order harmonics can be effectively mitigated. To avoid excessively bulky filters, while still meeting the relevant harmonic grid standards, optimized pulse patterns (OPPs) are employed and manipulated in a closed-loop fashion in real time. In doing so, the designed control algorithm ensures that low grid current harmonic distortions are produced, thus achieving excellent system performance. The effectiveness of the proposed control strategy is verified with an MV system consisting of a three-level neutral-point-clamped converter connected to the grid via an LCL filter.