A New Power Conversion System for Megawatt PMSG Wind Turbines Using Four-Level Converters and a Simple Control Scheme Based on Two-Step Model Predictive Strategy—Part I: Modeling and Theoretical Analysis

A new power conversion system is explored in this paper aiming wind turbines rated at the megawatt level. The proposed configuration consists of a medium-voltage, permanent magnet synchronous generator connected to a low-cost three-phase diode bridge rectifier, a dc-dc four-level boost converter as the intermediate stage, and a four-level diode-clamped inverter on the grid-side. The dc-link capacitor voltages are balanced by the boost converter, and thus the control complexity for the grid-tied inverter is greatly simplified. To control the boost converter and grid-tied inverter, a simple method based on a two-step model predictive strategy is presented. In the first part of this paper, the continuous- and discrete-time modeling of the proposed power conversion system is analyzed. The control objectives such as maximum power point tracking, dc-link capacitor voltages balancing, regulation of net dc-bus voltage, reactive power generation, lower switching frequency operation, and common-mode voltage minimization are considered in the design of controller.