Research On Generalized Modulation Algorithm And Capacitor Voltage Balance For Multilevel Inverter

With the concept of protecting the environment going popular, to improve energy efficiency, to reduce resource consumption, and to promote the use of renewable energy have become an important consensus about human development in the world. China has also made timely strategic objectives of construction of resource-saving and sustainable development society. The power electronics and automatic control technology are combined to develop various new power electronic devices, which provides strong support to build a green environment of power energy usage.With its outstanding advantages in performance and capacity Multilevel Converter is one of the abroad hotspots in research and application these years. Multilevel structure contains single DC unit type and multi DC unit type, as well as mixed type. One of the single DC unit type, the Neutral Point Diode-Clamped multilevel structure is the most widely studied and applied due to its simple structure and low cost.Modulation strategy for multilevel inverter is an important issue in the related field, which is the basic guarantee for the application of multilevel technique, and is a major component of this paper. The basic problem in the digital control of power electronic converters is analyzes in this paper. The mathematic model for multilevel system with any phases and levels is built from the view of the space vector meaning and of the sinusoidal modulation meaning. Two novel strategy, Extended 2-level Space Vector Modulation and Scalar Level Synthesis Modulation are put forward, which are effective for multilevel system with arbitrary phase or level, and their working mechanism, implementation steps and key technology issues are discussed in detail. A study on the working mechanism of the switching device among single-phase bridge arm was conducted, and these switches are divided into three groups according their characteristics, two-level switch, normally turn-on switch, and normally turn-off switch, so as to solve effective the problem of multilevel PWM signal distribution.As diode-clamped multilevel system has been widely studied and applied owing to its simple structure and low cost, capacitor voltage balance is one of the key issues involving operation safety and output performance for this type. With general discussing about causes and impacts of imbalance of mid-point level in three-level system, a deep analysis is conducted on the imbalance characteristics, and the instantaneous fluctuation, continuous fluctuation and peek fluctuation is calculated as its results, finally these results are extended to more level multilevel system. This research gives guidance for design and control of the Clamped type Multilevel system. Based on analysis of the imbalance, two novel capacitor voltage balance strategies for three-level system, Hysteresis Zoom of Capacitor Voltage strategy and Hysteresis Shift of Capacitor Voltage strategy, are presented. Furthermore the Flexible Balance of Capacitor Voltage strategy is presented to enhance the output performance of the foregoing two strategies, obeying the rule that output performance lies above capacitor voltage balance. Simulation results on the model built in Matlab verify the analysis and schemes above.With the goal of achieving the structure modularity and maximum utility, a three-level inverter prototype is designed based on the DSP+CPLD platform. All proposed strategies and analysis, Extended 2-level Space Vector Modulation strategy, Scalar Level Synthesis Modulation strategy, three-level capacitor voltage imbalance of the mid-point, the proposed Hysteresis Zoom of Capacitor Voltage strategy, Hysteresis Shift of Capacitor Voltage strategy, as well as the Flexible Balance of Capacitor Voltage, are studied on Matlab simulation platform and the prototype platform. Simulation and experimental results fully verify the correctness of theoretical research in this paper.