| The high-voltage direct current(HVDC) is rapidly developing. The voltage level and capacity of power quality control equipment are getting higher and higher. Frequency control of high power AC motor speed is increasingly promoted. All of the above trends raise high voltage and high power requirements for AC-DC and DC-AC converters. To make up for the gap between the maximum voltage tolerance of switching devices and high voltage requirement of converters, the early solution is to connect the switching devices in series and parallel directly. With the further development of technology and research, the birth of multilevel technology offers better solutions to aforementioned problem. After more than 30 years' study and development, multilevel technology has grown into a brand new power electronics course and been widely applied. Among kinds of multilevel converters, MMC becomes the better choice to construct medium and high voltage AD-DC or DC-AC converters because of its highly modular design and the structural flexibility for variable applications. This dissertation is aiming to research the operation and control of MMC, deeply studies the key issues such as modulation strategy, voltage balancing algorithm, the startup process control and inner circulating current suppressing. The following research is carried out in this paper:First, the basic operation characteristics of MMC are fully analyzed. Combined with the topology features of converter, the operation principles of MMC are introduced. By building the mathematical model of MMC, the advantages and disadvantages of different modulation strategies, voltage balancing algorithms are compared. An optimized modulation strategy is developed to improve the traditional nearest level modulation. The simulation results are used to verify the correctness of theoretical analysis.Second, the startup process of MMC is studied in detail. The startup of MMC is divided into uncontrolled pre-charging stage and controlled pre-charging stage. A configuration of pre-charging current limiting resistor is proposed by calculating the maximum current during the startup process. One controlled pre-charging unlock method is proposed and the simulation results indicate that the raised method can rapidly charge the capacitor to the required voltage, starting the MMC converter smoothly and safely.At last, the generation mechanism of circulating current in MMC is analyzed and the mathematical model of circulating current in MMC is built. Furthermore, an innovative circulating current suppressing strategy suitable for MMC is proposed. The simulation results show the rapidity and strong adaptability. The efficiency of converter is significantly improved. The detailed simulation model is established to verify the effectiveness of the raised strategy. |
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