In recent years, modular multilevel converters have become one of the pillars of high voltage direct current systems, and research on this issue has been expanded, especially from the aspects of network and converter reliability. In this thesis, considering the attractiveness of modular multilevel converters based on the half-bridge cell structure due to the cost of implementation and low losses and the lack of inherent DC fault blocking capability in this converter, we first try to modify and improve the performance of the converter in normal and fault conditions and then present a new structure for this converter is based on half-bridge cell structure with fault blocking capability. Nowadays, the criteria of the implementation cost and losses of a system, along with the performance speed of that system, are among the qualitative and quantitative eva‎luation criteria of a system, so research has always gone in this direction to solve the mentioned challenges with the least possible cost and losses. On the other hand, in recent years, modular multilevel converters have become one of the pillars of high voltage direct current power transmission systems, and research has been found on this issue, especially from the aspects of reliability and protection of these networks and converters, has been expanded in order to eliminate their errors as quickly as possible. Modular multilevel converters based on half-bridge cells, despite their attractiveness, do not have the ability to block DC fault intrinsically, and this is the main challenge of using this structure. Other structures for the modular multilevel converter cell have been proposed, such as the full-bridge structure and the hybrid structures that create the fault blocking capability intrinsically within the converter, But these structures need to be modified in terms of costs and losses. At first, by stating the advantages of modular multilevel converters and the challenges of implementing this converter, an attempt has been made to limit the circulating current without using a controller in order to reduce the cost. Then, through the optimization of the switching method of the converter in order to achieve the least number of switching, the switching losses have been reduced and in parallel, through the appropriate switching algorithm, the voltage balance of the cell capacitors in the converter has been achieved. After improving the performance and costs, the main goal of the research is to provide a new structure for modular multilevel converters based on half-bridge cells with the ability to control the DC fault intrinsically. After stating the theory of the proposed method and its implementation and simulation, the validity of the main idea has been investigated. In the following, the results obtained from the simulation of the proposed method and the full-bridge and hybrid structures presented so far have been compared and analyzed from different aspects such as implementation cost, losses and structure efficiency, fault blocking time. In this comparison, the proposed structure was able to minimize the number of semiconductor devices. reduce by about 10%, and from the point of loss, it was able to improve the efficiency of the converter by at least 7%, and also improve the performance by about 25% in terms of fault blocking time.

حميدرضا كارشناس

احمدرضا تابش , محمد ابراهيمي