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In the conventional capacitor voltage balancing control of modular multilevel converters (MMC), calculation quantity increases exponentially with the number of sub-modules (SMs). In the application with a great many SMs, it leads great difficulty and high cost to hardware design. To solve the problem, this paper proposed an improved capacitor voltage balancing strategy based on the merge sort to reduce computation quantity evidently. Meanwhile, according to the capacitor voltage variation rule of the nearest level modulation (NLM) method in the ideal condition, the paper optimized the merge sort to further decrease calculation quantity, analyzed variations of capacitor voltages in the non-ideal condition, and proposed a corrected optimized algorithm. In the end, the paper used the DSP controller TMS320F28335 to compare execution time of the sorting algorithms, and results of the optimized method indicate significant reduction of computation quantity. Using PSCAD/EMTDC to establish a 401-level MMC simulation model, validity of the optimized merge sort and the corrected algorithm are verified.

With the development of power system, the capacity has been increasing. So configuring large power supply in high load density grids and interconnections between power grids will cause enormous short circuit current, which will put forward a more stringent requirement for electrical equipment. If the short circuit current rating of the equipment in system is exceeded, the equipment must be replaced, which is a very cost- and time-intensive procedure. A fault current limiter (FCL) offers an alternative in such case. The purpose of this paper is to offer a reference of choosing FCL by analyzing the characteristics of several typical FCL and comparing the advantages and disadvantages between them. First, this paper attempts to give an overview of fault current limiting measures for medium and high voltage applications. Then several representative FCL are examined with an emphasis upon the solid-state fault current limiter (SSFCL). Finally, the demonstration of the FCL based on coupling autotransformer is introduced. The main circuit diagram and control strategy used are also presented.