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In order to improve operational stability of the island system after grid faults of local weak connection, a wind farm and grid power system model composed of doubly-fed induction generator is built. This paper studies the impact of faults on the wind farm output and the grid bus after the wind farm is connected to the grid, and sets three-phase short-circuit fault in wind farm synchronization. Moreover, we analysed voltage fluctuation of the wind farm connected to the grid and voltage fluctuation of grid bus, as well as changes in output frequency, active power and reactive power of the wind farm. The microgrid voltage and frequency changes are studied in island operation when the fault is removed. The research results lay the foundation for island operation when fault occurs after wind turbine is connected to the grid.

AC-AC converter is a piece of key equipment in the fractional frequency transmission system and both back-to-back MMC and M3C have great potential, but to date there are few works focusing on their reliability. A comparison of the reliability of these two types of converter is carried out in this paper, considering the SM redundancy. Firstly, the characteristics of two converters are discussed. Then the reliability model of SM and arm of two converters are presented. Based on it, the reliability models of the whole converters are proposed. The back-to-back MMC and M3C are described as the two-state model and three-state model respectively. Employing these models, the comparison of the reliability of the two types of converter and the analysis of the effect of SM redundancy are carried out in the case study.

Urban area is the major load centre of power system. To meet N-1-1 criterion is important for an urban grid with the requirement of high power supply reliability. The considerations of N-1-1 criterion in overseas developed areas are compared in this paper. The impact of the configuration of substations and the inter-connections on power supply reliability is analysed for the grid under N-1-1 condition. In addition, the urban network topology which can meet the N-1-1 criterion of British ER P2/6 guideline is discussed. A more stringent N-1-1 criterion and corresponding network topology is proposed to accommodate the development of urban grid.