| With the fast development of modern industry and the rapid increase in productivity, a large number of sensitive loads put into the grid, so power companies and power users have higher requirement of power quality. Power quality issues are attached great importance, especially voltage sag problems. Grid short circuit failure is the main cause of voltage sag. Voltage sag will cause plant shutdowns, sensitive equipment to stop working even damaged, bring huge economic losses, therefore, to analyze voltage sags and depression domain have important theoretical and practical significance. The essence of traditional depression domain analysis is through calculating voltage sags of the sensitive load when system failed, to determine the area of the failure point that make the sensitive load does not work. However, due to the many sensitive loads, the traditional depression domain analysis greatly increased the workloads. And it is not conducive to distinguish the responsibilities between the power supply enterprise and power users and it is hard to make the definition of the effective compensations clear that caused by voltage sag. Therefore, this paper based on the traditional depression domain of voltage sag proposed a new definition-voltage sag concentric relaxation concave domain, that is put the failure point as the centered, the area that the voltage sag value is below a certain voltage threshold which caused by failure. Voltage sag concentric relaxation concave domain can be more intuitive represents the scope of influence about voltage sag caused by the fault. And put the failure point as the center will greatly reduce the workload of the depression domain analysis and it can be more convenient for determining the area endured voltage sags caused by this failure. The analysis of voltage sag concentric relaxation concave domain can become an effective criterion of economic disputes between power companies and power users. Based on the voltage sags analytical formula method, the concentric relaxation concave domain analysis algorithm is studied.Secondly, this paper takes the actual distribution network with multiple voltage grade into account, and gives full consideration to the impact of the transformer wiring under different fault types for calculating voltage sag value. The different transformer wirings will change the transfer characteristics of voltage sag and mainly about the amplitude and phase. And then change the concentric relaxation concave domain. Therefore, this paper corrects the algorithm of voltage sag concentric relaxation concave domain with considering the different transformer wiring. And achieved the general package of voltage sag relaxation concave concentric domain in the MATLAB environment. The package uses a graphical interface and by entering the network parameters, fault information parameters and transformer parameters, as well as reading text files, it can easily analysis the voltage sag concentric relaxation concave domain. Set the IEEE-30 node test system as an example to obtain voltage sag per unit and concentric relaxation concave domain. The results from the generic package are compared with the results from PSCAD/EMTDC that the validity and accuracy of this generic package is verified.Finally, based on the current momentum for rapid development of large-scale distributed generation, photovoltaic power generation, for example, this paper analysis the influence of voltage sag value and concentric relaxation concave domain due to distributed photovoltaic power’s different capacity and grid positions. The studies have shown that:The distributed photovoltaic power interconnected to the power grid is able to support the voltage during the failure. But its role in supporting the grid voltage during fault is closely related to its capacity and grid position. The different capacities as well as the grid positions will have different effects on voltage sag and concentric relaxation concave domain. |
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