The Research On Stochastic Estimation Of Voltage Sags In Complex Power Grid

With the higher and higher requirement of power quality by modern load, the power quality problem caused by voltage sag has become the main cause of economic losses to consumer. The voltage sag is largely caused by the fault of line, with strong randomness, it brings a great difficulty to management, therefore, forecast the voltage sag caused by short circuit fault stochastically, and according to the results to evaluate the influence of the voltage sag is an important content on the study of current power quality.Based on the virtual node method, the research and analysis on the area of vulnerability will provide the analysis basis for the influence by voltge sag to the sensitive load connected to a certain specific bus. In this thesis, the theoretical deribation of virtual node method is analysised, the areas of vulnerability of voltage sag under four kinds of faults namely the single-phase earth fault, the phase-to phase-short-circuit fault, the two-phase earth fault and the three-phase short-circuit fault in the New England39bus system are abtained with simulation analysis, and the impacts of different operation modes of transformer neutrals in power grid and the start-up/shut-down of generating units during the voltage sag on the area of vulnerability are researched in the meantime.Simulation results verify the effectiveness of virtual node method and provide the basis of applying this method in complex power grids, with clear physical concept, fast computing speed. And it is known from the comparison of different results that appropriately changing operation mode of neutral points of partial transformer in power grid can bring favorable impact to sensitive load in the power grid, the start-up of generating units will increase the area of vulnerability, on the contrary, the shut-down of generating units will decrease the area of vulnerability.