Calculation And Analysis Methods Of Voltage Sags Due To Faults In Power System

With the rapid development of industrial technology and national economics, power quality has attracted more and more attentions in the field of power apartments and users. In recent years voltage sags have been the most serious problem in power quality and most of the interest in voltage sags is directed to voltage sags due to short-circuit faults which cause the majority of equipment trips. So the analysis and calculation of voltage sags due to various short-circuit faults is of great benefit to reduce the damage due to voltage sags.Considering the limitations of the traditional methods such as critical distance method and fault positions method, this paper presents a new method called analytical method for the analysis of voltage sags due to various short-circuit faults. The equations of voltage-sag magnitudes and phase-angle jumps due to faults along each line for three-phase fault, single-phase-to-ground fault, phase to phase faults and two-phase-to-ground fault are derived by using the impedance matrix. According to the equations, the voltage-sag magnitudes and phase-angle jumps at any bus of the power system can be calculated for any short-circuit fault position.The area of vulnerability for a sensitive load is essential for the estimation of the expected number of voltage sags. A new method based on the derived equations of the voltage-sag magnitudes due to faults along each line is proposed to determine the areas of vulnerability for various faults. And then, the durations and the phase-angle jumps of voltage sags should be considered to modify the area of vulnerability. Both the new method and the fault position method are used to determine the area of vulnerability in the IEEE 39-bus test system, the validity of such a method is testified by their comparison.This thesis not only presents a general way of calculating sag magnitudes at any bus according to the derived equations of voltage-sag magnitudes for any given short-circuit fault, but also proposes a way of analysis of voltage sags for complex faults based on the multiple-port network theory and the method of symmetrical components. Then the IEEE 39 bus test system is simulated by PSCAD/EMTDP, and several characteristics of voltage sags for complex faults are derived.