Voltage Sags In Distribution Networks And Relative Issues On Power Quality

With the rapid development of the electric and electronics equipments, control devices and electronics units based on computer systems are widely applied in power customers, great changes of the load character have taken placed in modern power system, power quality is becoming the common concern of power utility and customers, more attentions are paid on voltage sags by learners in the world. Voltage sag calculation and analysis is a fundamental task for other researches and it is also the starting point of the mitigation. This dissertation is focused on voltage sags in distribution networks and relative issues on power quality, voltage sag analysis is discussed in this dissertation, reasonable load model in voltage sag analysis is explored at the same time, at the end of this dissertation, voltage fluctuation and flickers caused by a small arc furnace are discussed. Above all, Main work that has been carried out in this dissertation is: Critical percentage method is presented to determine voltage sag exposed areas, a concept of critical percentage of feeder is defined, to a distribution system of one plant with sensitive loads, this method is applied, the result verify its effectivity. Voltage sag analysis software is designed in this dissertation. Fault position method is adopted to design this software; PSASP is used as a tool to do fault calculations. The sag analysis software are capable to search the buses which are affected by faults, to analyze the voltage sag exposed areas of sensitive loads, to predict the number of voltage sags each year. Thus, the voltage sag analysis software can provide a quantative description. A fault position sag coefficient (FPSC) is defined to evaluate the degree of fault position influence on the power system, based on the results of sag analysis, FPSC are determined; the fault area which have a serious effect to power system can be found. Many methods to restrict voltage sags are discussed. A second_order dynamic power recovery load model with the consideration of ITIC curve is presented in the dissertation. Second order differential equations are adopted to describe the power recovery, the solution reflect the accurate process of the dynamic power recovery, at the same time, nonlinearity of the static power is kept, consider the voltage tolerance curves of the sensitive loads, ITIC curve is contained by the dynamic load model, the new model is checked by EMTDC simulation, the results prove that it can provide a good approximation of dynamic loads with sensitive equipments. A voltage fluctuation and flicker analysis caused by a small electric arc furnace of a mechanical factory is given, the computation shows that the voltage fluctuation and flickers caused by the small arc furnace cannot be ignored, according to the characteristics and economics of the small arc furnaces, some methods for electric arc furnace flicker mitigation are held out in the dissertation.