Charactristc Analysis And Reliability Evaluation Of Voltage Sag In Distribution System

With the rapid development of information technology and other high-techs, a number of equipments which are sensitive to short disturbance of voltage magnitude have been put into use, the problems caused by voltage sag have attracted much attention in the world. Up to now, many research works have been done on voltage sag around many aspects.The definition, cause, characteristic quantities of voltage sag and some techniques that reduce the effect of voltage sag on customers are analyzed first. Then this paper studies the following two questions: 1) analyzing the characteristic of voltage sag and 2) assessing the reliability of voltage sag in distribution system.Firstly the paper brings forward a method for classifying the voltage sag based on characteristic phase, which simplifies existing seven types of sag into two types. Then the relation between the three-phase voltage magnitudes and the characteristic voltage, Positive-Negative factors has been established through the definition of relation between the characteristic voltage, Positive-Negative factors and the positive, negative sequence voltage. Meanwhile it investigates the relations between the voltage magnitude of sag and the location of the fault point and the relations with the impendence of system. Based on the probability distribution a method for stochastic prediction of voltage sag in radial distribution system has been put forward in this paper, in which the method of critical distanceis adopted, which is first proposed by M.H.J.Bollen and then recommended by the IEEE. The method can assess the probability of voltage sag in different voltage magnitude taking into account the influence of all kinds of fault type and different fault distribution function. The sequence characteristic of phase-angle shift during voltage sag has also been studied and it also discusses the transfer of voltage sag through different type of winding connection of transformer and conclusions have been made for the relation between the sag type and the transformer type.The paper presents a Monte-Carlo simulation for the evaluation of reliability of voltage sag in distribution systems. Through 5000 times fault simulations the fault information like fault type, fault line and the location of fault point is obtained. A short circuit program that has the ability of stochastic selection of fault information is developed for calculating the voltage magnitude of the sensitive load customers. The duration of voltage sag and the frequency of voltage sag are also obtained at every sensitive load customer through the program. A systematic set of indices for assessment of reliability of voltage sag in distribution system, including the customer-orientated and global system-orientated indices is established. The indices system is concise, yet complete. Theresults of the evaluation through calculating the indices for the IEEE DRTS indicate that the indices can allow both the utility and the end-user to, objectively and accurately, assess the impact of voltage sag on electric system, the performance of the system and the cost of voltage sag on end-user. It also can be used by the end-user planning engineers to choose the best location in the supply system, where new sensitive equipments are to be connected.