21-01-2016, 12:11 PM
Abstract
THE main topic of this licentiate thesis is fast fault detection. The thesis summaries the work performed in the project “Fast fault detection for distribution systems”.In the first chapters of the thesis the term “fast” is used in a general manner. The term is later defined based upon considerations and conclusions made in the first chapters and then related to a specific time.To be able to understand and appreciate why fast fault detection is necessary, power system faults and their consequences are briefly discussed. The consequences of a fault are dependent of a number of different factors, one of the factors being the duration of the fault. The importance of the speed of the fault detection depends on the type of equipment used to clear the fault. A circuit breaker which interrupt currents only when they pass through a natural zero crossing might be less dependent on the speed of the fault detection than a fault current limiter which limits the fault current before it has reached its first prospective current peak. In order to be able to detect a fault in a power system, the power system must be observed, i.e., measurements of relevant quantities must be performed so that the fault detection equipment can obtain information of the state of the system. The fault detection equipment and some general methods of fault detection are briefly described. Some algorithms and their possible adaptation to fast fault detection are described. A common principle of many algorithms are that they assume that either a signal or the power system object can be described by a model. Sampled data values are then fitted to the model so that an estimate of relevant parameters needed for fault detection is obtained. An algorithm which do not fit samples to a model but use instantaneous current values for fault detection is also described and evaluated. Since the exact state of a power system never is known due to variations in power production and load, a model of the power system or of the signal can never be perfect, i.e., the estimated parameter can never be truly correct. Furthermore, errors from the data acquisition system contribute to the total error of the estimated parameter. Two case studies are used to study the performance of the (modified) algorithms. For those studies it has been shown that the algorithms can detect a fault within approximately 1ms after fault inception and that one of the algorithms can discriminate between a fault and two types of common power system transients (capacitor and transformer energization). a directional algorithm to discriminate between faults inside or outside the protection zone. It is concluded that under certain assumptions it is possible to detect power system faults within approximately 1ms and that it is possible to discriminate a power system fault from power system transient that regularly occurs within power systems but which not are faults.