Research On New Single-end Fault Location Methods For Medium Voltage Distribution Systems

By the grounding principles, power system can be divided into large current grounding system and small current grounding system. Due to the different conditions in different countries, medium voltage (MV) distribution systems have multiple grounding principles. In. large current grounding system, the steady or transient fault information is evident and can be used for fault location. However, in small current grounding system, the steady fault current is weak, which makes fault location methods based on steady electrical quantity invalid, but the transient electrical quantity is evident and can be used for fault location. Furthermore, the characteristics of MV distribution system, such as the bus bar configuration, laterals, intermediate loads, different types of lines and so on, make the fault location challenging. In this paper, locating fault point after fault line selection is concerned. And the steady electrical quantity based fault location methods in solidly grounded distribution system and transient electrical quantity based fault location methods in small current grounding system are discussed. The main contents of research are as follows.The paper presents an improved method based on steady electrical quantity. Utilizing fault admittance matrix, the method takes direct circuit analysis in phase domain, and make a derivation of nonlinear fault location equations which can be solved by Newton-Raphson iterative method. Compared with former methods using simple iterative method, the proposed method has high stability and accuracy. This method is applicable to solidly grounded distribution systems.A fault section estimation method based on characteristic wave is proposed in this paper. When a fault happens in distribution system, the travelling waves observed at measurement point contain the first wave and following waves which include the reflected waves with fault section information. Considering to the number, propagation path and energy of these reflected waves, three characteristic waves is defined in this paper. The existence of characteristic waves is relevant with fault section, and the combination of characteristic waves in each fault section is unique. After analyzing transient fault travelling waves by wavelets, the combination of characteristic waves is obtained and can be used to judge the fault section. Furthermore, the effect of superposition of disturbing waves and characteristic waves is discussed, and counter measures are put forward to minimizing the effect. The fault section estimation method is applicable to distribution system with any grounding principle.The propagation characteristics of modal components of travelling wave in distribution system is discussed in this paper. And then two fault location methods utilizing zero mode component are proposed. The key problem of zero mode component is the estimation of its velocity. The relationship between velocities of modal components and fault distance can be described by fitting function. Utilizing these fitting functions and time delay between modal components, the fault distance can be solved by iterative process. On the other hand, the modal components have many differences in time domain, frequency domain and energy domain if the fault distance is different. Thus, support vector machine is used to considering this characteristic information comprehensively, while the estimation process of zero mode velocity is avoided. The trained support vector machine can be used to locate fault. The effect of earth resistivity is discussed in this paper, and counter measures are put forward for above two fault location methods. The two methods are applicable to distribution system with any grounding principle, but both have the problem of fake fault points. Referring the result of fault section estimation method, this problem can be fixed.