Research On Broken-Line Fault Protection Scheme Of Distribution System

With the continuous increase of electricity consumption in our country,the requirements of users for the power quality are also growing,and the safety and stability of distribution networks is getting more and more attention.In recent years,however,due to meteorological disasters,the transmission line aging,and other external damage,broken-line faults tend to be more frequent.Broken-line faults will cause the asymmetrical three-phase voltage and current of the load side,and the negative-sequence and zero-sequence components caused by the fault will lead to serious damage to various loads,especially rotating equipment.In addition,broken-line fault is often accompanied with the grounding fault,developing into complex fault,which will easily lead to fire and electrical shock on humans and animals.And it can even develop into inter-phase short-circuit faults,expanding the range of outages.Thus,in 10kV neutral point ungrounded system,the electrical quantity characteristics of broken-line fault is analyzed in detail when it happens on both traditional transmission lines and those with distribution generation(DG)under PQ control mode.Accordingly,a set of fault diagnosis and protection method is proposed in this thesis.For the traditional distribution lines,the fault current is analyzed in this paper by the symmetry component method and the current characteristics of the distribution network,and the neutral point voltage change in the small current neutral grounding system is analyzed based on the system structure to analyze the phase voltage changes at both ends of the line.The fault electrical characteristics of various broken-line fault types are studied through the above methods,the characteristics and distribution laws of the fault electrical variables are summed up,and the general rules of fault current and fault voltage change at both ends are obtained in this paper.On this basis,the protection start criteria based on current and zero-sequence voltage,the fault detection criteria based on three-phase voltage and current,and the fault isolation scheme based on the change of phase voltage amplitude and line voltage amplitude are designed,these three parts constitute a complete protection program.In addition,the current situation of transformer configuration in the small-current grounding system is also considered in this paper.The protection scheme is optimized by combining the various operating conditions of the power distribution line and fault conditions such as disconnection of the transformer and non-full-phase operation.For distribution lines with DG under PQ control mode,firstly,the control model is briefly introduced according to the relevant grid connection standards,and its fault characteristics are studied.Then,using the principle of superposition,the output of DG and its effect on the fault characteristics of the distribution line are investigated after a broken-line fault occurred on the distribution line with DG.The relevant conclusions are obtained and summarized,and the general law are summed up.Finally,on the basis of the traditional protection scheme,relevant improvements are made to the fault detection criteria,and a method to judge and protect the broken-line faults of distribution lines with DG is proposed.Models of a lOkV distribution line with or without PQ controlled DG are established using the PSCAD/EMTDC.The examples for various types of broken-line fault and various operating conditions of the distribution network are simulated,the simulation data is summarized and processed,verifying the correctness of the theoretical analysis,as well as the effectiveness of fault diagnosis methods and protection schemes.