Study On Weak Fault Grounding Protection And Location In Distribution Network

The reliability of power supply is one of the most important issues in distribution network security.Since the distribution network is the end of the power system,it has the characteristics of wide network coverage,complicated transmission lines and many operation modes,which leads to the distribution network failures frequently.Therefore,fault dectection,fault location and fault isolation is an important way to reduce the numbers and time of power failure in the real system.However,the weak fault problem is not only the most frequent occurrence but also the most difficult identification in all the faults occur in the distribution system,that including single-phase ground fault of small current grounding system,high-impedance fault caused by various types of grounding medium,intermittent arc fault caused by aging of cables and so on.Therefore,this dissertation concerns the grounding fault protection and fault location and focuses on solving the problems of fault line selection and fault location identification of the weak fault in distribution network.The main work of the dissertation is as follows:(1)In order to solve the grounding protection of the neutral non-effectively grounded system,a single-phase ground fault feeder identification method based on the feature between voltage and integration current is proposed in this dissertation.Firstly,phase-frequency characteristics depend on the parameters of distribution feeders have been analyzed for the characteristic frequency band.And the relation between voltage and integration of current is given in detail.Then,the value of the average ratio of zero sequence voltage in the bus and integration of current in feeders can be employed to establish the detection criterion to discriminate between the healthy feeders and the faulty one.Finally,the reliability and the validity of this algorithm have been extensively studied,in which a wide range of fault conditions with different fault resistance take into account.Specially,the proposed method has good performance for the two lines faults and the faults with high value fault resistance.(2)In order to solve the high-impedence fault section identification of the neutral non-effectively grounded system,this dissertation tests the time-frequency energy characteristics of high-impedence fault generated by grass,sand,brick,reinforced concrete,stone and branch firstly.And the criterion of fault detection is made of a sequence of the time-frequence energy proportion in two cycles after the high-impedence fault.And then,a fault section identification method based on the transient center frequency is proposed in this dissertation.It depends on the difference of the center frequency of the zero-sequence transient current in the both sides of the fault point.When the fault occurs in the complex topological network,the center frequency is difficult to extract due to the problem of spectrum aliasing.In this condition,the fault area can be obtained based on the maximum level of time-frequency energy distribution.The simulation results show that the proposed methods only requires the information of zero-sequence current without the time synchronization for each measurement points and they are also suitable to are high-impedence fault.(3)In order to solve the fault distance estimation for arc fault in the cables,a fault location method of self-clearing fault in three phase single core cables based on double impedance model is proposed in this dissertation.This method concerns the periodical self-clearing fault during the failure process of the underground cable.And considering the impact of the metal sheath,the circuit equations of the core-sheath-ground self-clearing fault and the core-sheath self-clearing fault have been derived based on the double impedance model.The peoposed method uses the time-domain information of the current and voltage waveforms monitored in the head-end of cables.The self-clearing fault can be located by the least squares regress method during a short time.Extensive simulations show that the algorithm,considering the behavior of the metal sheath of cables,can locate the self-clearing fault accurately.And it is more cinsistent with actual structure of cables than the methods only consider the impedance of the core and it also can explain the electrical behavior between the conduction and metal sheath of coaxial cables.(4)In order to solve the multi-estimation problem of the fault location in complex distribution networks,the fautlt location method based on the wide-area power frequency information is proposed in this dissertation.In this chapter,the voltage charecteristics of the transmission lines after a fault is analyzed firstly.And then the equations among the sending terminal,fault distance,receiving terminal are established by the circuit relationship of the three points.The proposed method makes use of the difference between the calculated voltage and measured voltage at the end of the lines to locate the fault.A deviation sequence and a tendency factor are defined to search for the fault point.Since this method involved the the amplitude of the voltage at the receiving terminal,it does not need the synchronization strictly among the monitoring points.This dissertation focuses on the study of high-speed railway distribution system with simple transmission lines model,IEEE-34 nodes network and IEEE-134 nodes network with complex branches radial distribution system.And a fault location method based on the voltage sag profiles in high speed railway distribution system and a grounding fault location method in radial distribution networks involving distributed voltage measurement are proposed in th is chapter.In the deviation equation of voltage from the proposed methods,it avoids the error from the fault resistance estimation due to the fault type and fault resistance are not involved in the calculation process.Therefore,the methods have good performance on the fault with high value resistance.The theoretical research and simulation results of the dissertation show that the theory system of protection and fault location has formed including grounding protection and fault section identification in the neutral non-effectively grounded system,high-impedence fault detection and fault location,incipient fault location in the underground cables and fault location in the complex distribution system.It improves the reliability of weak fault grounding protection and fault location in the distribution network effectively.