Research On On-line Inspection And Accurate Fault Location Methods For Power Lines

With the development of the smart grid, the number of the EHV/UHV transmission lines with high capacity and long distance increases obviously, as well as the load density in the distribution system. As the main artery of the electrical power system, power lines play an important role in the transmission system and distribution system. The safe operation of the power lines is the key technology to guarantee the safeness and reliability of the power supply, and it is the foundation of the functions in the smart grid.In order to improve the security of power lines, the main technological means are accurate fault location for the power lines after the fault occurs and the on-line inspection/condition monitoring before the fault occurs.The accurate fault location is the key technology to repair the fault lines quickly and restore the non-trouble area power supply after a fault occurs. Fault location based on the fault-generated traveling wave has the advantages of less influence by saturation characteristic of current transformer, fault resistance, fault type and the operating mode of system. It has already become a focus in recent years and has been successfully applied into transmission network. But the reliability of the traveling-wave-based fault location needs to be improved further. Meanwhile, comparing with the fault location in the transmission system, the meaning of the fault location in the distribution system is broader, and the realization is more complex. It mainly includes faulty line selection, fault section location and fault position location. Faulty line selection and fault section location have been a challenge when a single phase-to-ground occurs in the non-effectively distribution system. And because of the complex structure and the multiple laterals, traveling-wave-based fault-location methods used in the transmission system cannot be directly applied in the distribution system. The novel fault location methods based on the fault-generated traveling wave should be studied to solve the fault location problem in the distribution system.The on-line inspection and condition monitoring for the power lines can help the maintainer to obtain the operation conditions and the environmental information of the power lines in real time. Then the abnormal state and the defect of the power lines and their attachments can be found before the fault occurs. The existing methods mainly include manual inspection, helicopter inspection, robot inspection and the assistant on-line condition monitoring. Currently, these methods cannot realize the on-line inspection and remote-control-based inspection required by the power utilities in the field.To overcome the problems mentioned above, the application of the traveling-wave-based fault-location methods in medium distribution system and transmission system is studied in this thesis, as well as a novel on-line inspection system for transmission lines using optical-fiber ground wires.Firstly, traveling-wave-based fault-location method based on the time difference between the first aerial-mode and zero-mode wavefronts of the traveling wave is improved and is applied into the fault position location in the distribution system. About the speed instability of zero-mode components, the thesis discovers that the detected zero-mode wave velocity would be more stability by selecting appropriate large wavelet decomposition scale to detect the arrival time of traveling wave. Meanwhile, an iterative algorithm is designed based on the correspondence of the frequency component at the arrival time of traveling wave to fault distance and the correspondence of zero-mode velocity to the frequency. It can improve the accuracy and anti-interference ability for the extraction of detected zero-mode wave velocity. Based on the iterative algorithm, the thesis presents a new two-ended method of single phase-to-ground fault location based on the gap between zero mode and aerial mode traveling wave propagation time. The theoretical analysis and the simulation result have proved that this method does not need time synchronization, not only can locate the fault position at trunk lines, but it can also locate the fault position at braches.Secondly, faulty line section and fault section location based on the single phase current in the non-effectively distribution system are studied. Considering that only two-phase current transformers are installed in the Phases A and C and Feeder Terminal Units only provide line-to-line voltage information in some distribution systems, the characteristics of phase current traveling waves are analyzed at busbars. And then faulty-feeder fore-identification algorithm is proposed based on the single phase current traveling waves. Combined with the propagation characteristics of the phase current, the fault section location can be realized based on the both-ended faulty-feeder fore-identification results. Meanwhile, the fault position can be located based on the arrival times of the traveling waves of phase currents and line-to-line voltages. The proposed fault-location method can simultaneously realize fault section location and fault position location.Thirdly, to improve the reliability of the fault location in the transmission system, an optimal configuration for traveling-wave fault-location units (TWFLUs) based on the principle of characteristic non-singular set is proposed. The topological graph of the power system can be equivalent to a maximum non-singular set using Dijkstra algorithm, and then the characteristic non-singular set of each line can be obtained. Combined with the principle of network-based traveling-wave fault-location methods (NBTWFLs), the optimal configuration can be obtained according to the characteristic non-singular set. Meanwhile, the corresponding NBTWFL is studied. The proposed method can locate the faults on any line when the number of installed TWFLUs is minimal, even if the status of the breaker is unknown. It can improve the reliability and practicality of NBTWFLs.Fourthly, the oscillation in the transient response of electro-magnetic transformers is found by the analyses of the transfer characteristics in time domain. Due to the oscillation, the analysis results show that the sampling rate should be greater than1MHz to obtain acceptable fault location results. And continuous wavelet transform (CWT) is more suitable to identify the wave-front arrival time than discrete wavelet transform (DWT) because the appreciate wavelet decomposition level should be selected.Fifthly, the inspection tasks and essential condition monitoring functions are analyzed. Then combining the recent splicing technology of optical fiber composite overhead ground wire (OPGW) with Ethernet passive optical network (EPON), the network schemes for communication system are analyzed and proposed. Based on the proposed communication system, a novel on-line inspection system for transmission lines is proposed, which consists of tower terminal units, EPON and master stations. The prototype is developed and the pilot run is carried out. Meanwhile, the fault current in OPGW is analyzed based on mesh-analysis method, and a novel fault location method based on the phase comparison of the fault currents in OPGW at two adjacent towers. The proposed on-line inspection system can realize automatic inspection on schedule and remote-controlled inspection, and the proposed fault location method can realize accurate fault location with an error of one tower span. Finally, the full-text work and innovation points were summarized, and the future research work was prospected.