Off-line Earth Faults Location Method For Overhead Distribution Lines

The fault current caused by single phase to earth faults in non-effectively earthed systems is very small and the failure of the fault point is not obvious,so it is very difficult to find the fault point by visual inspection under the state of power outage of the overhead line.Therefore,new fault location techniques need to be developed in order to meet the requirement of noneffectively earthed systems.At present,the commonly used online fault location methods can only locate the fault section,it is still necessary to find the location of the fault point.Due to the complex structure of the distribution network,there are laterals,intermediate loads,different types of lines,which makes the fault location methods based on traveling waves and line impedances have poor reliability.The AC signal injection method cannot overcome the signal attenuation of the distributed capacitance.The DC signal injection method requires the operator to perform detection on the pole,the operation which has low safety and reliability is cumbersome.Foreign scholars have proposed an offline fault location method based on pulse signal injection.This method injects a high-voltage pulse signal in the faulty line,and uses the difference in the amplitude and flow direction of the line current front and back the fault point to locate the fault point.This method lacks the analysis of the line current distribution characteristics,only relying on the operator to identify the characteristic changes of the current signal subjectively.This method ignores the influence of different magnetic field measurement methods and the selection of signal reference points on the positioning accuracy,and this method has poor operability and low positioning accuracy.Based on the composition and working principle of the high-voltage pulse impulse fault location system,this paper establishes an equivalent model of the pulse discharge circuit.By analyzing the current(magnetic field)characteristics and waveform differences of the fault line,an overhead distribution powerline automatic fault location method based on the principle of waveform comparison is proposed.This method uses a high-voltage pulse signal generator to inject a DC pulse signal into the fault line.After the fault is clicked through and the fault is reproduced,there is a difference in waveform similarity between the upstream and downstream currents(magnetic field)of the fault point.The current(magnetic field)difference is used to develop the application the waveform comparison algorithm for offline fault location.This method defines the correlation coefficient as the basis for judging the degree of waveform similarity.By analyzing the similarity difference between the current waveform at different measurement points and the reference current waveform,the automatic calculation of offline fault location is realized,and the traditional offline fault is solved.The positioning method has problems such as poor positioning accuracy and low sensitivity.Taking into account that the shortcomings of cumbersome operation and poor safety in the direct measurement of current signals,this paper designs a non-contact magnetic field measurement method based on the magnetic rod coil for the magnetic field difference between the upstream and downstream line currents,and analyzes the influence of different magnetic field measurement methods and the selection of reference signal on the fault location accuracy.This method completes the offline fault location system,and improves the location accuracy and the operability of practical applications.Finally,the simulation software EMTP-ATP and MATLAB are used to simulate and analyze the off-line fault location method of overhead distribution lines.This method is verified by the fault data of the actual test system.Through the analysis of simulation and actual test data,it is verified that the automatic calculation method for offline fault location based on the waveform comparison principle which proposed in this paper has high reliability and can effectively realize the accurate location of the fault point.