Grounding Fault Line Selection And Location Method On Transient Feeder

Distribution network connects the power system most directly, and its wide coverage leads to high failure probability, in which single-phase fault is the major type. Most distribution networks in our country are small current neutral grounding system. With the proportion of city cable network grows year by year, the capacitive current of feeders increase, resulting much more difficulty in arc extinction. In addition, faults caused by falling to the ground wires and the contact between trees and overhead lines lead to the missing and mistaken selection. At the same time, most distribution systems are radial networks with many branches and complex structure. Thus, the parameters of feeders vary with the environmental factors and are difficult to obtain, and the compensation of conductive currents weaken the characteristic quantity, leading to the fault location becomes the puzzle even have completed the fault line selection. In practical, fault location largely depends on the artificial line patrol with huge workload. It has been a problem for years, for technology and economy factors determine the particularities of fault line selection and fault location.The arc-suppression coil impairs the ground residues, although it contributes to arc extinction, it leads to the difficulty in ground current detection and location afterwards. Post-fault transient components are free from the effects of arc suppression devices and contains extensive fault information making the methods based on transient signals become preferred approaches. This paper analyzes the transient process after fault, and built a small current grounding system simulation model. Based on this, common phenomenon of unreal grounding in practical is analyzed in theory and simulated, and a transient energy based method of distinguish unreal grounding from single-phase fault is proposed. There is no transient high frequency component when unreal grounding occurs, but there are large amount in grounding fault. Extract one power cycle post-fault data, by constructing half cycle transient energy ratio to detect fault. Meanwhile, the comparison between practical data and simulation data lays the foundation for building a better simulation platform in lab. The study and analysis of the practical data and the devices development and improvement make a constructive comment on comprehensive methods of fault line selection. As single-phase fault is the most common fault type in distribution system, the paper extract fault transient zero-sequence current data of each feeder from the last quarter of the frequency cycle time window and then use wavelet packet to decompose and reconstruct them to different frequency band to get the time-frequency characteristics. The characteristics in a certain time-frequency window can fully reflect the time-frequency characteristics of the signal. By using the correlation coefficients to characterize the distribution low in time-frequency characteristics of transient zero-sequence current and its variation regularity and combining the complex correlation coefficient to form the line selection criterion, implementing a line selection method of single-phase grounded fault. Based on this, a time-frequency based fault line selection method is proposed.However, the actual field experiments show that most devices can start correctly and have a high performance. But the fault location in small current grounding system has always been the difficulty, especially location in single-phase fault situations. This paper researches the failure mechanism extensively and makes a detailed discussion about the transient process. Firstly, based on the transition character of transient traveling wave, voltage in fault point drop quickly after fault happens, producing aerial mode component transmitting between wires and zero mode component flows between wires and the earth and a detailed analysis of the propagation laws and their velocities are given. Given to the transmission characteristic of the traveling wave, a periodic natural frequencies caused by the fault point, feeder extremity, beginning end and each point in lines form, leading to the differences in the frequency spectrum distribution of the transient signals. Hence, a fault location method in distribution system based on transient energy distribution is proposed.