Research On Extremely Low Frequency Signal Based Faulty Line Identification Method Of Low-resistance Grounding System

Due to the feature of being able to operate with a single-phase-to-ground fault for 2hours,isolated grounding and resonant grounding are mainly adopted in China.As the proportion of cables in the urban distribution network increases,the fault capacitance current increases.A single-phase-to-ground fault can easily evolve into interphase short-circuit fault,leading to the expansion of the fault range.Therefore,single-phase-to-ground faults need to be removed quickly.The low-resistance grounding system is increasingly used in urban distribution networks due to the obvious characteristics of metal grounding faults and low resistance grounding faults.However,when a high-resistance grounding fault occurs in low-resistance grounding system,the characteristic amplitude of the fault is significantly reduced.Undiscovered grounding faults that are not removed for a long time may cause major accidents such as equipment damage,fire accidents,and casualties.The discrimination of characteristic values of conventional frequency methods are low,and it is difficult to distinguish the small difference of fault characteristic between the faulty line and healthy lines.Existing methods can only identify high-resistance grounding faults of 2-3 kΩ.Improving the sensitivity of fault identification and improving the reliability of fault identification hinder each other,making it difficult to improve the ability of high-resistance grounding fault identification in low-resistance grounding systems.Therefore,the identification of single-phase-to-ground fault in low-resistance grounding systems,especially the identification of high-resistance grounding fault,needs further study.This paper studies the single-phase-to-ground fault characteristics of steady state and transient state in low-resistance grounded systems.The analysis and calculation results are verified by simulation.The analysis shows the limitation that the conventional frequency method has insufficient discrimination of fault characteristic value and the conventional transformers cannot accurately measure the weak fault characteristic values in high-resistance grounding fault situation.Aiming at the problem of insufficient discrimination of the fault characteristic values in the conventional frequency methods,a novel method based on the system’s own extremely low frequency signal is proposed.To remedy the problem of the system’s own extremely low frequency signal amplitude is affected by the initial phase angle,a fault identification method based on artificially injected extremely low frequency signals is proposed,which injects stable extremely low frequency signal into the system by connecting passive injection sources at the neutral point.A typical low-resistance grounding system model is built,and single-phase-to-ground faults with different fault locations,different initial phase angles and different fault resistance are simulated.Aiming at the problem that conventional transformers cannot measure weak fault characteristic values,a fully digital magnetic modulator for micro-ampere extremely low frequency signal measurement is designed and developed.Finally,the proposed method were verified in the half-truth system and the true-type system.Simulation and experimental results in the half-truth system and the true-type system show that the discrimination of characteristic signal increases from 31 obtained by power frequency method to more than 500,the high-resistance fault of 10 k? is successfully identified,which shows that the proposed method has good selectivity,reliability and has practical application prospects.