3D near-surface P-wave velocity structure imaging with Distributed Acoustic Sensing and electric hammer source

Distributed Acoustic Sensing(DAS) is an emerging technique for ultra-dense seismic observation, which provides a new method for high-resolution sub-surface seismic imaging. Recently a large number of linear DAS arrays have been used for two-dimensional S-wave near-surface imaging in urban areas. In order to explore the feasibility of three-dimensional(3D) structure imaging using a DAS array, we carried out an active source experiment at the Beijing National Earth Observatory. We deployed a 1 km optical cable in a rectangular shape, and the optical cable was recast into 250 sensors with a channel spacing of 4 m. The DAS array clearly recorded the P, S and surface waves generated by a hammer source. The first-arrival P wave travel times were first picked with a ShortTerm Average/Long-Term Average(STA/LTA) method and further manually checked. The P-wave signals recorded by the DAS are consistent with those recorded by the horizontal components of short-period seismometers. At shorter source-receiver distances, the picked P-wave arrivals from the DAS recording are consistent with vertical component recordings of seismometers, but they clearly lag behind the latter at greater distances.This is likely due to a combination of the signal-to-noise ratio and the polarization of the incoming wave. Then,we used the Tomo DD software to invert the 3D P-wave velocity structure for the uppermost 50 m with a resolution of 10 m. The inverted P-wave velocity structures agree well with the S-wave velocity structure previously obtained through ambient noise tomography. Our study indicates the feasibility of 3D near-surface imaging with the active source and DAS array. However, the inverted absolute velocity values at large depths may be biased due to potential time shifts between the DAS recording and seismometer at large source-receiver distances.

Traveling wave solutions in a nonlocal dispersal SIR epidemic model with nonlocal time-delay and general nonlinear incidences

This paper investigates a nonlocal dispersal epidemic model under the multiple nonlocal distributed delays and nonlinear incidence effects.First,the minimal wave speed c*and the basic reproduction number Ro are defined,which determine the existence of traveling wave solutions.Second,with the help of the upper and lower solutions,Schauder's fixed point theorem,and limiting techniques,the traveling waves satisfying some asymptotic boundary conditions are discussed.Specifically,when Ro>1,for every speed c>c^(*) there exists a traveling wave solution satisfying the boundary conditions,and there is no such traveling wave solution for any 0<c<c^(*) when R_(0)>1 or c>0 when R_(0)<1.Finally,we analyze the effects of nonlocal time delay on the minimum wave speed.

A novel fault location method for distribution networks with distributed generations based on the time matrix of traveling-waves

To improve location speed,accuracy and reliability,this paper proposes a fault location method for distribution networks based on the time matrix of fault traveling waves.First,an inherent time matrix is established according to the normalized topology of the target distribution network,and a post-fault time matrix is obtained by extracting the head data of initial waves from traveling wave detection devices.A time determination matrix is then obtained using the difference operation between the two matrices.The features of the time determination matrix are used for fault section identification and fault distance calculation,to accurately locate faults.The method is modified by considering economic benefits,through the optimal configuration of detection devices of traveling waves when calculating fault distances.Simulation results show that the proposed method has good adaptation with higher fault location accu-racy than two other typical ones.It can deal with faults on invalid branches,and the error rate is under 0.5%even with connected DGs.

Application of Fault Location Mode Based on Travelling Waves for Neutral Non-effective Grounding Systems

Fault location for distribution feeders short circuit especially single-phase grounding fault is an important task in distribution system with non-effectively grounded neutral.Fault location mode for distribution feeders using fault generated current and voltage transient traveling waves was investigated.The characteristics of transient traveling waves resulted from each short circuit fault and their transmission disciplinarian in distribution feeders are analyzed.This paper proposed that double end travelling waves theory which measures arriving time of fault initiated surge at both ends of the monitored line is fit for distribution feeders but single end traveling waves theory not.According to different distribution feeders,on the basis of analyzing original traveling waves reflection rule in line terminal, Current-voltage mode,voltage-voltage mode and current-current mode for fault location based on traveling waves are proposed and aerial mode component of original traveling waves is used to realize fault location.Experimental test verify the feasibility and correctness of the proposed method.

基于时域波形特征认知的输电线路近端故障辨识与定位

针对现有单端行波故障测距对近端故障存在测距盲区、双端行波故障测距对近端故障测距误差较大,无法满足工程需要的不足,提出基于波形特征认知的近端故障辨识与定位方法。首先,分析了线路故障行波传播规律,以固定分辨率显示波形。发现线路近端故障时,初始行波及其后续波形在长时窗整体宏观观测下呈堆叠缓变特征,而在短时窗局部放大观测下呈周期性变化特征,且周期与故障距离相关。不同线路的近端故障历史样本能统一作为参照基准为测距提供提示。进而提出基于波形密度和突变分布的近端故障辨识方法。最后,对辨识出的近端故障进行周期估计,利用近端故障与线长的无关性以及历史样本突变周期和故障位置已知性,搜索周期最近邻历史样本,并由已知故障距离插值实现故障位置确定。基于大量实测数据进行仿真测试,结果表明所提方法能够显著提升单端行波测距可靠性和成功率。

可实现故障快速响应和全面覆盖的有源高压配电网单端暂态量阶段式保护方案

在新型电力系统架构下,分布式电源的广泛接入给现有高压配电网保护的性能带来了巨大挑战。由于分布式电源接入导致故障特征复杂,传统工频量就地主保护方案正面临性能下降的风险。因此,对于应对新型电力系统下传统工频量就地主保护动作性能明显下降问题,构建基于暂态量的保护判据是较为合理的替代方案。为此,以首个前行波为参考,首先分析区内、区外故障下首个同极性波与首个反极性波到达时刻之和的关系,发现区内、区外故障下首个同极性波与首个反极性波到达时刻之和满足不同的关系式,且具有鲜明的区分度。以此为基础,提出一种基于后续首个同极性波到达时刻与后续首个反极性波到达时刻之和的单端测距式行波保护新判据;进一步针对其存在的动作死区和配电网运行对近、远后备保护的需求,提出一种单端暂态量阶段式保护方案。该方案基于上述设计的保护Ⅰ段能够覆盖本级线路的95%,引入二次振荡波相继速动判据设计的保护Ⅱ段可以覆盖其余范围,保护Ⅰ、Ⅱ段协同工作可保护本级线路全长;同样基于上述判据设计的单端暂态量保护Ⅲ段可仅以1个时间级差延时,实现对下游线路大多数位置故障的快速响应,配合反时限过电流保护可保护下级线路全长。最后在PSCAD平台搭建仿真模型,验证了所提保护判据及方案的有效性和优越性。

基于信号频谱特性的配电网故障行波检测方法

针对配电网干扰情况下微弱故障信号特征不明显导致行波采集设备难以有效检测故障行波信号的问题,提出一种基于信号频谱特性的配电网故障行波检测方法。首先,通过分析配电网故障行波的传输特征与频率特性,建立基于波形增量比值的启动判据,对设备采样数据进行预处理,减少行波定位装置的误启动。然后,引入鲁棒性局部均值分解(robust local mean decomposition,RLMD)方法处理采样数据,滤除采样过程中的干扰信号,减少噪声信号的影响。最后,根据行波低频含量衰减较小而高频含量衰减快的性质,建立故障行波辨识判据,辨识配电网故障行波信号。仿真表明,所提方法能够有效检测微弱故障时的行波信号。

基于变电站高频滤波边界特性的配电网线模行波选线方法

受母线并联电容器对于线模行波的高频滤波作用的启发,该文跳出基于零模行波设计选线方法的视角,提出利用变电站边界特性的配电网线模行波选线方法。首先,研究证实了电子式电压互感器、金属氧化物避雷器与母线其余设备杂散电容共同组成的智能变电站线路边界具备强健的高频滤波特性,其对于线模行波的折射衰减作用为后续选线方法的设计奠定了基础;然后,基于健全线路与故障线路上线模行波高频瞬时能量的差异构造了选线新判据,其在高阻故障与小相角接地故障等工况下具备较高的灵敏度;最后,基于PSCAD/EMTDC仿真平台验证了所提选线方法的有效性、灵敏性与可靠性,该方法以线模行波作为所选模量,为故障选线问题的解决提供了全新的思路。

基于双端行波频率变化特性的中低速磁浮交通供电轨故障定位方法

中低速磁浮供电轨供电过程中容易发生直流接地故障,由于其供电距离较短、接地网结构复杂,发生直流接地故障后不易准确定位故障点。为此针对以上问题,根据列车运行过程中车体与轨道无接触的特点,建立中低速磁浮牵引供电系统,分析双端故障测距与单端故障测距的优缺点,研究供电轨发生接地故障后故障行波时域分布与频域分布的关系;并通过计算式计算分析行波频谱的产生机理,得出故障行波的频率分布不受车站内阻抗、过渡电阻的影响,只与故障点位置和故障行波到达线路两端的时间相关;基于此,提出基于双端频率变化特性的中低速磁浮交通供电轨故障定位方法,根据故障行波频率特性与行波速度计算故障点位置。并且通过仿真验证,该方法不受故障点位置、过渡电阻大小影响,且适用于单处接地故障、多处接地故障类型,故障误差始终保持在1%以内,相比传统的直流牵引供电系统故障定位方法,测量精度更高,适用于中低速磁浮交通供电轨接地故障。

多级多分支配电线路雷击故障定位方法

针对监测终端受多级多分支配电线路负荷小,无法布置在线路末端导致监测范围小的问题,以及雷击次级分支时缺乏有效识别方法的现状,提出一种适用于多级多分支配电线路的雷击故障定位方法。首先通过电流极性判断矩阵识别监测区段内外故障,并针对区段外故障采用改进型双端行波定位方法进行故障定位。然后,针对监测区段内故障,基于改进型双端行波原理形成故障支路判断矩阵实现一级多分支线路的故障定位,而针对多级多分支线路,需要先采用矩阵变换将多级多分支变换为一级多分支线路。最后,PSCAD/EMTDC仿真结果表明,该方法可以有效定位监测区段外故障,并能有效定位多级多分支配电线路中次级线路雷击故障,扩大分布式监测系统的监测范围,对于示范线路目前的布点方案,将监测范围扩大了57.46%。

基于多端行波频率矩阵的复杂配电网故障定位方法

针对配电网线路参数受依频变化影响导致现有故障定位方法实现复杂以及行波传输速度无法准确计算的问题,基于多端行波频率矩阵,提出一种不依赖波头时间信息的复杂配电网故障定位方法。通过对故障行波固有频率主成分与传输距离的关系进行分析,定义各分支节点的参考端,并计算真实故障发生前的基准固有频率矩阵与真实故障发生后的故障固有频率矩阵之间的差值,最终得到相应的故障分支判定原理。在判定故障分支的基础上,按照故障点到参考端的路径不经过分支节点或经过分支节点数量最少为原则选取参考端,同时计算对应参考端故障固有频率主成分下的波速度,进而对故障点位置实现精确定位。仿真结果表明,所提方法无须检测行波波头时间,通过构建多端频率矩阵准确刻画配电网拓扑任一分支发生故障的情形,在保证故障分支可靠判定的同时实现了频率分量与波速度相互匹配,大大提高了故障定位精度,且定位结果不受故障位置、类型、过渡电阻、初相角影响。

基于自适应VMD和WVD的配电网故障行波检测方法

精确检测故障行波信号是保证故障定位结果准确可靠的重要因素,针对Wigner Ville分布(Wigner Ville distribution,WVD)在检测故障行波时易产生交叉项的问题,提出一种基于改进变分模态分解(variational mode decomposition,VMD)和WVD的故障行波检测方法。首先,通过波形相似性确定VMD参数的选取,利用改进的VMD对线模行波信号自适应分解,实现对故障行波不同频段信号提取的同时保留重要故障信息;然后,分别求取各分量信号的WVD后叠加得到原始信号的WVD,由此解决WVD交叉项带来的影响;最后求取VMD-WVD能量谱密度分布,利用WVD良好的时频聚集性检测故障行波[150,170]kHz频段波头位置,从而实现故障行波波头的精准检测。仿真实验结果表明,该文所提方法相比小波和HHT的故障行波检测方法更加快速、准确。

基于行波法的配网混合线路故障定位研究

为实现配网混合线路故障的精准定位,提出了一种行波极性判断与单端故障定位提取相结合的方法.首先,基于小波变换模极大值的方法,计算不同区段的行波电流极性,确定故障点所在的区间;接着,仿真计算故障区间内的行波波速,并基于小波变换提取波头时间,实现混合线路的故障定位;最后,在10kV动模试验平台搭建混合线路故障模型,开展不同影响因素的定位精度研究.研究结果表明:当采样频率和故障初相角增加时,定位精度也会逐步增加;故障区段为电力电缆时,定位精度优于架空线路的故障区段;该方法可精准地实现故障定位,具有一定的推广意义.

Traveling Wave Based Single-Phase-to-Ground Protection Method for Power Distribution System

Correct detection and identification of single-phase to-ground faults not effectively grounded in distribution systems is a major challenge for protection engineers.This paper proposes a novel traveling wave based protection method to solve this problem.The proposed method compares the polarities of current and voltage traveling waves measured immediately after the fault inception to determine the fault direction.Nuisance tripping is avoided by using the power frequency voltages detected on the busbar to inhibit operation.The power frequency voltages ensure that the system does not mal-operate due to noise and also provide discrimination for phase-to-phase and three-phase faults.The wavelet transform and modulus maxima theories are used to extract the polarity of traveling waves measured at the relaying point.The simulation studies demonstrate correct operation of protection,which is independent of fault distance,fault inception angle,fault path resistance,and the method used for neutral grounding.

基于多端行波的10 kV配电网线路故障定位方法应用分析

由于进行10 kV配电网线路故障定位时,没有判定故障区段,致使故障定位误差率较大,定位效果不佳,为此,提出基于多端行波的10 kV配电网线路故障定位方法。通过计算线路参数与行波频率,获取配电网行波特征,并构建10 kV配电网线路故障距离矩阵,对故障区段进行判定。根据区段判定结果,结合故障行波定位原理,计算配电网线路故障点距离,以计算结果为基础,对10 kV配电网线路故障定位。试验结果表明,所提方法进行10 kV配电网线路故障定位的结果与预设结果之间的误差率在6%以下,可有效提升配电网线路故障定位精度,定位效果较好。

Experimental Investigation of Self-sustained Oscillation in a Traveling Wave Thermoacoustic System

A traveling wave thermoacoustic engine consisting of a loop tube with a resonator has been tested. The onset characteristic together with the transition of oscillation mode from traveling wave to standing wave and the periodic shifting between modes in this system are investigated experimentally. The process of self-sustained thermoacoustic oscillation in this heat engine is described and analyzed through phase space distribution reconstructed from the time series of acoustic signal.

配电网故障行波定位综述

快速准确地进行配电网故障定位是保证供电可靠性的基础,是实现快速恢复区段供电的关键技术。本文主要结合近些年国内外学者的研究成果,对当前影响配电网行波故障定位技术应用的因素进行归纳,并对当前主流的定位方法进行总结分类,以及对后期工作的开展进行展望。

Multi-Branch Fault Line Location Method Based on Time Difference Matrix Fitting

The distribution network exhibits complex structural characteristics,which makes fault localization a challenging task.Especially when a branch of the multi-branch distribution network fails,the traditional multi-branch fault location algorithm makes it difficult to meet the demands of high-precision fault localization in the multi-branch distribution network system.In this paper,the multi-branch mainline is decomposed into single branch lines,transforming the complex multi-branch fault location problem into a double-ended fault location problem.Based on the different transmission characteristics of the fault-traveling wave in fault lines and non-fault lines,the endpoint reference time difference matrix S and the fault time difference matrix G were established.The time variation rule of the fault-traveling wave arriving at each endpoint before and after a fault was comprehensively utilized.To realize the fault segment location,the least square method was introduced.It was used to find the first-order fitting relation that satisfies the matching relationship between the corresponding row vector and the first-order function in the two matrices,to realize the fault segment location.Then,the time difference matrix is used to determine the traveling wave velocity,which,combined with the double-ended traveling wave location,enables accurate fault location.

配电网集中式馈线故障自动化定位方法研究

配电网线路众多且纷杂,加上恶劣的环境,查找故障的难度大,需要耗费大量的人力和物力。为此,研究了一种配电网集中式馈线故障自动化定位方法。利用录波器采集行波信号并利用小波变换方法对信号进行去噪处理。利用VMD方法分解行波信号,确定分解后每个模态分量的边际谱,提取每个模态分量的特征,包括频谱均值、方差、带宽、最高幅值对应的频率,组成特征量集合。输入特征,利用多分类SVM模型定位故障类型并通过行波法确定故障位置,完成配电网集中式馈线故障自动化定位。结果表明,所研究方法的定位误差达到相对最小值,定位结果更准确。

基于行波时差法的配电网故障诊断方法

针对配电网结构复杂故障难以精确诊断的问题,提出了一种基于故障行波时差的配电网故障诊断方法,实现对配电网故障点的精确定位及故障类型的判断。根据线路行波传输特性及故障点距离计算理论,构建配电网线路状态方程,将计算的理论值代入状态方程,输出故障搜索矩阵,利用矩阵变化的特征来诊断线路的故障情况。最后,利用行波到达多端时刻差解决线路中波速变化造成的故障定位不准确问题。该方法经配电网仿真模型验证具有有效性,可以准确诊断出不同阻值接地故障。