外文资料及其翻译李超(3)

1、AWaveletBasedApproachforFastDetectionofInternalFaultinPowerTransformers,ithasbeenseenthatthefaultcurrentcancontainhighersecondorderharmonicsthantheinrushcurrentduetononlinearfaultresistance,CTsaturation.thedistributedcapacitanceinthetransmissionline,whichtransformerisconnectedto,orduetotheuseofextra

2、highvoltageundergroundcables.Variousmethodshavebeensuggestedforovercomingthisprotectionsystemmal-operation.5kHz.Then,thedifferentialcurrentsofthethreephasesaredecomposedintotwodetailsandonlythesecondlevelwillbeconsideredbyusingdb5motherwavelet.DiscreteWaveletTransformThewavelettransformisapowerfulto

3、oltoextractinformationfromthenon-stationarysignalssimultaneouslyinbothtimeandfrequencydomains.Theabilityofthewavelettransformtofocusonshorttimeintervalsforhigh-frequencycomponentsandlongintervalsforlow-frequencycomponentsimprovestheanalysisoftransientphenomenasignals.Variouswaveletfunctions,suchasSy

4、mlet,MorlertandDaubechiesareusedtoanalyzedifferentpowersystemphenomena.Themotherwaveletmustbeselectedperformedbasedonitsapplicationandthefeaturesofsignal.whichshouldbeprocessed.Inthispaper,Daubechieswaveletisused.Therearethreetypesofwavelettransform.WhichareContinuousWaveletTransform(CWT.DiscreteWav

5、eletTransform(DWTandWaveletPacketTransform(WPT.DWTisderivedfromCWT.Assumethatx(tisatomevariablesignal,thentheCWTisdeterminedby(1:(1Where,and_iaretranslatingandscalingparameters,respectively.Also,一isthewaveletfunctionand一isthecomplexconjugateof一.Waveletfunctionmustsatisfy(2andshouldhavelimitedenergy:

6、XI(2Then,thediscretizedmotherwaveletisasfollows:Where,1andI0andtheyarefixedrealvalues.Also,andarepositiveintegers.DWTisexpressedby(4:(4Where,_listhecomplexconjugateof丨.In(4,themotherwaveletisdilatedandtranslateddiscretelybyselectingand).一IandI(5DWTcanbeeasilyandquicklyimplementedbycomplementarylowpa

7、ssandhigh-passfilters.ProposedAlgorithmIntheproposedalgorithm,theDWTisappliedtothedifferentialcurrentsofthreephases.TheDaubechiesDb-5typewaveletisusedasthemotherwaveletandthesignalsaredecomposeduptothesecond-level.Then,thespectralenergyandstandarddeviationofthedecomposedsignalsinthelevelarecalculate

8、d.Theproposedmethodconsistsoftwosteps。detectionanddiscrimination.DisturbaneeDetectionUndernormalconditionsandexternalfaults,thedifferentialcurrentshavesmallervaluesthaninternalfaults.Howeverinsomeoperatingconditions,theexternalfaultscanresultinhighdifferentialcurrentsduetoratiomismatchofCTsortapchan

9、gesofpowertransformer.Then,theseconditionsmaycausemal-operationoftherelay.Therefore,athresholdcurrentisusedinordertopreventmalfunctionscausedbynon-faultycurrents.Ifoneofdifferentialcurrentsexceedsthisthresholdvalue,itwillbeidentifiedasafault.Thethresholdvalueiddefined,asfollows:JWhereIandIaretheseco

10、ndaryandprimaryCTcurrents,respectively,andistheslopeofthedifferentialrelaycharacteristic.If,thenthedetectionalgorithmdefinesitasaninternalfault.DisturbanceDiscriminationInordertoclassifydisturbances,thedifferentialcurrentsaredecomposeduptothesecondlevel,usingDaubechiesDb5typewaveletwithdatawindowles

11、sthanthehalfofthepowerfrequencycycle.Asamplingrateof5kHz,isconsideredforthealgorithm(i.e.100samplesperpowerfrequencycyclebasedon50Hz.Then,theenergyandstandarddeviationintheseconddetailarecalculatedforeachdifferentialcurrent.Itisseenthatthespectralenergyaswellasthestandarddeviationinleveltendstohaveh

12、ighvaluesduringinrushcurrents.Then,adiscriminationindex(一canbecalculatedbymultiplyingthespectralenergybystandarddeviationintheseconddetailforeachdifferentialcurrent,asfollows:Where,STDisthestandarddeviationin二detailandEisitsspectralenergy.TheSTDcanbedeterminedusingthefollowingequation:Where,一isn-thc

13、oefficientfromdetail2.IisitsmeanvalueandMisthetotalTlnumberofexistedcoefficients.Then,thespectralenergyofthewaveletsignalinthe一leveliscalculatedby(9:(9ElThen,thendiscriminationindex(willbecomparedwithathresholdvalue(.Therelaywillbeactivated,ifanyoneofthethree-phasedifferentialcurrentsexceedsthisthre

14、sholdvalue(一.译文:一个基于小波变换对电力变压器内部故障快速检测的方法电力变压器是电力系统中最重要的部分之一，并且其保护是整个系统的保护策略中重要组成部分。差动保护为电力变压器提供最好的保护。其工作原理是差动电流在内部故障时的值高于正常状态，但是一个大的瞬态电流（涌流会导致差动继电器的误动。对变压器保护的改进的研究集中在变压器内部短路电流和冲击电流的识别。与内部故障相比，励磁涌流有大量二阶谐波分量。因此，一些变压器保护系统会在检测到大量的二阶谐波分量判断涌流而中断变压器工作。现代大型电力变压器因为电力变压器的核心材料的改进使励磁涌流的二次谐波分量变得相对较小。同时，故障电流会比涌流

15、包含更高的二阶谐波分量，原因有非线性故障电阻、CT饱和、连接变压器输电线路的分布电容、或因使用过高压地下电缆。已经提出了各种方法来克服这一保护系统的误动。本文提出了一种在判别涌流、内部短路、外部短路与励磁的基于小波变换的方法，它不受CT饱和的影响并且当变压器励磁时它可以探测到内部故障。不同与人工神经网络和模糊逻辑基础算法。该方法不依赖系统。该方案的响应时间少于10毫秒。基小波采样频率为5kHz。那么，三项电的差动电流分解成两个步骤，并且只有第二阶段会考虑使用db5基小波离散小波变换。小波变换在同时在时域和频域提取非平稳信号的信息是一个功能强大的工具。小波变换能够在高频时时频窗变窄和低频时变宽来

16、提高对瞬变电流的分析。有多种小波变换，例如Symlet、Morlert和Daubechies小波变换用于分析不同的电力系统现象。基小波的选择必须依赖于他的应用及信号的特点。在本文中，使Daubechies小波变换有三种小波变换方式。连续小波变换（CWT、离散小波变换（DWT和小波包变换（WPT，DWT来源于CWT。假设x（t是一个时变信号，那么CWT就由（1式决定：是小波函数和.二：是厂共轭复(1其中，和分别是转换和尺度参数。同时,数。小波函数必须满足（2并且应该有限能量：*.1（2那么，离散基小波如下:如下:其中，厂是|：“共轭复数。在4）中，基小波通过选定和来表述和转化：一工.1and一乂

17、-（5DWT可以由互补的低通和高通滤波器方便而快速地实现。处理算法：在处理算法中，DWT用于三相电的差分电流。Daubechies的Db-5类型的小波为作为基小波，信号分解到第二层。然后分解信号的光谱能量和标准偏差在L水平进行了计算分析。该方法有两个步骤。检测和鉴别。干扰检测：在正常的条件和外部故障时的差分电流比在内部故障时要小。但是，在某些条件下，因为CT比率不匹配或电力变压器的振动变化会导致较高的差分电流。然后，这些状况可能引起继电器的误动。因此，一个阈值电流用于防止非故障电流引起的误动。如果差分电流超过阈值，它将被认定为发生了故障。阈值定义如下：(6其中，丨和尸|分别是CT的次级和初级电流，是差动继电器特征斜率。如果11，然后检测算法将它定义成为一个内部错误。干扰识别：为了对干扰进行分类，我们把差分电流分解到第二层，用窗口少于一半工频周期的DaubechiesDb5型基小波