電氣外文資料翻譯---一個基于小波變換對電力變壓器內(nèi)部故障快速檢測的方法

1、<p>　　A Wavelet Based Approach for Fast Detection of Internal Fault in Power Transformers</p><p>　　The power transformer is one of the most expensive elements of power system and its protection is an es

2、sential part of the overall system protection strategy. The differential protection provides the best protection for power transformer. Its operation principle is based on this point that the differential current during

3、an internal fault is higher than normal condition. But, a large transient current (inrush current) can cause mal-operation of differential relays. Then, studies for the improvem</p><p>　　This paper presents

4、a wavelet based method for discrimination among inrush current, internal short circuit ,external short circuit and energizing and it is not affected by CT saturation and it is able to detect internal faults while transfo

5、rmer energization. Unlike Artificial Neural Network and Fuzzy logic based algorithms. This approach is not system dependent. The operating time of the scheme is less than 10ms. The Daubechies mother wavelet is used wi

6、th a sample rate of 5 kHz. Then , the di</p><p>　　Discrete Wavelet Transform</p><p>　　The wavelet transform is a powerful tool to extract information from the non-stationary signals simultaneous

7、ly in both time and frequency domains. The ability of the wavelet transform to focus on short time intervals for high-frequency components and long intervals for low-frequency components improves the analysis of transien

8、t phenomena signals. Various wavelet functions ,such as Symlet,Morlert and Daubechies are used to analyze different power system phenomena. The mother wavelet must be selec</p><p><b>　　(1)</b><

9、;/p><p>　　Where, and are translating and scaling parameters, respectively. Also , is the wavelet function and is the complex conjugate of . Wavelet function must satisfy(2) and should have limited energy:&l

10、t;/p><p><b>　　(2)</b></p><p>　　Then ,the discretized mother wavelet is as follows :</p><p><b>　　(3)</b></p><p>　　Where, >1 and >0 and they ar

11、e fixed real values. Also , and are positive integers. DWT is expressed by (4):</p><p><b>　　(4)</b></p><p>　　Where, is the complex conjugate of . In (4), the mother wavelet is dilat

12、ed and translated discretely by selecting and .</p><p>　　and (5)</p><p>　　DWT can be easily and quickly implemented by complementary low pass and high-pass filters.<

13、;/p><p>　　Proposed Algorithm</p><p>　　In the proposed algorithm, the DWT is applied to the differential currents of three phases. The Daubechies Db-5 type wavelet is used as the mother wavelet and

14、the signals are decomposed up to the second-level. Then , the spectral energy and standard deviation of the decomposed signals in thelevel are calculated. The proposed method consists of two steps; detection and discrimi

15、nation.</p><p>　　Disturbance Detection </p><p>　　Under normal conditions and external faults, the differential currents have smaller values than internal faults. However in some operating condi

16、tions, the external faults can result in high differential currents due to ratio mismatch of CTs or tap changes of power transformer. Then ,these conditions may cause mal-operation of the relay. Therefore ,a threshold cu

17、rrent is used in order to prevent malfunctions caused by non-faulty currents. If one of differential currents exceeds this threshold va</p><p><b>　　(6)</b></p><p>　　Where and are t

18、he secondary and primary CT currents, respectively, and is the slope of the differential relay characteristic. If , then the detection algorithm defines it as an internal fault.</p><p>　　Disturbance Discrim

19、ination</p><p>　　In order to classify disturbances, the differential currents are decomposed up to the second level, using Daubechies Db5 type wavelet with data window less than the half of the power frequen

20、cy cycle. A sampling rate of 5 kHz, is considered for the algorithm(i.e .. 100 samples per power frequency cycle based on 50 Hz). Then, the energy and standard deviation in the second detail are calculated for each diffe

21、rential current. It is seen that the spectral energy as well as the standard deviation in </p><p><b>　　(7)</b></p><p>　　Where, STD is the standard deviation in detail and E is its s

22、pectral energy. The STD can be determined using the following equation:</p><p><b>　　(8)</b></p><p>　　Where, is n-th coefficient from detail 2. is its mean value and M is the total n

23、umber of existed coefficients. Then , the spectral energy of the wavelet signal in the level is calculated by (9):</p><p><b>　　(9)</b></p><p>　　Then , then discrimination index () wi

24、ll be compared with a threshold value (). The relay will be activated, if any one of the three-phase differential currents exceeds this threshold value().</p><p><b>　　譯文：</b></p><p>

25、　　一個基于小波變換對電力變壓器內(nèi)部故障快速檢測的方法</p><p>　　電力變壓器是電力系統(tǒng)中最重要的部分之一并且其保護是整個系統(tǒng)的保護策略中重要組成部分。差動保護為電力變壓器提供最好的保護。其工作原理是差動電流在內(nèi)部故障時的值高于正常狀態(tài)。但是,一個大的瞬態(tài)電流(涌流)會導(dǎo)致差動繼電器的誤動。然后,對變壓器保護的改進的研究集中變壓器內(nèi)部短路電流和沖擊電流的識別。與內(nèi)部故障相比勵磁涌流有大量二階諧波分量。因此

26、,一些變壓器保護系統(tǒng)會在檢測到大量的二階諧波分量判斷涌流而中斷變壓器工作?，F(xiàn)代大型電力變壓器由于電力變壓器的核心材料的改進使勵磁涌流的二次諧波分量變得相對較小。同時,故障電流會比涌流包含更高的二階諧波分量，原因有非線性故障電阻、CT飽和、連接變壓器輸電線路的分布電容、或因使用過高壓地下電纜。已經(jīng)提出了各種方法來克服這一保護系統(tǒng)的誤動。</p><p>　　本文提出了一種在判別涌流、內(nèi)部短路、外部短路與勵磁的基于小

27、波變換的方法,它不受CT飽和的影響并且當(dāng)變壓器勵磁時它可以探測到內(nèi)部故障。不同與人工神經(jīng)網(wǎng)絡(luò)和模糊邏輯基礎(chǔ)算法。該方法不依賴系統(tǒng)。該方案的響應(yīng)時間少于10毫秒?；〔ú蓸宇l率為5 kHz。那么,三項電的差動電流分解成兩個步驟并且只有第二階段會考慮使用db5基小波。</p><p><b>　　離散小波變換</b></p><p>　　小波變換在同時在時域和頻域提取非平

28、穩(wěn)信號的信息是一個功能強大的工具。小波變換能夠在高頻時時頻窗變窄和低頻時變寬來提高對瞬變電流的分析。有多種小波變換,例如Symlet Morlert和Daubechies小波變換用于分析不同的電力系統(tǒng)現(xiàn)象。基小波的選擇必須依賴于他的應(yīng)用及信號的特點。在本文中, 使Daubechies小波變換。有三種小波變換方式。連續(xù)小波變換(CWT)。離散小波變換(DWT)和小波包變換(WPT)。DWT來源于CWT。假設(shè)x(t)是一個時變信號,那么C

29、WT就由 (1)式?jīng)Q定:</p><p><b>　　(1)</b></p><p>　　其中，和分別是轉(zhuǎn)換和尺度參數(shù)。同時, 是小波函數(shù)和是共軛復(fù)數(shù)。小波函數(shù)必須滿足(2)并且應(yīng)該有限能量:</p><p><b>　　(2)</b></p><p>　　那么，離散基小波如下:</p>

30、<p><b>　　(3)</b></p><p>　　其中，>1和>0并且它們是固定實數(shù)。另外，和是正整數(shù)。DWT由（4）表示如下:</p><p><b>　　(4)</b></p><p>　　其中，是共軛復(fù)數(shù)。在（4）中，基小波通過選定和來表述和轉(zhuǎn)化：</p><p&g

31、t;　　and (5)</p><p>　　DWT可以由互補的低通和高通濾波器方便而快速地實現(xiàn)。</p><p><b>　　處理算法</b></p><p>　　在處理算法中，DWT用于三相電的差分電流。Daubechies的Db-5類型的小波為作為基小波,信號分解到第二層。然后, 分解信號

32、的光譜能量和標(biāo)準(zhǔn)偏差在水平進行了計算分析。該方法有兩個步驟;檢測和鑒別。</p><p><b>　　干擾檢測</b></p><p>　　在正常的條件和外部故障時的差分電流比在內(nèi)部故障時要小。但是,在某些條件下,由于CT比率不匹配或電力變壓器的振動變化會導(dǎo)致較高的差分電流。然后,這些狀況可能引起繼電器的誤動。因此,一個閾值電流用于防止非故障電流引起的誤動。如果差分電

33、流超過閾值,它將被認(rèn)定為發(fā)生了故障。閾值定義如下:</p><p><b>　　(6)</b></p><p>　　其中，和分別是CT的次級和初級電流, 是差動繼電器特征斜率。如果,然后檢測算法將它定義成為一個內(nèi)部錯誤。</p><p><b>　　干擾識別</b></p><p>　　為了對干擾進

34、行分類,我們把差分電流分解到第二層,用窗口少于一半工頻周期的Daubechies Db5型基小波，考慮到算法，采樣速率為5 kHz, (例如, 每工頻周期100點基于50Hz)。然后,在第二層計算每一個差分電流的能量和標(biāo)準(zhǔn)偏差。我們可以發(fā)現(xiàn)的光譜能量以及標(biāo)準(zhǔn)偏差水平會在涌流保持較高的值。然后,鑒別指數(shù)可以由下式計算出來:</p><p><b>　　(7)</b></p>&l

35、t;p>　　其中，STD是層次的標(biāo)準(zhǔn)偏差，STD可以有下式確定：</p><p><b>　　(8)</b></p><p>　　其中，是層的n相式的系數(shù)，是它的平均值M是總系數(shù)和。那么，在層的小波信號的光譜能量可以由9式計算：</p><p><b>　　(9)</b></p><p>