潘俊文畢業(yè)設(shè)計開題報告、外文翻譯、撰寫規(guī)范doc

1、畢業(yè)設(shè)計/論文開 題 報 告課 題 名 稱 院 系 專 業(yè) 班 姓 名 評 分 指 導(dǎo) 教 師 華中科技大學(xué)武昌分校畢業(yè)設(shè)計開題報告撰寫要求1. 開題報告主要內(nèi)容1）課題設(shè)計的目的和意義；2）課題設(shè)計的主要內(nèi)容；3）設(shè)計方案；4）實施計劃。 5）主要參考文獻：不少于5篇，其中外文文獻不少于1篇。2撰寫開題報告時，所選課題的課題名稱也不得多于25個漢字，課題設(shè)計份量要適當(dāng)， 設(shè)計中必須是自己的設(shè)計內(nèi)容。3. 開題報告的字?jǐn)?shù)不少于2000字（藝術(shù)類專業(yè)不少于1000字），格式按華中科技大學(xué)武昌分校本科畢業(yè)設(shè)計/論文撰寫規(guī)范的要求撰寫。4. 指導(dǎo)教師和責(zé)任單位必須審查簽字。5開題報告單獨裝訂，本附件

2、為封面，后續(xù)表格請從網(wǎng)上下載并用a4紙打印后填寫。6. 此開題報告適用于全校各專業(yè)，部分特殊專業(yè)需要變更的，由所在系在基礎(chǔ)上提出調(diào)整方案，報學(xué)校審批后執(zhí)行。華中科技大學(xué)武昌分校學(xué)生畢業(yè)設(shè)計開題報告學(xué) 生 姓 名 潘俊文 學(xué) 號20101131281專業(yè)班級電氣1006 系別電氣指導(dǎo)教師郭磊職稱 課題名稱500kv變電站gis設(shè)備接地電流的計算研究 1課題設(shè)計的目的和意義 gis（gas insulated switchgear,全封閉式氣體絕緣開關(guān)設(shè)備）和hgis（hybrid gas insulated switchgear,復(fù)合組合電器）設(shè)備廣泛應(yīng)用于我國電力系統(tǒng)各電壓等級的變電站中。我國

3、已投運的1000kv特高壓交流示范工程和正在規(guī)劃的特高壓交流工程也都采用了gis或hgis的設(shè)計。 為了解決gis母線外殼環(huán)流帶來gis設(shè)備局部發(fā)熱，加速絕緣的老化，特別是gis外殼與地網(wǎng)回路中感應(yīng)的入地電流造成的安全問題，通過對500kv分體結(jié)構(gòu)gis穩(wěn)態(tài)運行情況下外殼感應(yīng)電流的計算，研究了gis基本設(shè)計參數(shù)對外殼感應(yīng)電流的影響。在設(shè)計時要合理設(shè)定gis接地裝置的型式和布置，以降低gis場地的接觸電位差和跨步電壓差，保證工作運行人員和場內(nèi)設(shè)備的安全。一般來說gis所需的場地面積僅為常規(guī)變電站的10%25%。在整個gis中，母線被密封于接地的金屬殼內(nèi)，金屬殼內(nèi)充滿了sf6壓縮氣體，母線與外殼之

4、間的距離很近。正是由于gis的這種緊湊結(jié)構(gòu)特征和短尺寸，使gis的接地顯得尤為重要。根據(jù)我國已建成的gis變電站的運行經(jīng)驗，許多gis設(shè)備的故障往往都與其外殼不平衡電流有關(guān)。目前國內(nèi)還沒有與gis金屬外殼環(huán)流相關(guān)的定量判斷標(biāo)準(zhǔn)，因此有必要研究分析gis金屬外殼環(huán)流的特性，以保證gis變電站能安全可可考地運行，并為特高壓gis變電站的優(yōu)化設(shè)計提供理論依據(jù)。2課題設(shè)計的主要內(nèi)容 (1)查閱大量參考文獻，理解課題的意義目的，掌握設(shè)計的原理，認(rèn)真完成開題報告，綜合運用所學(xué)的理論知識和實驗經(jīng)驗確定比較完善的課題設(shè)計方案并擬定實施計劃。 (2)應(yīng)用atp計算程序計算500kv變電站工況運行時gis設(shè)備外殼

5、的接地電流。通過仿真軟件建立變電站按一線一變方式運行的情況下的變電站模型和gis設(shè)備外殼的接地模型。計算500kv變電站正常運行時，gis設(shè)備外殼電流和外殼電流通過接地裝置流入地網(wǎng)的接地電流分布情況，并將計算值與有關(guān)部的實地測量數(shù)據(jù)進行比較分析。 (3)弄明白每個環(huán)節(jié)的具體實施方案，選擇合適的控制策略。并根據(jù)選擇的控制策略，設(shè)計出合適的軟件系統(tǒng)。要重點掌握atp的操作方法制定方案。 (4) 根據(jù)設(shè)計需要完成相應(yīng)軟件設(shè)計，并借助工具完成調(diào)試。 (5) 總結(jié)本課題設(shè)計過程中的經(jīng)驗，結(jié)合設(shè)計實踐，編寫完整的設(shè)計論文。3設(shè)計方案 （1）分析gis外殼感應(yīng)電流的產(chǎn)生原理 （2）gis外殼感應(yīng)電流的等效計

6、算 等效計算模型的建立 計算結(jié)果 （3）gis外殼感應(yīng)電流影響因素的計算分析 相間距離對外殼接地段環(huán)流的影響 外殼半徑對外殼接地段環(huán)流的影響 外殼接地點間距對外殼接地段環(huán)流的影響 （4）得出結(jié)論4實施計劃 （1） 第 1-2 周：查閱資料，翻譯外文文獻，撰寫開題報告。 （2） 第 2 周：開題報告答辯。 （3） 第 3-9 周：根據(jù)擬定的課題實施方案，進行深入研究。 （4） 第10-12周：撰寫并提交畢業(yè)設(shè)計論文初槁，指導(dǎo)教師批改論文，給出指導(dǎo)意見，然后修改論文，定稿。 （5） 第 13 周：畢業(yè)設(shè)計答辯資格審查。 （6） 第13-14周：答辯前專項檢查各教研室自查、院（系）復(fù)查與學(xué)校檢查相結(jié)

7、合，未通過學(xué)校檢查的專業(yè)不得進入答辯環(huán)節(jié)。 （7） 第14-15周：進行畢業(yè)設(shè)計/論文修改，完成答辯。5主要參考文獻（不少于5篇，其中外文文獻至少1篇）1 何金良，曾嶸.電力系統(tǒng)接地技術(shù).科學(xué)出版社，2007.2 梁曦東，陳昌漁，周遠翔.高電壓工程.清華大學(xué)出版社，2003.3 劉振亞，國家電網(wǎng)公司750kv輸變電示范工程建設(shè)總結(jié).北京：中國電力出版社，2006.4 吳云飛，汪濤，林志偉，500kv變電站的hgis和gis設(shè)備接地電流測量結(jié)果分析.電網(wǎng)技術(shù).2007,12:4749.5 ieee std 1243-1997. ieee guide for improving the light

8、ning performance of transmission lines.1997指導(dǎo)教師意見指導(dǎo)教師簽字： 年 月 日答辯小組意見：組長簽字： 年 月 日教研室審查意見：教研室負責(zé)人簽字：年 月 日院系審查意見：院系負責(zé)人簽字： （公 章） 年 月 日（此表由學(xué)生填寫，指導(dǎo)教師、答辯小組、教研室、院系簽署意見）畢業(yè)設(shè)計/論文外 文 文 獻 翻 譯院 系 專 業(yè) 班 級 姓 名 原 文 出 處 評 分 指 導(dǎo) 教 師 華中科技大學(xué)武昌分校20 年 月 日畢業(yè)設(shè)計/論文外文文獻翻譯要求：1外文文獻翻譯的內(nèi)容應(yīng)與畢業(yè)設(shè)計/論文課題相關(guān)。2外文文獻翻譯的字?jǐn)?shù)：非英語專業(yè)學(xué)生應(yīng)完成與畢業(yè)設(shè)計/論文

9、課題內(nèi)容相關(guān)的不少于2000漢字的外文文獻翻譯任務(wù)（其中，漢語言文學(xué)專業(yè)、藝術(shù)類專業(yè)不作要求），英語專業(yè)學(xué)生應(yīng)完成不少于2000漢字的二外文獻翻譯任務(wù)。格式按華中科技大學(xué)武昌分校本科畢業(yè)設(shè)計/論文撰寫規(guī)范的要求撰寫。3外文文獻翻譯附于開題報告之后：第一部分為譯文，第二部分為外文文獻原文，譯文與原文均需單獨編制頁碼（底端居中）并注明出處。本附件為封面，封面上不得出現(xiàn)頁碼。4外文文獻翻譯原文由指導(dǎo)教師指定，同一指導(dǎo)教師指導(dǎo)的學(xué)生不得選用相同的外文原文。金屬氧化物避雷器在氣體絕緣變電站中的響應(yīng)以及改善其動態(tài)性能的方法摘要眾所周知，金屬氧化物避雷器（moa）已無力對付快速暫態(tài)過電壓（vfto）。本文的

10、目的是研究金屬氧化物避雷器動態(tài)性能的氣體絕緣變電站（gis）。230 kv gis是從當(dāng)?shù)氐膶嵱眯院拖到y(tǒng)的模擬是非常快的瞬態(tài)。對避雷器的動態(tài)性能從當(dāng)前不同前時代潮，從微秒到納秒級使用電磁暫態(tài)程序進行了分析。動態(tài)特性可以得到超過1毫秒前的時間。然而，在當(dāng)前的時間有一個偏差小于1毫秒，說明避雷器傳導(dǎo)失敗,因為在其最初的響應(yīng)延遲，尤其是在vfto。動態(tài)特性的特性提高是通過該系統(tǒng)中的陽離子以及避雷器。構(gòu)造一個動態(tài)磁滯曲線確定殘余電壓和電流之間的延遲。通過降低延遲，有可能使避雷器的有效利用，可以提高vfto應(yīng)用提高避雷器的動態(tài)特性。1 簡介快速暫態(tài)過電壓（vftos），與前時間5 ns 命令，發(fā)生在氣體

11、絕緣變電站間（gis）和真空斷路器中的開關(guān)操作期；也有對變電站雷電流上升很快的速度發(fā)生明顯增大，陡瞬變的振幅是依賴于開關(guān)的類型，真空產(chǎn)生的比sf6振幅越大，也依賴于系統(tǒng)條件。245千伏的gis,vfto的最大可能值水平為2.72單位。vfto的主要問題是對地閃絡(luò)隔離開關(guān)接觸和失敗的電子控制電路連接到gis 的vfto下，該設(shè)備在gis絕緣經(jīng)受漸進而逐步惡化，在gis及其配件的合理設(shè)計需要減少這些vftos的影響。目前電力網(wǎng)vftos存在已經(jīng)建立；也對金屬氧化物避雷器的設(shè)計來防止vftos不足。值得注意的是，這些避雷器不能在短上升時間有效的對應(yīng)瞬變。電壓時間行為的無間隙避雷器在非?？斓乃矐B(tài)下主要

12、取決于它的打開行為這些行為受其設(shè)計和施工材料。所有這些都是一種很好的協(xié)調(diào)器防vftos的重要性。這項工作的目的是研究和金屬氧化物避雷器的響應(yīng)行為，連接到gis，當(dāng)受到電流不同前時代潮，微秒到納秒級。2 避雷器的動態(tài)特性金屬氧化物避雷器的高度非線性行為在這樣一種方式，所施加的電壓增加它們的電阻急劇減小。這個通常表示為 q是一個幾何因子由非線性的尺寸決定，是一個變化范圍從30至40的常數(shù)，并給出了一個衡量或感覺存在的電壓和電流之間的非線性關(guān)系。這一價值估計所需的電流的大小和相應(yīng)的電壓之間的關(guān)系式 v1和v2的電壓電流i1和i2（i2>i1）。正確的值的計算來自于避雷器的vi特性的使用。避雷器

13、的行為不同的各種浪涌波形，根據(jù)每次的幅度和上升的上升率。殘壓的測量，目前超過1毫秒前的激增，表明和特定的動態(tài)特性,剩余電壓隨電流前時間減小，殘余電壓達到其最大浪涌電流達到其峰值之前。它們的動態(tài)特性是非常重要的表演的絕緣配合研究對快速波前過電壓浪涌。快速前激增，那些在微秒的范圍內(nèi)的上升時間，電壓波的峰值電流波的峰值之前。然而，非?？斓乃矐B(tài)浪涌電流，用納秒級的時間面前，電壓波的峰值電流波的峰值后。因此瞬間的殘余電壓的初始峰值出現(xiàn)的金屬氧化物避雷器對vftos 動態(tài)性能的研究十分重要的,所以它是衡量殘余電壓上升時間的初始的本質(zhì)，除了其峰值，尤其對vfto。3 gis系統(tǒng)的描述 變電站電氣設(shè)備的配置和

14、設(shè)計是基于用戶的決策。這是更經(jīng)濟的使用傳統(tǒng)的避雷器和變壓器上的架空線相鄰，地理信息系統(tǒng)，而不是氣體絕緣避雷器和氣體絕緣變壓器的連接。避雷器與變壓器之間的距離來確定變壓器的高電壓端子過電壓的大小。一個有助于vfto作用下變壓器故障的因素是由于較高的非線性電壓分布沿繞組由于其電容可能導(dǎo)致在轉(zhuǎn)身之間的高電壓。由于出現(xiàn)在初始電壓分布的電位梯度大，在高壓繞組匝間絕緣的開始往往是不成比例地強調(diào)的更多。如通過架空線路連接到gis變壓器設(shè)備，電纜或直接導(dǎo)入gis將由vftos 影響。盡管vfto幅值低于基本脈沖電平（bil）的系統(tǒng)，由于頻繁的發(fā)生，它們有助于系統(tǒng)中的絕緣壽命的降低。根據(jù)的vfto及其頻率內(nèi)容的

15、大小，可導(dǎo)致應(yīng)力超限vftos變壓器繞組絕緣的。這是在文獻 1，4，1419 日報道，有vfto作用下變壓器故障由于金屬氧化物避雷器（moa）可能在非導(dǎo)通狀態(tài)下vfto，但它是一個很好的保護裝置的時間超過1毫秒前的浪涌電流（雷電和開關(guān)浪涌）。所以，避雷器動態(tài)性能的研究是一個試圖提高vfto的應(yīng)用。 一個從泰米爾納德邦電力局的方案系統(tǒng)，印度當(dāng)?shù)氐男в檬欠治?。圖1顯示了230 kv gis的單線排列和連接的設(shè)備。230 kv gis與14米長的氣體絕緣母線（bc）是連接到一個100 mva，230 / 110 kv變壓器通過20米長的架空線路（ohl）表現(xiàn)為ce。2公里長的奧爾（ba）連接到另一端

16、的氣體絕緣母線。避雷器分級一分級環(huán)放置10米（d）從230 kv側(cè)變壓器的非線性指數(shù)（）為避雷器是50，這是從vi特性的避雷器獲得由廠家提供。 圖1一230 kv gis單線圖 所安裝的避雷器的額定值如下：mcov=181 kvrms。 電流=10 ka。 雷電防護等級=525 kvpeak。開關(guān)浪涌保護水平=445 kvpeak。 暫時過電壓耐受水平=230200 kvrms的100.01 s，分別。三類放電線 3.1 系統(tǒng)建模 gis組件和vfto計算模型是由povh et al.和vinod kumar et al.提出的。由于vfto主要含有高頻分量，從幾百千赫到幾十兆赫，大多數(shù)的模型

17、都有其電容支配其他參數(shù)。一個地理信息系統(tǒng)總線的典型長度遠小于普通的變電站，與此同時在高頻率，在幾百khz到mhz范圍，gis總線的作用類似于一個有限的運輸時間，傳播速度和波阻抗傳輸線。對gis總線被建模作為傳輸線可以從關(guān)系得到的波阻抗值 “a”是在高壓母線和“b”的直徑為外殼的內(nèi)徑。對于給定的230 kv gis，波阻抗值被發(fā)現(xiàn)是78 。在ohl的波阻抗值為388 ，它認(rèn)為激增的ohl以光速并且光的速度0.9倍在gis中的傳播。電力變壓器，終止gis母線，表示為集總電容4 nf 。變壓器電容對過電壓的產(chǎn)生有非常重要的意義。表1顯示了圖1的等效構(gòu)件表示。 基于這次研究的重點是納秒級電流浪涌避雷器

18、的動態(tài)性能，避雷器的精確建模與分布式參數(shù)。moa的vi特性對于一個給定的電流激增循環(huán)趨勢。循環(huán)是由于在金屬氧化物避雷器的響應(yīng)時間滯后，環(huán)的大小取決于電流沖擊的幅度和波形，這種特性表明有等價與非線性電阻串聯(lián)電感。由于電容起主要作用的非?？斓乃矐B(tài)條件下，對于vfto的研究實際電容將是非常必要的。因此，避雷器是作為一個組合的非線性電阻，電感和電容的模型。塊電容（電容的非線性塊）和雜散電容（電容接地）避雷器提取用有限元法。這些提取的值是用避雷器的vfto模型。建模過程中對moa非?？斓乃矐B(tài)在討論。3.2 短連接的重要性導(dǎo)致 連接的總和導(dǎo)致架空線路和地墊之間創(chuàng)建一個電感約1 hm ，它影響避雷器電流并且

19、因此引起變電站的過電壓。避雷器的殘余電壓隨電流的振幅非常微小的變化，由于連接的引線電感，陡峭的電流會產(chǎn)生一個電壓，避雷器的殘余電壓增加。因此，為了使這些重要的引線盡可能短，如圖2所示，特別是在短的變電站。電壓應(yīng)力對變壓器絕緣并聯(lián)避雷器和避雷器的殘余電壓和所導(dǎo)致的感應(yīng)電壓也在平行的保護絕緣。如果避雷器的長度減少，故此，變壓器絕緣應(yīng)力可以減少。 因此，gis系統(tǒng)參數(shù)如避雷器引線和分離距離對系統(tǒng)中的過電壓的計算很重要。延遲線的建模（dc），地下電纜有意引入電路造成避雷器響應(yīng)的改進，電容式電壓互感器（cvt）在表1中給出。完整的230 kv gis系統(tǒng)的模擬是準(zhǔn)確的vfto過電壓在不同的陡度使用電磁暫

20、態(tài)程序（emtp）計算的浪涌電流。電流浪涌尾時間不影響避雷器的初始反應(yīng)并且在電壓曲線尾部的下降是不重要的絕緣配合研究 。因此，下面的討論僅基于當(dāng)前潮前的時間。翻譯原文：response of metal oxide arrester in gas-insulated substation and methods to improve its dynamic characteristicsabstract: it is well known that the metal oxide arrester (moa) has the inability to operate against very

21、fast transient overvoltage (vfto). the aim of this paper is to study the dynamic performance of moa in gas-insulated substation (gis). a 230 kv gis is taken from a local utility and the system is modelled for very fast transients. the dynamic performance of arrester is analysed for current surges of

22、 different front times, varying from microsecond to nanosecond using electromagnetic transient program. the dynamic characteristics can be obtained for front time, more than 1 ms. however, there is a deviation in it when front time is less than 1 ms, showing failure of arrester conduction because of

23、 delay in its initial response, especially under vfto. the dynamic characteristic is improved by making modications in the system as well as in the arrester. a dynamic hysteresis curve is constructed to conrm the delay between the residual voltage and current surge. by decreasing the delay, it is po

24、ssible to improve the dynamic characteristics of the arrester so that the effective utilisation of moa can be enhanced in vfto applications.1 introduction very fast transient overvoltages (vftos), with the front times in the order of 5 ns 1, occur during the switching operations in gas-insulated sub

25、station (gis) and vacuum circuit breakers; there has also been increased evidence of the occurrence of very fast rate of rise of lightning currents at substations 2, 3. the amplitude of the steep fronted transients is dependent on the type of switch, with vacuum producing higher amplitudes than sf6,

26、 and also on system conditions 4. in the case of a 245 kv gis, the maximum possible value of the vfto level is 2.72 per unit. the main problems associated with the vfto are ashover to ground at the disconnector switch contacts and failure of electronic control circuits connected to gis under vfto 5.

27、 consequent to the higher frequency of the occurrence of vftos, the insulation of the equipment in gis is subjected to gradual but progressive deterioration 6. proper design of the gis and its accessories are required to reduce the impact of these vftos. the presence of vftos in the power networks h

28、as now been well established; so also the inadequacy of the present design of metal oxide arresters to protect against vftos 1. it is noted that these arresters are not effective against the transients with very short rise time. the voltage time behaviour of gapless arresters under very fast transie

29、nts depends chiey on its turn on behaviour which is inuenced by its design and construction materials 7. all these attest to the importance of a well-coordinated arrester protection against vftos. this work aims to study the behaviour and response of metal oxide arrester, connected to gis, when subj

30、ected to current surges of different front times, microsecond to nanosecond level.2 dynamic characteristics of arrester the highly non-linear metal oxide arresters behave in such a way that their resistance decreases sharply as the voltage applied across them increases. this characteristic is usuall

31、y expressed as in i = qv a (1)where q is a geometrical factor, determined by the dimensions of the non-linear resistor and a is a constant varies from 30 and 40, and gives a measure or a feel of the non-linear relationship that exists between current and voltage 8. the a value is estimated between t

32、wo desired magnitudes of current and corresponding voltage by (2) where v1 and v2 are the voltages at currents i1 and i2 (i2 . i1). correct value of a is calculated from v i characteristics of the arrester using (2). surge arresters behave differently for various surge waveforms, depending each time

33、 on the magnitude and the rate of rise of the surge 9, 10. the measurements of the residual voltage, for current surges of front time more than 1 ms, indicate dynamic characteristics and specically, the residual voltage increases as the current front time decreases and residual voltage reaches its m

34、aximum before the current surge reaches its peak. their dynamic characteristics are important for performing insulation coordination studies against fast front overvoltage surges. for fast front surges, those with rise times in the range of microseconds, the peak of the voltage wave occurs before th

35、e peak of the current wave. however, for very fast transients, with front time of nanosecond current surge, the peak of the voltage wave occurs after the peak of the current wave. therefore the instant at which the initial peak of the residual voltage occurs is very important for the study of dynami

36、c performance of the metal oxide surge arrester against vftos 11. so it is essential to measure the initial rise time of the residual voltage, in addition to its peak value, especially for vfto.3 description of the gis system the conguration of substation and design of electrical equipment are based

37、 on the decision of the user. it is more economical to use conventional arrester and transformer on the overhead line adjacent to gis, rather than connecting as gas-insulated arrester and gas-insulated transformer. the distance between the arrester and the transformer will determine the overvoltage

38、magnitude at transformer high- voltage terminals. one of the factors that contribute to transformer failure under vfto is due to higher non-linear voltage distribution along the winding owing to its capacitance, which could result in high voltage between turns. the insulation between turns at the be

39、ginning of the high-voltage winding is often disproportionally more stressed because of the large potential gradient appearing in the initial voltage distribution 12. equipments such as transformers connected to the gis via overhead lines, cables or direct gis ducting will be affected by the vftos 1

40、3. even though vfto magnitudes are lower than basic impulse level (bil) of the system, they contribute to reduction in the life of insulation in the system because of their frequent occurrences 6. depending upon the magnitude of the vfto and its frequency content, vftos can cause overstressing of th

41、e transformer winding insulation. it is reported in the literature 1, 4, 14 19 that there is failure of transformer under vfto since the metal oxide arrester (moa) may be in non-conduction state under vfto, but it is a good protective device for current surge of front time more than 1 ms (lightning

42、and switching surges). so, an attempt is made to study and improve the dynamic performance of arrester for vfto applications. a modied system from tamil nadu electricity board, india for the local utility is considered for the analysis. fig. 1 shows the single-line arrangement of the 230 kv gis and

43、the connected equipment. the 230 kv gis with 14 m length of gas-insulated busbar (bc) is connected to a 100 mva, 230/110 kv transformer through 20 m length of overhead line (ohl) shown as ce. a 2 km length of ohl (ba) is connected to the other end of gas-insulated busbar. a surge arrester graded wit

44、h one grading ring is placed 10 m (d) away from the transformer in 230 kv side. the non-linearity index (a) for the arrester is 50, which is obtained from v i characteristics of the arrester as furnished by the manufacturer. fig. 1 single-line diagram of a 230 kv gisthe rating of the installed surge

45、 arrester is as follows: mcov ¼ 181 kvrms.current ¼ 10 ka.lightning protection level ¼ 525 kvpeak. switching surge protection level ¼ 445 kvpeak.temporary overvoltage withstand level ¼ 230 200 kvrms for 10 0.01 s, respectively.line discharge class iii.3.1 system modelling mo

46、delling of gis components and vfto computation are presented by povh et al. 3 and vinod kumar et al. 6. since the vfto contain predominantly high-frequency components ranging from hundreds of khz to tens of mhz, most of the models have their capacitances dominating the other parameters. the typical

47、length of a gis bus is much smaller than that of an ordinary substation and also, at high frequencies, in the range of few hundreds of khz to mhz, the gis bus acts like a transmission line with a nite transit time, propagation velocity and surge impedance. the value of surge impedance of gis bus whi

48、ch is modelled as transmission line can be obtained from the relation where a is the diameter of the high-voltage bus and b is the inner diameter of the enclosure. for the given 230 kv gis, the surge impedance value is found to be 78 v. the surge impedance value for ohl is 388 v. it is assumed that

49、the surge propagates in ohl with a velocity of light and in gis, 0.9 times the speed of velocity of light 20. the power transformer, which terminates the gis bus, is represented as lumped capacitance to ground of 4 nf 21, 22. the transformer capacitance has a signicant impact on overvoltage generati

50、on 20. table 1 shows the equivalent component representation for fig. 1. since this study is focused on dynamic performance of arrester for nanosecond current surge, the arrester is modelled accurately with distributed parameters. the v i characteristics of moa for a given current surge has a loopin

51、g tendency. the looping is due to the time lag in the response of metal oxide arrester. the size of loop depends on the magnitude and wave shape of current surge. this characteristic suggests that there is equivalently an inductance in series with the non-linear resistance. since capacitance plays m

52、ajor role under very fast transients, the incorporation of actual capacitance is very much essential for vfto study. so the surge arrester is modelled as a combination of non-linear resistance, inductance and capacitance. the block capacitance (capacitance of non- linear block) and stray capacitance

53、 (capacitance to ground) of arrester are extracted using nite element method. these extracted values are used in the vfto model of the arrester. the modelling procedure for moa under very fast transients is discussed in 11.3.2 importance of short connecting leads the sum of the connection leads betw

54、een the overhead line and the ground mat will create an inductance of about 1 mh/m 7, 21, which inuences the surge arrester current and consequently the overvoltage in the substation. the residual voltage of the arrester varies very moderately with current amplitude, but the steep current will generate a voltage additional to the residual voltage of the arrester because of the inductance of the connection lead. it is therefore important to make these leads as short as possible as shown in fig. 2, and espec