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正常情況下，零(ling)序(xu)(xu)(xu)電(dian)(dian)流(liu)(liu)(liu)保(bao)(bao)護(hu)(hu)(hu)不動(dong)作(zuo)(zuo)，發生接地短路時出現零(ling)序(xu)(xu)(xu)電(dian)(dian)流(liu)(liu)(liu)，當它(ta)大于(yu)保(bao)(bao)護(hu)(hu)(hu)的(de)動(dong)作(zuo)(zuo)電(dian)(dian)流(liu)(liu)(liu)時，電(dian)(dian)流(liu)(liu)(liu)繼電(dian)(dian)器ka動(dong)作(zuo)(zuo)，經kt延(yan)時后，跳開變壓器兩側(ce)斷路器。零(ling)序(xu)(xu)(xu)電(dian)(dian)流(liu)(liu)(liu)保(bao)(bao)護(hu)(hu)(hu)的(de)動(dong)作(zuo)(zuo)電(dian)(dian)流(liu)(liu)(liu)，應大于(yu)該側(ce)出線零(ling)序(xu)(xu)(xu)電(dian)(dian)流(liu)(liu)(liu)保(bao)(bao)護(hu)(hu)(hu)后備段(duan)的(de)動(dong)作(zuo)(zuo)電(dian)(dian)流(liu)(liu)(liu)。保(bao)(bao)護(hu)(hu)(hu)的(de)動(dong)作(zuo)(zuo)時限(xian)也要比后者大一個δt。

變電(dian)(dian)(dian)站是(shi)(shi)混跡電(dian)(dian)(dian)源、升(sheng)(sheng)降(jiang)(jiang)電(dian)(dian)(dian)壓(ya)(ya)和(he)分配(pei)電(dian)(dian)(dian)力(li)的場(chang)所(suo)(suo)，是(shi)(shi)聯系發電(dian)(dian)(dian)廠和(he)用戶的中間環節(jie)。變電(dian)(dian)(dian)站有升(sheng)(sheng)壓(ya)(ya)和(he)降(jiang)(jiang)壓(ya)(ya)之分。升(sheng)(sheng)壓(ya)(ya)變電(dian)(dian)(dian)站通常是(shi)(shi)發電(dian)(dian)(dian)廠升(sheng)(sheng)壓(ya)(ya)站部(bu)分，緊靠發電(dian)(dian)(dian)廠。降(jiang)(jiang)壓(ya)(ya)變電(dian)(dian)(dian)所(suo)(suo)通常遠離(li)發電(dian)(dian)(dian)廠而靠近負荷中心。根據變電(dian)(dian)(dian)站在電(dian)(dian)(dian)力(li)系統中所(suo)(suo)處的地(di)位和(he)作用，可分為以下幾類(lei)。

樞紐變電(dian)(dian)(dian)站(zhan)(zhan)位于電(dian)(dian)(dian)力系(xi)統的相紐點.電(dian)(dian)(dian)壓等級一般(ban)為330kv及(ji)以上.聯系(xi)多(duo)個電(dian)(dian)(dian)源.出線(xian)回路多(duo).變電(dian)(dian)(dian)容皿(min)大。全站(zhan)(zhan)停電(dian)(dian)(dian)后將造成(cheng)(cheng)大面積停電(dian)(dian)(dian).成(cheng)(cheng)系(xi)統瓦解(jie).樞紐變電(dian)(dian)(dian)站(zhan)(zhan)對電(dian)(dian)(dian)力系(xi)統運行的往(wang)定和(he)可靠性起(qi)著重(zhong)要作用(yong)。

(2)中間變電站

中間(jian)變電(dian)站(zhan)位于(yu)系(xi)統主(zhu)干環線(xian)(xian)或(huo)系(xi)統主(zhu)要于(yu)線(xian)(xian)的(de)接口處(chu)，電(dian)壓等級一般為220 kv-330 kv.匯集2-3個電(dian)譚和若干線(xian)(xian)路，高壓側以(yi)穿越功串(chuan)為主(zhu).同時(shi)降壓向地區(qu)用(yong)戶供電(dian).全站(zhan)停電(dian)后(hou)將引起(qi)區(qu)城(cheng)電(dian)網的(de)解(jie)列。

(3)地區變電站

地(di)(di)區變(bian)電站是一個地(di)(di)區和一個中、小城市的主要(yao)變(bian)電站，電壓(ya)等級一般為(wei)110 kv-220 kv，全站停電后將遺成該地(di)(di)民或城市供電的紊(wen)亂(luan)。

(4)企業變電站

企(qi)業(ye)變(bian)電(dian)站(zhan)是(shi)大、中(zhong)盛企(qi)業(ye)的專用變(bian)電(dian)站(zhan)，電(dian)爪等級一般為35 kv-220 kv.1--2間進線。

(5)終端變電站

終端變電(dian)站位(wei)于(yu)配電(dian)線路(lu)的(de)終端.接近附和處，高壓側(ce)10 kv--11o kv引(yin)人線，經降(jiang)壓后向用戶供(gong)電(dian)。

電氣(qi)設備的(de)預(yu)防性試驗可(ke)以通(tong)過以下幾方(fang)面進行。

1、測量絕緣電阻

它是一種常用(yong)而又簡單的(de)試驗方法，通(tong)常用(yong)兆歐表進行測量(liang)。根據測得的(de)試品(pin)在1分鐘時(shi)的(de)絕(jue)緣電阻的(de)大小(xiao)，可以(yi)檢測出絕(jue)緣是否有貫通(tong)的(de)集中性缺(que)陷、整體受潮或貫通(tong)性受潮。

2、測量泄漏電流

它與(yu)測量(liang)絕(jue)緣電(dian)(dian)阻(zu)的(de)原理基(ji)本(ben)上是相同的(de)，而且(qie)檢出缺陷的(de)性質(zhi)也大致相同。但由于泄(xie)漏(lou)電(dian)(dian)流(liu)測量(liang)中所用的(de)電(dian)(dian)源一般(ban)均由高壓(ya)整流(liu)設備提供，并用微安表直(zhi)接讀取泄(xie)漏(lou)電(dian)(dian)流(liu)。它有試驗電(dian)(dian)壓(ya)可隨意調(diao)節；靈敏度高，測量(liang)重復性較好；換算絕(jue)緣電(dian)(dian)阻(zu)值；可用測量(liang)吸收比來(lai)判斷(duan)絕(jue)緣缺陷等特點(dian)。

3、測量介質損失角

它是一項靈敏度很高(gao)的試(shi)驗項目(mu)，它可以(yi)發(fa)現電氣(qi)設(she)備絕緣整(zheng)體受潮、劣化、變質以(yi)及小體積被試(shi)設(she)備貫通或未貫通的局部缺(que)陷。但當被試(shi)品體積較大，而缺(que)陷所占(zhan)的體積又較小時，用(yong)這(zhe)種方法就難以(yi)發(fa)現了。它廣泛應用(yong)在(zai)電工制造和電氣(qi)設(she)備交接和預防性試(shi)驗中。

4、交流耐壓試驗

它是破壞性試驗中的(de)(de)一(yi)種，它能進一(yi)步診(zhen)斷出(chu)電氣(qi)(qi)設(she)備的(de)(de)絕緣(yuan)(yuan)缺陷。交(jiao)流耐壓(ya)試驗是鑒定電氣(qi)(qi)設(she)備絕緣(yuan)(yuan)強度的(de)(de)嚴格、有(you)效和直接的(de)(de)試驗方法，它能檢出(chu)絕緣(yuan)(yuan)在正常運行時的(de)(de)弱點，對判斷電氣(qi)(qi)設(she)備能否繼續投入運行具有(you)決(jue)定性的(de)(de)作用。

5、直流耐壓試驗

它除了能(neng)(neng)發現(xian)(xian)設備絕緣(yuan)(yuan)受(shou)潮、劣化外，對發現(xian)(xian)絕緣(yuan)(yuan)的(de)某些局部缺陷具有(you)特(te)殊(shu)的(de)作用(yong)。直流(liu)耐(nai)(nai)壓試(shi)驗能(neng)(neng)夠(gou)發現(xian)(xian)某些交流(liu)耐(nai)(nai)壓試(shi)驗所不(bu)能(neng)(neng)發現(xian)(xian)的(de)缺陷，交流(liu)與直流(liu)耐(nai)(nai)壓試(shi)驗這兩種試(shi)驗不(bu)能(neng)(neng)互相代替，必須(xu)同時(shi)應(ying)(ying)用(yong)于(yu)預防性試(shi)驗中，特(te)別是(shi)電機、電纜等(deng)更應(ying)(ying)當做直流(liu)耐(nai)(nai)壓試(shi)驗。

變壓(ya)器(qi)電(dian)(dian)(dian)(dian)流(liu)(liu)差(cha)動(dong)(dong)保(bao)護(hu)主要用(yong)來(lai)保(bao)護(hu)雙繞(rao)組或(huo)三繞(rao)組變壓(ya)器(qi)繞(rao)組內部及其(qi)引出(chu)線上(shang)(shang)發(fa)生的(de)(de)各種相間短(duan)路(lu)(lu)故障(zhang)，同時也可以用(yong)來(lai)保(bao)護(hu)變壓(ya)器(qi)單相匝間短(duan)路(lu)(lu)故障(zhang)。在繼(ji)(ji)電(dian)(dian)(dian)(dian)器(qi)線圈中(zhong)流(liu)(liu)過(guo)(guo)的(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)是(shi)兩側電(dian)(dian)(dian)(dian)流(liu)(liu)互感(gan)器(qi)的(de)(de)二(er)次電(dian)(dian)(dian)(dian)流(liu)(liu)差(cha)，也就是(shi)說差(cha)動(dong)(dong)繼(ji)(ji)電(dian)(dian)(dian)(dian)器(qi)是(shi)接在差(cha)動(dong)(dong)回(hui)(hui)路(lu)(lu)的(de)(de)。從理論上(shang)(shang)講，正常運(yun)行及外(wai)部故障(zhang)時，差(cha)動(dong)(dong)回(hui)(hui)路(lu)(lu)電(dian)(dian)(dian)(dian)流(liu)(liu)為(wei)零。實際上(shang)(shang)由于兩側電(dian)(dian)(dian)(dian)流(liu)(liu)互感(gan)器(qi)的(de)(de)特性不(bu)(bu)可能完全一(yi)致(zhi)等原因，在正常運(yun)行和(he)外(wai)部短(duan)路(lu)(lu)時，差(cha)動(dong)(dong)回(hui)(hui)路(lu)(lu)中(zhong)仍有不(bu)(bu)平衡電(dian)(dian)(dian)(dian)流(liu)(liu)Iumb流(liu)(liu)過(guo)(guo)，此時流(liu)(liu)過(guo)(guo)繼(ji)(ji)電(dian)(dian)(dian)(dian)器(qi)的(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)IK為(wei) Ik=I1-I2=Iumb 要求不(bu)(bu)平衡點流(liu)(liu)應盡量的(de)(de)小，以確保(bao)繼(ji)(ji)電(dian)(dian)(dian)(dian)器(qi)不(bu)(bu)會誤(wu)動(dong)(dong)。當變壓(ya)器(qi)內部發(fa)生相間短(duan)路(lu)(lu)故障(zhang)時，在差(cha)動(dong)(dong)回(hui)(hui)路(lu)(lu)中(zhong)由于I2改變了方向或(huo)等于零（無電(dian)(dian)(dian)(dian)源(yuan)側），這是(shi)流(liu)(liu)過(guo)(guo)繼(ji)(ji)電(dian)(dian)(dian)(dian)器(qi)的(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)為(wei)I1與I2之和(he)，即(ji) Ik=I1+I2=Iumb 能使繼(ji)(ji)電(dian)(dian)(dian)(dian)器(qi)可靠動(dong)(dong)作。

變壓器差(cha)動(dong)(dong)保(bao)(bao)護(hu)(hu)(hu)(hu)的范(fan)圍是構成變壓器差(cha)動(dong)(dong)保(bao)(bao)護(hu)(hu)(hu)(hu)的電流互感器之(zhi)間(jian)的電氣設備、以及連接(jie)這些設備的導線(xian)。由于差(cha)動(dong)(dong)保(bao)(bao)護(hu)(hu)(hu)(hu)對保(bao)(bao)護(hu)(hu)(hu)(hu)區(qu)外故(gu)障不會動(dong)(dong)作(zuo)(zuo)(zuo)，因此差(cha)動(dong)(dong)保(bao)(bao)護(hu)(hu)(hu)(hu)不需要與保(bao)(bao)護(hu)(hu)(hu)(hu)區(qu)外相鄰元件保(bao)(bao)護(hu)(hu)(hu)(hu)在動(dong)(dong)作(zuo)(zuo)(zuo)值和動(dong)(dong)作(zuo)(zuo)(zuo)時限上相互配合(he)，所以在區(qu)內故(gu)障時，可(ke)以瞬時動(dong)(dong)作(zuo)(zuo)(zuo)。

變壓(ya)器差動(dong)保(bao)護和零(ling)序電流(liu)保(bao)護區別用途不一(yi)樣。

差(cha)動保護(hu)的性(xing)能非常好，可以瞬(shun)時切除全線(xian)范圍的故障，一般只(zhi)用于元件保護(hu)，如變壓器和發電機(ji)等(deng)。其原理(li)是比(bi)較元件兩側的電流大(da)小和方向。

零(ling)序電(dian)(dian)流(liu)保(bao)護(hu)反(fan)應接地(di)短路故障(zhang)，只有接地(di)時才出現零(ling)序電(dian)(dian)流(liu)，引起該(gai)保(bao)護(hu)動作。當然要構成該(gai)保(bao)護(hu)，需要用零(ling)序電(dian)(dian)流(liu)濾過器（電(dian)(dian)纜的話要用零(ling)序電(dian)(dian)流(liu)互感器）來獲(huo)得(de)零(ling)序電(dian)(dian)流(liu)。

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