在實際運行(xing)中(zhong)(zhong)，部分(fen)變(bian)(bian)壓(ya)(ya)(ya)器的(de)中(zhong)(zhong)性(xing)(xing)點(dian)是直接(jie)(jie)(jie)接(jie)(jie)(jie)地(di)(di)的(de)。但(dan)還有(you)部分(fen)變(bian)(bian)壓(ya)(ya)(ya)器的(de)中(zhong)(zhong)性(xing)(xing)點(dian)不(bu)接(jie)(jie)(jie)地(di)(di)運行(xing)。當系統發生接(jie)(jie)(jie)地(di)(di)故障(zhang)(zhang)，中(zhong)(zhong)性(xing)(xing)點(dian)接(jie)(jie)(jie)地(di)(di)的(de)變(bian)(bian)壓(ya)(ya)(ya)器裝設(she)的(de)零序(xu)電(dian)流保(bao)護(hu)，短時限(xian)動作(zuo)(zuo)于斷開(kai)母聯或(huo)分(fen)段斷路器，縮小故障(zhang)(zhang)影(ying)響范圍，長時限(xian)動作(zuo)(zuo)于斷開(kai)變(bian)(bian)壓(ya)(ya)(ya)器各側斷路器。中(zhong)(zhong)性(xing)(xing)點(dian)接(jie)(jie)(jie)地(di)(di)的(de)變(bian)(bian)壓(ya)(ya)(ya)器跳開(kai)后(hou)，電(dian)網零序(xu)電(dian)壓(ya)(ya)(ya)升高會(hui)危(wei)及(ji)中(zhong)(zhong)性(xing)(xing)點(dian)不(bu)接(jie)(jie)(jie)地(di)(di)的(de)變(bian)(bian)壓(ya)(ya)(ya)器中(zhong)(zhong)性(xing)(xing)點(dian)絕緣。因此，中(zhong)(zhong)性(xing)(xing)點(dian)不(bu)接(jie)(jie)(jie)地(di)(di)的(de)變(bian)(bian)壓(ya)(ya)(ya)器應裝設(she)零序(xu)電(dian)壓(ya)(ya)(ya)保(bao)護(hu)。

中性點放(fang)電(dian)(dian)(dian)間(jian)隙的擊穿電(dian)(dian)(dian)壓難以做到完全(quan)準確，因(yin)其(qi)放(fang)電(dian)(dian)(dian)電(dian)(dian)(dian)壓受大氣條(tiao)件(jian)、調整精(jing)度以及(ji)連(lian)續放(fang)電(dian)(dian)(dian)次數的影響可(ke)能出現間(jian)隙過(guo)流保(bao)護(hu)該動作(zuo)而不(bu)能動作(zuo)的情況。為此，還應裝(zhuang)設(she)間(jian)隙過(guo)壓保(bao)護(hu)，作(zuo)為放(fang)電(dian)(dian)(dian)間(jian)隙拒(ju)動時的后(hou)備保(bao)護(hu)。

一(yi)般而言(yan)，間隙過壓(ya)保護采用母(mu)線電(dian)(dian)壓(ya)互感器開(kai)口三(san)角繞(rao)組的電(dian)(dian)壓(ya)。大電(dian)(dian)流接地系(xi)統開(kai)口三(san)角繞(rao)組額定相電(dian)(dian)壓(ya)為100V。

為確保(bao)(bao)系統接地(di)中性(xing)點(dian)(dian)存在時，間隙保(bao)(bao)護不誤動，其(qi)整定值應(ying)躲過(guo)系統單(dan)相(xiang)接地(di)時不平衡電(dian)(dian)壓U0。由單(dan)相(xiang)接地(di)故障序網圖可(ke)知當零序阻(zu)(zu)抗與正序阻(zu)(zu)抗比值最(zui)大時，故障點(dian)(dian)的(de)零序電(dian)(dian)壓最(zui)大。

一(yi)般認(ren)為由(you)于(yu)(yu)(yu)輸電線路零(ling)序(xu)阻(zu)抗約等(deng)于(yu)(yu)(yu)正(zheng)序(xu)阻(zu)抗的(de)3倍，中(zhong)性點接(jie)(jie)地主變壓(ya)器的(de)零(ling)序(xu)阻(zu)抗小(xiao)于(yu)(yu)(yu)等(deng)于(yu)(yu)(yu)正(zheng)序(xu)阻(zu)抗，因此中(zhong)性點直接(jie)(jie)接(jie)(jie)地系統綜合零(ling)序(xu)阻(zu)抗小(xiao)于(yu)(yu)(yu)等(deng)于(yu)(yu)(yu)3倍正(zheng)序(xu)阻(zu)抗，故取(qu)X0∑=3X1∑分析單(dan)相接(jie)(jie)地故障(zhang)時可(ke)能出(chu)現的(de)最大零(ling)序(xu)電壓(ya)。

由(you)于(yu)開口(kou)三角繞組(zu)位于(yu)母線電壓互(hu)感(gan)器，因此在線路發生單相接地故障時，其感(gan)應(ying)電壓中的零序分量必然要小于(yu)180V，可保(bao)證間隙保(bao)護不誤動。

當接地(di)(di)變壓(ya)器跳開后，系(xi)統(tong)接地(di)(di)中性點失去，此時系(xi)統(tong)變為(wei)不接地(di)(di)系(xi)統(tong)。故障相(xiang)(xiang)(xiang)電壓(ya)降為(wei)零(ling)，非故障相(xiang)(xiang)(xiang)電壓(ya)升(sheng)高為(wei)原來(lai)的倍，兩(liang)者夾角(jiao)由(you)120°縮(suo)小為(wei)60°。此時感應至母線壓(ya)變開口(kou)三角(jiao)繞組的相(xiang)(xiang)(xiang)電壓(ya)數值(zhi)理(li)論上應為(wei)173.2V。

但開(kai)(kai)口(kou)三角繞(rao)(rao)組(zu)(zu)額定相(xiang)電壓為100V，考(kao)慮開(kai)(kai)口(kou)三角繞(rao)(rao)組(zu)(zu)飽和的情(qing)況下，實際上僅能輸出130V左右的相(xiang)電壓。此時，開(kai)(kai)口(kou)三角繞(rao)(rao)組(zu)(zu)感應電壓U為U=1.732*130=225V

由于在系(xi)統接(jie)地(di)中性點失去時，開口三角繞組感(gan)應(ying)電壓僅(jin)存在零序(xu)分量(liang)3U0=225V＞180V

由(you)此可見，間(jian)隙(xi)過壓保護整(zheng)定180V能(neng)在系(xi)統接(jie)地(di)中(zhong)性(xing)點(dian)失去的(de)情況下，發生(sheng)單相接(jie)地(di)故障時，能(neng)夠可靠動作。