在(zai)高(gao)壓輸(shu)(shu)電(dian)線路(lu)中只有相線，具體原因需要(yao)從零線設置目(mu)的(de)、輸(shu)(shu)電(dian)系統結(jie)構(gou)及(ji)輸(shu)(shu)電(dian)的(de)目(mu)的(de)三個方(fang)面(mian)有關。

       首(shou)先，零(ling)線(xian)設(she)(she)(she)置目的(de)(de)(de)(de)有二(er)，其一為了給單相(xiang)(xiang)用(yong)(yong)電(dian)(dian)(dian)設(she)(she)(she)備通(tong)過(guo)電(dian)(dian)(dian)流用(yong)(yong)的(de)(de)(de)(de)，比如我們家里的(de)(de)(de)(de)各種用(yong)(yong)電(dian)(dian)(dian)器(qi)具(ju)都(dou)要有零(ling)線(xian)參與(yu)才能(neng)工(gong)作。而(er)在高壓輸電(dian)(dian)(dian)中(zhong)不需要接用(yong)(yong)電(dian)(dian)(dian)設(she)(she)(she)備，主要是(shi)提(ti)高輸電(dian)(dian)(dian)的(de)(de)(de)(de)效率；其二(er)零(ling)線(xian)在三(san)相(xiang)(xiang)電(dian)(dian)(dian)路(lu)中(zhong)主要用(yong)(yong)來通(tong)過(guo)不平衡的(de)(de)(de)(de)電(dian)(dian)(dian)流或(huo)者(zhe)通(tong)過(guo)三(san)次諧波(bo)電(dian)(dian)(dian)流的(de)(de)(de)(de)。而(er)不平衡電(dian)(dian)(dian)流或(huo)者(zhe)三(san)次諧波(bo)電(dian)(dian)(dian)流都(dou)是(shi)我們用(yong)(yong)電(dian)(dian)(dian)終端的(de)(de)(de)(de)負載引起的(de)(de)(de)(de)，在發電(dian)(dian)(dian)和高壓輸電(dian)(dian)(dian)端電(dian)(dian)(dian)壓和電(dian)(dian)(dian)流是(shi)非常對稱的(de)(de)(de)(de)；其三(san)作用(yong)(yong)是(shi)為了使(shi)不平衡的(de)(de)(de)(de)三(san)相(xiang)(xiang)設(she)(she)(she)備上的(de)(de)(de)(de)電(dian)(dian)(dian)壓相(xiang)(xiang)等，而(er)我們高壓輸電(dian)(dian)(dian)端電(dian)(dian)(dian)壓對稱且相(xiang)(xiang)等。

       其次，輸(shu)電(dian)(dian)(dian)(dian)的(de)(de)目的(de)(de)是盡可(ke)能(neng)高效地傳(chuan)輸(shu)電(dian)(dian)(dian)(dian)能(neng)，電(dian)(dian)(dian)(dian)能(neng)的(de)(de)傳(chuan)輸(shu)過程中(zhong)由于電(dian)(dian)(dian)(dian)壓比較(jiao)高，加(jia)了零線不(bu)但起不(bu)到傳(chuan)輸(shu)電(dian)(dian)(dian)(dian)能(neng)的(de)(de)作(zuo)用反而又(you)增加(jia)了耗材，增加(jia)了輸(shu)電(dian)(dian)(dian)(dian)的(de)(de)成本。

       再者，從輸電(dian)(dian)(dian)系統結構(gou)來說，電(dian)(dian)(dian)是由發(fa)(fa)電(dian)(dian)(dian)站(zhan)通過(guo)發(fa)(fa)電(dian)(dian)(dian)機生產出(chu)來的(de)(de)(de)，發(fa)(fa)電(dian)(dian)(dian)機發(fa)(fa)出(chu)的(de)(de)(de)電(dian)(dian)(dian)是對(dui)稱的(de)(de)(de)三相交流(liu)電(dian)(dian)(dian)，這個電(dian)(dian)(dian)壓(ya)(ya)(ya)相對(dui)比較低，一(yi)(yi)般(ban)只有6.3KV或者10.5KV的(de)(de)(de)電(dian)(dian)(dian)壓(ya)(ya)(ya)，為了實現遠距離傳送電(dian)(dian)(dian)能，一(yi)(yi)般(ban)要(yao)(yao)把(ba)這個對(dui)稱的(de)(de)(de)三相電(dian)(dian)(dian)通過(guo)升壓(ya)(ya)(ya)變(bian)壓(ya)(ya)(ya)器升高到110千(qian)伏的(de)(de)(de)高壓(ya)(ya)(ya)，甚至更高的(de)(de)(de)220KV的(de)(de)(de)電(dian)(dian)(dian)壓(ya)(ya)(ya)。在升壓(ya)(ya)(ya)變(bian)壓(ya)(ya)(ya)器中所輸送的(de)(de)(de)三相高壓(ya)(ya)(ya)電(dian)(dian)(dian)是嚴格對(dui)稱的(de)(de)(de)，況且當把(ba)電(dian)(dian)(dian)輸送到目的(de)(de)(de)地時(shi)，當需(xu)要(yao)(yao)降壓(ya)(ya)(ya)時(shi)，在降壓(ya)(ya)(ya)變(bian)壓(ya)(ya)(ya)器的(de)(de)(de)初級繞組都是用三角形的(de)(de)(de)連接方式，因此就不(bu)需(xu)要(yao)(yao)用零線了