答:是由于主降低壓(ya)(ya)側分為(wei)(wei)△接(jie)線(xian),為(wei)(wei)不(bu)接(jie)地(di)系(xi)統。當發生單相(xiang)接(jie)地(di)時(shi),不(bu)會(hui)產生短路電(dian)流,三相(xiang)仍對(dui)稱運(yun)行(xing)(xing),僅使電(dian)壓(ya)(ya)互感器二次側中性(xing)點電(dian)壓(ya)(ya)偏(pian)移(yi),中性(xing)點電(dian)壓(ya)(ya)偏(pian)移(yi)保(bao)護的構成原理即基于此(ci)。但不(bu)接(jie)地(di)系(xi)統發生單相(xiang)接(jie)地(di)時(shi),健全相(xiang)相(xiang)對(dui)地(di)電(dian)壓(ya)(ya)升高,長期運(yun)行(xing)(xing)對(dui)絕緣不(bu)利等多種不(bu)利因(yin)素。基于此(ci),不(bu)接(jie)地(di)系(xi)統單相(xiang)接(jie)地(di)時(shi)不(bu)可長期運(yun)行(xing)(xing)。規程規定不(bu)能超過(guo)2小時(shi),必須盡快處理。中性(xing)點電(dian)壓(ya)(ya)偏(pian)移(yi)保(bao)護的作(zuo)用即在(zai)發生單相(xiang)接(jie)地(di)故障時(shi),動作(zuo)警(jing)告,通知運(yun)行(xing)(xing)人員給予(yu)處理。
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