6kV及以上(shang)的電纜一(yi)般(ban)都(dou)有導體屏(ping)蔽(bi)層和(he)絕緣屏(ping)蔽(bi)層，也稱為內(nei)屏(ping)蔽(bi)層和(he)外屏(ping)蔽(bi)層。

屏(ping)蔽，是能夠將電(dian)(dian)場控(kong)制在絕(jue)(jue)(jue)緣(yuan)(yuan)內部，同時能夠使(shi)得(de)絕(jue)(jue)(jue)緣(yuan)(yuan)界(jie)面處表面光滑，并(bing)借此消除界(jie)面空隙(xi)的導(dao)(dao)電(dian)(dian)層(ceng)(ceng)。電(dian)(dian)纜導(dao)(dao)體(ti)由多根導(dao)(dao)線絞合而成(cheng)，它(ta)與(yu)絕(jue)(jue)(jue)緣(yuan)(yuan)層(ceng)(ceng)之間(jian)易形成(cheng)氣(qi)隙(xi)，導(dao)(dao)體(ti)表面不(bu)光滑，會造成(cheng)電(dian)(dian)場集中。在導(dao)(dao)體(ti)表面加一層(ceng)(ceng)半導(dao)(dao)電(dian)(dian)材料的屏(ping)蔽層(ceng)(ceng)，它(ta)與(yu)被屏(ping)蔽的導(dao)(dao)體(ti)等電(dian)(dian)位，并(bing)與(yu)絕(jue)(jue)(jue)緣(yuan)(yuan)層(ceng)(ceng)良好(hao)接觸，從而避免在導(dao)(dao)體(ti)與(yu)絕(jue)(jue)(jue)緣(yuan)(yuan)層(ceng)(ceng)之間(jian)發生局(ju)部放電(dian)(dian)。這一層(ceng)(ceng)屏(ping)蔽，又稱為內屏(ping)蔽層(ceng)(ceng)。

在(zai)(zai)(zai)絕(jue)(jue)(jue)緣(yuan)表(biao)面(mian)(mian)和護套(tao)接觸處，也可能存(cun)在(zai)(zai)(zai)間隙，電(dian)纜(lan)(lan)彎曲時(shi)，油紙電(dian)纜(lan)(lan)絕(jue)(jue)(jue)緣(yuan)表(biao)面(mian)(mian)易造(zao)成裂(lie)紋(wen)或皺折(zhe)，這些(xie)都(dou)是引起局(ju)部放(fang)電(dian)的(de)因素。在(zai)(zai)(zai)絕(jue)(jue)(jue)緣(yuan)層(ceng)(ceng)表(biao)面(mian)(mian)加一.層(ceng)(ceng)半導電(dian)材(cai)料的(de)屏(ping)蔽層(ceng)(ceng)，它(ta)與被(bei)屏(ping)蔽的(de)絕(jue)(jue)(jue)緣(yuan)層(ceng)(ceng)有良好接觸，與金屬護套(tao)等電(dian)位，從而避免在(zai)(zai)(zai)絕(jue)(jue)(jue)緣(yuan)層(ceng)(ceng)與護套(tao)之間發(fa)生局(ju)部放(fang)電(dian)，又稱為(wei)外屏(ping)蔽層(ceng)(ceng)。

銅芯電力電纜(lan)對于XLPE絕(jue)緣(yuan)電纜(lan)來說，半(ban)導電屏蔽層具有抑制樹枝(zhi)生長和(he)熱(re)屏障(zhang)的作用。

1)當(dang)導(dao)(dao)體表面(mian)金(jin)(jin)屬(shu)毛(mao)刺(ci)直接刺(ci)人絕緣(yuan)層時，或者在(zai)絕緣(yuan)層內部存在(zai)雜質顆粒、水(shui)氣(qi)、氣(qi)隙時，這些(xie)將引起(qi)尖端產生高電(dian)場(chang)、導(dao)(dao)致發(fa)(fa)射(she)而(er)引發(fa)(fa)樹(shu)枝。對(dui)于金(jin)(jin)屬(shu)表面(mian)毛(mao)刺(ci)，半導(dao)(dao)電(dian)屏(ping)蔽(bi)將有(you)(you)效地(di)減(jian)弱(ruo)毛(mao)刺(ci)附近的(de)場(chang)強，減(jian)少(shao)場(chang)致發(fa)(fa)射(she)，從(cong)而(er)提高耐電(dian)樹(shu)枝放電(dian)特性(xing)。若在(zai)半導(dao)(dao)電(dian)屏(ping)蔽(bi)料中加入(ru)能捕捉水(shui)分的(de)物質，就能有(you)(you)效地(di)阻擋由(you)線芯引人的(de)水(shui)分進入(ru)絕緣(yuan)層，從(cong)而(er)防止絕緣(yuan)中產生水(shui)樹(shu)枝。

2)半導電(dian)(dian)屏蔽(bi)層(ceng)有(you)(you)一定(ding)熱(re)阻，當(dang)線芯溫(wen)(wen)度瞬時升高時，電(dian)(dian)纜有(you)(you)半導屏蔽(bi)層(ceng)的(de)熱(re)阻，高溫(wen)(wen)不會立即沖擊到絕緣層(ceng)，通過(guo)熱(re)阻的(de)分溫(wen)(wen)作用，使絕緣層(ceng)上的(de)溫(wen)(wen)升下降。

(2)材(cai)料(liao)要求。屏(ping)(ping)蔽(bi)層(ceng)的(de)材(cai)料(liao)是(shi)半導電(dian)(dian)(dian)材(cai)料(liao)，其體(ti)積電(dian)(dian)(dian)阻率為10~10°2●m。油紙電(dian)(dian)(dian)纜(lan)的(de)屏(ping)(ping)蔽(bi)層(ceng)為半導電(dian)(dian)(dian)紙。半導電(dian)(dian)(dian)紙還(huan)有吸附離(li)子(zi)的(de)作用(yong)，有利于改善絕緣電(dian)(dian)(dian)氣性能。擠(ji)包(bao)絕緣電(dian)(dian)(dian)纜(lan)的(de)屏(ping)(ping)蔽(bi)層(ceng)材(cai)料(liao)是(shi)加人碳黑粒子(zi)的(de)聚合物。

(3)結構、種(zhong)類。塑(su)料、橡(xiang)皮(pi)絕緣電(dian)(dian)(dian)纜的(de)屏蔽(bi)層(ceng)(ceng)材料分別為半(ban)導電(dian)(dian)(dian)塑(su)料和半(ban)導電(dian)(dian)(dian)橡(xiang)皮(pi)。對(dui)于無金屬護套的(de)擠(ji)包(bao)絕緣電(dian)(dian)(dian)纜，除半(ban)導電(dian)(dian)(dian)屏蔽(bi)層(ceng)(ceng)外(wai)，還要(yao)增(zeng)加用(yong)(yong)銅(tong)帶或(huo)銅(tong)絲繞包(bao)的(de)金屬屏蔽(bi)層(ceng)(ceng)。其作用(yong)(yong)為，在正常運(yun)行(xing)時(shi)(shi)通過電(dian)(dian)(dian)容電(dian)(dian)(dian)流;當系統發生短路時(shi)(shi)，作為短路電(dian)(dian)(dian)流的(de)通道，同時(shi)(shi)也起(qi)到屏蔽(bi)電(dian)(dian)(dian)場的(de)作用(yong)(yong)。在電(dian)(dian)(dian)纜結構設計中，要(yao)根據系統短路電(dian)(dian)(dian)流的(de)大小(xiao)，采用(yong)(yong)相應截(jie)面的(de)金屬屏蔽(bi)層(ceng)(ceng)。