d.自由導電微粒和固體導體上金屬突起放電的相位分布有著明顯不同。這個特征通常可以用來區分缺陷的類型。GIS設備中自由導電微粒有積累電荷能力,在交流電壓作用下,靜電力可使導電微粒在GIS筒內跳動,如直立旋轉、舞動運動等。這種運動與放電的出現在很大程度上是隨機的,這一過程與所加電壓大小以及微粒的特性有關。如果一個跳動的微粒接近或運動至GIS設備中的高場強區時,伴隨產生的局部放電有可能形成導電通道,造成絕緣擊穿。相對而言,GIS設備內殘留的金屬碎屑或金屬顆粒產生的各種效應是最為嚴重的,因此,金屬顆粒的放電對GIS設備的危害相對較大。
2.3 檢測方法
GIS設備中的局部放電會在GIS設備外殼上產生流動的電磁波,使接地線上有高頻放電脈沖電流流過,從而導致外殼對地呈現高頻電壓并向周圍空間傳播。局部放電還會使通道氣體壓力驟增,在GIS設備氣體中產生縱波或超聲波,并在金屬外殼上出現各種聲波,如縱波、橫波和表面波等。GIS設備中的局部放電也可導致SF6氣體分解或發光。這些物理和化學變化的特征,有助于GIS設備局部放電的檢測。目前,國內外普遍采用如下幾種方法。
a.外殼電極法 在GIS設備外殼上敷設絕緣薄膜和金屬電極,使外殼與金屬電極間構成一個小電容。通過電容將放電信號耦合至檢測及處理單元,最終顯示GIS設備局部放電水平。這種方法的原理與傳統的局部放電檢測方法類似,優點是檢測靈敏度高,但其致命缺點是易受外界干擾,特別是電網中自然存在的各種脈沖型信號(如電網中的電暈干擾等)的干擾。
b.內部電極法 該方法是將GIS設備法蘭稍加改造,在法蘭內部加裝金屬電極,該電極與外殼構成耦合電容,以此電容傳感器提取局部放電的脈沖信號。當采用2個電容傳感器時即可進行GIS設備局部放電定位,由GIS設備局部放電信號到達2個傳感器的時間差確定放電點。另一種內電極法是在盆式絕緣子內靠近接地端預先埋設一個電極,由此與外殼構成耦合電容。內部電極法受客觀條件限制,內電極只能在廠家生產過程中預埋設,現場安裝往往不易實現。另外,內部電極法與外殼電極法都存在現場抗干擾問題。
c.外接電流傳感器 上述方法的改進形式是在GIS設備的接地線上加裝脈沖電流傳感器。由于現場GIS設備通常是多處接地,不易從外殼接地線上檢測脈沖電流,因此該方法的應用具有很大的局限性。
d.振動法 G
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