由於近年來備轉容量率偏低，加上配電系統故障事件頻傳，除了造成經濟損失也衍生民眾恐慌，因此高壓電力設備的維護及監測將愈來愈顯得重要。本文針對地下電纜系統中較常發生事故的直接接頭，進行局部放電的試驗和資料分析，並嘗試建立一個可適用在即時監測系統之絕緣診斷規則，作為判斷絕緣劣化程度和預先維護的參考。
首先針對9個地下電纜接頭絕緣劣化的完整數據資料進行分析，從觀察發現由放電次數累加而成的序列，其斜率的變化可反映出狀態變化程度，而且變化次數愈多代表需要汰換設備的急迫性越高。同時，若藉由放電區域之起始相位及相位重心將可判斷是否進入危險期，且此兩個門檻值會隨著的狀態變化程度增加而調整。例如起始相位及相位重心的初始門檻值分別為8度及25度，而各電纜經由不同劣化歷程最終門檻值也會有不同的調升。
經測試得知，本文提出的狀態變化診斷規則可適用在電纜6條瑕疵A(絕緣層有空隙)及3條瑕疵B(絕緣層空洞)兩種瑕疵上，在其絕緣破壞前判斷是否進入危險期，平均使用率也能達到80%。另外，本診斷規則相較簡單且具可理解性，可適用在局部放電分散式的監測設備上運行。

Due to the low reserve capacity ratio in recent years and the frequent transmission failures in the power distribution system, they cause economic losses and public panic. Therefore, the maintenance and supervising of high-voltage power equipment will become more and more crucial. The thesis will focus on the partial discharge test and information analysis which are performed for the cable joint in the underground cable system where accidents occur more frequently. An insulation diagnosis rule applicable to the real-time monitoring system is attempted as a reference for judging the level of insulation degradation and for pre-maintenance.
　　First, the complete data of the insulation degradation of 9 underground cable joints was analyzed. From the observation, it was found that the sequence of the cumulative number of discharges. The change of the slope can reflect the degree of state change. When the slopes changes more frequent, it means that it is more urgent to replace equipment. Meanwhile, the initial phase and the phase center of the discharge region will be used to determine whether to enter the dangerous period. The two threshold values will be adjusted as the degree of state change increases. For example, the initial threshold values of the initial phase and the phase center are 8 degrees and 25 degrees, respectively. The final threshold value of each cable through different degradation will have a different increase.
According to the tests, the state change diagnosis rule can be applied to the cable 6 defects A (Insulation Layer Gaps) and 3 defects B (Insulation Layer Holes) to determine whether to enter the dangerous period before its destruction. The average utilization rate can also reach more than 80%. In addition, this diagnostic rule is relatively simple and understandable and can be applied to operate on a partial discharge distributed monitoring device.

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