The main purpose of this paper is to evaluate electrical treeing degradation for cable insulation. To effectively deal with the currently facing issues, I endeavor to find the most optimal methods by means of applying signal process. First, we made three type models of electrical tree for PD generation to show the distribution characteristics and applied voltage to acquire data by using a PD detecting system. These acquired data presented distribution and four 2D distributions. Hn(q), Hn(), Hqn(), and Hqmax() were derived from the distribution of partial discharge. From the analysis of these distributions, each PD model is proved to hold its unique characteristics and the results were then applied as basic specific qualities for insulation conditions. In order to recognize the progresses of an electrical tree, we proposed methods using scale parameter by means of Weibull distribution. We measured the time of tree propagation for 16 specimens of each model from initiation stage, middle stage, and breakdown respectively, using these breakdown data, we estimated the shape parameter, scale parameter and MTTF(Mean Time To Failure). The results of this study recognize the sources of PD by applying acquired data from PD signals to pre-acquired data. If the cause of PD is degradation, in other words, electrical tree, we can determine the replacement time of devices at the initiation stage of tree growth progress or no later than the middle stage and use it as a basic methods analysis diagnosis system. That is, pattern recognition and Weibull distribution can be employed to get the reliability of diagnosis.

The number of XLPE cables being used near or beyond their designed lifespan is increasing. The importance of timely cable replacement is increasing. Therefore, research is actively being conducted using VLF Tan δ diagnostic technology to assess the insulation condition of cables. There is a lack of studies measuring and analyzing VLF Tan δ of service aged XLPE cables. Additionally, there is a lack of research considering the operating environment. Therefore, research is needed to diagnose and analyze the insulation condition of the same serviced aged cable when it is operated in a different environment. This paper evaluates and analyzes the insulation condition of cables installed in the BFP, cooling tower, and deaerator booster pump of a combined heat and power plant. Each cable was analyzed by measuring VLF Tan δ, dielectric breakdown test of the cable, and tensile strength, elongation at break, crystallinity, and dielectric strength of XLPE specimens. Additionally, the correlation between VLF Tan δ and other characteristics was also analyzed. It was found that degradation progressed in the order of BFP, cooling tower, and deaerator booster pump. Therefore, it was confirmed that even for the same cable, deterioration varies depending on the installation location.