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This research investigates partial discharge phenomena of on-site power transmission cables of 69 kV and 161 kV by inspecting the data measured under various environments. Effective procedures and methods are suggested for estimating insulation condition of power transmission cables. To tackle the problem of interference of the environmental noises to the on-site power transmission cables, a detection method is presented based on the relationship among the amplitude, phase, and focused area of electrical signals. The difference between the outer and inner electrical signals in power transmission cable is analyzed to improve the accuracy of pattern recognition. In addition, the insulation conditions of the cable joint and the cable terminator are evaluated by comparing results of electrical and non-electrical detections. Finally, a standard procedure with some key points of inspection processes under various environments is suggested to evaluate the cable insulation degradation based on the inspection data obtained from various environments.