By using OWTS, the existence of partial discharge defects simultaneously among the three intermediate cabling connectors was discovered. Statistics to the relationship among discharge frequency characteristic and test voltage for the three positions, respectively, found that the discharge pulse number presented terraced rising trend with the increase of test voltage, which is exactly the characteristics of typical discharge. To reveal the cause of concentrated discharge, a variety of advanced state detection technology were applied on the second defective intermediate joint. Partial discharge type identification with pulse current method suggested that: when phase A was at U0, recognition showed two discharge types, surface discharge and internal discharge. However, when the voltage came to 1.73U0, surface discharge gradually disappeared, left surface discharge only. These two discharge types always existed in phase B. The signal in phase C was weak, with intermittent and 180° phase characteristics. The subsequent disintegration of the second defective joint revealed that: the existence of the defect inside the joint partial discharge risks, and this is due to the obvious result of making bad process. At the same time, this case also shows that the oscillation detection technology effectively, as well as field testers judged entirely correct.