Diagnosis of common defects inside cast Resin current transformers by digital partial discharges acquisition

This work presents the outcome of a systematic experimental campaign about the recognition and the morphological analysis of defects inside the insulation of medium voltage measurement current transformers. Different kinds of common defects (voids, delaminations, etc.) have been intentionally produced inside the mass insulation and on the surface of the test objects. External influence parameters, such as the surface pollution, have been taken into account in order to spot false alarms of PD threshold overcoming that are actually due to improper handling of the transformer. All the test pieces are subjected to several PD tests in order to determine the inception threshold and to collect the digital PD pattern. The same tests are carried out on a batch of normal production transformers.