Partial discharge inception voltage in insulating liquids dependence from the definition used

The Partial Discharges Inception Voltage (PDIV) determination is believed to be an important test able to describe the behavior of insulating liquids when subjected to high electrical stress. In IEC standardization body, this test procedure is described by the IEC TR 61294 (Determination of the Partial Discharge Inception Voltage). In particular, according to this standard, the inception voltage is defined as the voltage at which at least one single partial discharge (PD) occurs of apparent charge equal to or greater than 100 pC. On the other hands, partial discharges in insulating liquids are random events which occurs within vapor phase regions located somewhere in the proximity of the cathode. These vapor phase regions may be different in number and in dimension in function of the local field so that partial discharge phenomena appear in terms of PD amplitude distribution in which a single pulse greater than 100 pC cannot be relevant to the overall phenomena. With the aim to establish when a significative PD activity is present in insulating liquids, in this paper different criteria for PDIV determination have been adopted and compared. The results of these tests may be of interest when revising the IEC TR 61294 or considering the actual preparation of an ASTM Standard (Project WK65707-New Test Method for Partial Discharge Inception Voltage Measurement of Insulating Liquids Using Plane and Needle Electrodes).