The correlation between Dissipation Factor (DF) and Partial Discharge (PD) test results in the evaluation of the stator insulation of ac rotating machines, is discussed in this paper. It is well known that DF provides information on the global losses occurring in insulation systems but it does not provide any indication of the distribution of these losses. Above PD Inception Voltage (PDIV), increments of DF values are mainly due to PD in localized defects. Thanks to PD-signal separation and PD sub-pattern analysis, the causes of DF increments can be explained with more accuracy. Moreover, by comparing DF values recorded below the PDIV and PD patterns due to distributed micro-voids, the quality of the impregnation process in terms of void content, can be better evaluated. These results clearly show that the joint use of these two test methods can significantly improve the information on the conditions and the quality of the insulation system.
