Insulation failure in power systems plant may be caused by manufacturing defects or by electrical, mechanical, thermal and chemical process which occur during operation. These processes create defects, including voids in solids, which locally reduces the dielectric insulation strength. Electrical discharge may occur in these regions of reduced dielectric strength without discharge occuring in the regions of non-degraded insulation. The resulting process is referred to a Partial Discharge (PD) since it is a localized electrical discharge that only partially bridges the insulation between conductors and which can or can not occur adjacent to a conductor [1]. PD is normally characterized by low-level current pulses with fast rise-time and short duration; typically no more than a few hundred nanoseconds. The character of PD may be useful in distinguishing different types (configurations), and degrees, of insulation defect. For many years PD recognition has been performed by observation, and (human) expert interpretation using, for example, patterns on the power frequency ellipse as displayed by an oscilloscope. The aim of this paper is present a methodology for the automatic assessment of insulation degradation and its characteristics based on the PD measurements. © 2012 IEEE.