Partial Discharges Acceptance Criteria for AC Rotating Machines Insulation Systems

A new pass/fail criterion based on the shape analysis of phase-resolved Partial Discharges (PRPD) patterns is proposed in this study to overcome the problems generated by the use of partial discharge (PD) amplitude threshold levels. Insulation systems based on mica tapes are considered to be in good condition when they are affected only by distributed micro-void PD and no other type of defect incept discharges. This definition implies the possibility of discriminating between PRPD patterns owing to different defect typologies. With this approach, the diagnostic information includes the defect typology that affects a specific insulation component, not the PD amplitude. Consequently, the PRPD pattern must address the same defect typology because the defect remains the same even if different PD instruments and couplers are used while testing different insulation systems and under different test conditions. A wide number of PRPD patterns associated with well-known defect typologies are presented and discussed in this paper to empirically demonstrate that the PRPD patterns associated with distributed micro voids are clearly distinguishable from the other PRPD patterns. Significant applications of the method in the condition assessment of the insulation system of coils, bars, and complete machines are discussed, highlighting the important role played by the separation phase prior to the identification of defects.