PDC Characteristics of Modern Stator Insulation Systems

Two of the mostly used electrical insulation systems for large turbo-generators are Resin-Rich and VPI systems. Insulation systems of electrical machines are the most critical parts of any rotating machines. There are some main characteristics which play determinative roles to judge about the quality of insulation system and to estimate the residual life time of the machine. One of the popular insulation characteristics of medium and large power range rotating machines in recent decade which has been considered in OEMs and research centers is Polarization and Depolarization Current Analyzing (PDC) method. In this paper, a base line of PDC characteristics for Resin-Rich (R.R.) and VPI insulation systems of large turbogenerators has been developed. All turbo-generators which have been tested by PDC test equipment are in the same range of power and the tests have been performed in OEM's workshop. Developing the base pattern for new machines and comparing and analyzing the patterns from old ones, made us able to recognize intensive degradation of insulation system in turbogenerators before any catastrophic failure occurrence. The effects of two R.R. and VPI insulation systems on PDC tests results (with representing the effect of stress grading system) are considered.