New insights and complications in determining generator stator insulation absorption current exponents and constants from Polarization Index test data

Significant errors may occur in calculating the generator insulation Polarization Index (PI), absorption current exponent (n), and the insulation constant (K), from standard PI test data. The paper addresses a previously undocumented PI response that occurs which is the cause of the subject errors. A simple approach is described to eliminate these errors especially when using the IEEE Standard 95-2002^TM to correct for leakage current. IEEE Standard 43-2000^TM is also useful regarding associated PI and insulation resistance calculations. In addition laboratory test data will be presented showing the relation between stator bar insulation temperature and the insulation constants K and n.