Contents I. Introduction
The reliability of rotating machine in service depend largely on the stator winding insulation system which experiences continuous stresses like thermal, electrical and thermomechanical stresses and undergoes ageing. Ageing process is complicated, and takes place under stresses simultaneously or sequentially. Nondestructive techniques like measurement of Insulation Resistance (IR), Polarization Index (PI), Dielectric loss angle and Capacitance (C), Partial Discharge (PD) measurements are used for condition assessment studies. However, determination of remaining life is the most difficult part of analysis because of lack of well defined deterioration models, multiplicity of failure mechanisms which are not well understood in quantitative form and are highly dependent on environmental and operating conditions. Electrical endurance studies at higher frequencies and thermal ageing studies of insulation system are well accepted accelerated testing techniques. Higher stresses are necessary for any study of long term ageing and life time of practical insulation systems whose working life at normal stresses is a matter of years. Studies carried out by Izeki, Herstad et al [1 , 2] at frequencies 50 Hz, 500 Hz, 1000 Hz, 1500 Hz and 3000 Hz have revealed that linearity of the frequency acceleration is almost valid till a frequency level of 3000 Hz. However Arnold Wichmann [3] has experimentally concluded that with insulation thickness of 2 to 3 mm, the test frequency should not exceed 1000 Hz to avoid the undesirable side effect like local overheating. Laboratory studies were undertaken to identify ageing indicators of stator winding insulation during ageing under elevated frequency, voltage and thermal stresses. Studies were made on epoxy-mica resin poor stator bars by measuring the properties using electrical, thermo-analytical and spectroscopic techniques. The results are presented and discussed.
