Analysis of Weibull Distribution for Electrical Breakdown Voltage o Stator Windings

Because plant systems are growing, motors need to be more reliable. Preventaive maintenance has therefore become more important. The authors reported nondestructive measurements for insulation diagnosis and recommended the optimum time of rewinding in earlier papers [1-2]. As part of this research series, this paper analyses how the Weibull distribution of the breakdown voltage changes in relation to operating years. This analysis was performed on 75 motors which had been used for 15 to 23 years in many thermal power plants. Results show that the deterioration of windings begins mainly wit thermal stress; then vibration, a form of mechanical stress, makes some delaminations and/or hair cracks in the insulation. Finally, moisture absorption, an environmental stress factor, affects the system and results in electrical breakdown paths. Through these steps, the Weibull shape parameter changes drastically from an initial high value to a final low value. The details are described below.