Takeshi Hakamada

A Proposal for Insulation Diagnosis of 3 KV Motor Stator Windings

This paper presents a proposal for insulation diagnosis of 3 kV induction motor stator windings of mica splittings and solvent-type thermosetting resin insulation. Nondestructive and destructive tests were conducted on 29 windings it operation for 15 years or more. The average breakdown voltage (BDV), Vave of the coils was estimated using 10 to 12 coils from each winding. The relationship between Vave and Vmin, the BDV of the winding and the BDV of the worst coil in the winding, was determined. The criterion for insulation renewal, generally Vmin¿7 kV (the withstand test voltage), could also be expressed in terms of Vave, with Vave¿l5 kV. Several nondestructive measurements were also examined for their ability to forecast Vave or Vmin. Although the dissipation factor tip-up (¿tan¿) should be related to Vave, no useful correlation was found. Partial discharge starting voltage (Vi) and the first turning point voltage (Pil) on the I-V curve in an ac current test proved to have comparatively good correlation with Vmin. A criterion for insulation diagnosis of that insulation is proposed, i.e. when Vi or Pil has decreased to 3 kV, the rated voltage, the insulation of the winding should be replaced.

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.

Analysis and Experiments for Thermal Stress of Totally Impregnated Stator Windings

Recently, medium-capacity electrical rotating machines usually have been made by the total impregnation process, through which the stator windings and core are rigidly bonded together. When such machines start or are in the heat cycle, thermal stress takes place among conductor, insulation wall, and core. This paper theoretically analyzes the phenomena of thermal stress and confirms the results experimentally. A procedure to reduce stress is also presented.