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CIGRÉ 1996 : 15/21/33-02



1.0 SUMMARY Accelerated aging tests or type tests are now routinely used to ensure reliable life time performance of ground-wall insulation in stator coils andbars. However, many groundwall failures in multiturn coils are initiated by a breakdown of the turn insulation. At present no accelerated aging test is available for checking the long term performance of the turn insulation. The need for such a test is becoming even more important with the increasing trend to apply variable speed drives to many motors. Results from extensive work on developing atype test for turn insulation in motor coils are presented here. In the accelerated aging tests, new motor coils were subjected to enhanced stresses and then tested to breakdown under fast fronted (0.1/50 µs) surges after aging. The aging or degradation of turn insulation was measured as the decrease of breakdown voltage compared to unaged coils. Coils aged up to 1000 hours under enhanced voltagestress at 3 kHz and moderate thermal stress showed no degradation of the turn insulation. However, thermal cycling of coils, i.e., repetitive fast alternate raising and lowering of the temperature, which subjects the insulation system of coils to thermo-mechanical stress, degraded the turn insulation significantly. Class F coils with polyestermica groundwall insulation and three different types ofturn insulation (enamel + daglass, double daglass, and micatape) were subjected to 400 thermal cycles, each cycle having 15 minutes for heating and 15 minutes for cooling. Two peak temperatures of 155°C and 185°C were tried. The decreases in impulse breakdown voltages (compared with unaged coils) were different for the two peak temperatures and for the three types of turn insulations. Aging coils inan oven at the same constant temperatures produced no or little decrease in impulse breakdown voltages. The results demonstrated clearly that the mechanical stress present in thermal

cycling is quite important for degrading the turn insulation. Further work was carried out to study the effect of various parameters, eg, peak temperature, aging duration, and cycling period. The results are usedto identify optimum parameters for an accelerated aging test to ensure life time reliable performance of the turn insulation in motor coils. Keywords: Machine - Motor - Coil - Turn Insulation - Accelerated Aging - Type Test - Life

2.0 INTRODUCTION A significant proportion of costly forced outages of motors in generating stations and industrial plants are caused by premature failures of theinsulation system in the stator winding [1,2]. The voltage across the ground-wall insulation can be as high as the full line-to-ground rated voltage of machine. Although the voltage across the relatively thin interturn insulation is small (typically 10 to 100 V), steep fronted voltage transients can puncture the turn insulation, leading eventually to a failure of the groundwall insulation [3-5]. Thepresence of partial discharges near the turn insulation and any mechanical distortion or damage during the coil manufacturing or installation process may further reduce the withstand of the thin turn insulation. Many of the reported groundwall failures may have been initiated by the breakdown of the turn insulation. Accelerated aging or type tests are useful for ensuring the long term reliabilityof insulation systems. At present type tests, such as the one described in IEEE Standard 1043 [6], are used widely for the groundwall insulation. However, no such accelerated aging test exists for the turn insulation. The need for a type test for the turn insulation has increased further because of the current trend towards applications of adjustable speed drives which subject

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