Mechanical Degradation of High Voltage Rotating Machine Insulation

Several kinds of insulation systems, which were developed for high voltage coil insulation for motors and generators, are examined for insulation damage caused by mechanical stresses. Tests have been carried out on electrical properties under bending stress, i. e. the critical deformation characteristics which are defined as the maximum deformations able to withstand a fixed proof voltage, the fatigue characteristics under repeated bending stresses, and the voltage life characteristics under bending stress. Breakdown voltage, BDV, decreases with an increase of bending stress. Moreover, a remarkable increase of the base tan6 value, tan6o, and tan6 tip-up can be seen due to the delamination of mica-splittings, especially in the case of the insulation using mica splitting. An insulation system in which a flexible binder is used has a far greater critical deformation. The decrease of residual breakdown voltage RBDV, and the increase of tan6o and tan6 tip-up are seen at less repeated bending stress in the insulation system with mica-splitting tapes than in the insulation system with only mica-paper, because of the weakness of mica-splittings under repeated bending stress. Effects of the temperature on the repeated bending fatigue characteristics from -55 to 1550C are greatly influenced by the binder of the mica tape and/or the impregnating resin. Moreover, voltage life under a static bending stress becomes extremely short beyond a certain stress. Lastly, experimental equations are proposed for estimating the relationship between RBDV and the numbers of repeated bending cycles, and the voltage life under a static bending stress.