Voltage life characteristics of epoxy micaceous insulation systems under flexural stress

Voltage endurance tests under flexural stress were carried out on high-voltage rotating machine insulations with various mica tapes. The following interesting observations were obtained from the test data. The relationship between flexural stress and voltage life is comparatively differentiated by the mica tape construction. In a region exceeding a certain level of flexural stress, voltage lives are shortened abruptly for the insulation with mica papers only. On the other hand, for the insulation with mica splittings only, voltage lives are shortened more slowly with an increase of flexural stress, compared with the case for the insulation with mica papers only. Generally, breakdown occurs on the tensile side for the insulation with mica papers only, but on the compressive side for the insulation with mica splittings only. In the case of insulation using a combination of mica papers and mica splittings, breakdown occurs mostly on the compressive side, some-times on the tensile side when voltage is rather low. As the displacement becomes larger, the breakdown path becomes longer for the insulation with mica papers only, and shorter for the insulation with mica splittings only. It becomes longer with the increase of voltage, when the displacement is constant. From an examination of the relationships between nondestructive insulation characteristics and voltage life, almost no correlation was found between conventionally known characteristics (tan δ0, Δ tan δ and ΔC/C0) and voltage life. However, comparatively clear correlation was found between the void formation coefficient K and voltage life.