Gaseous Ionization in Built-up Insulation---II

It has been suggested frequently that the failure of high-voltage armature bars might be due to deterioration caused by gaseous ionization in entrapped air spaces. In an earlier paper a series of tests on a number of 6600-voltage mica folium armature bars made up with different degrees of mica content was described. The variations of the dielectric losses with voltage and with temperature were studied and by means of the application of pressure it was shown to what extent losses due to internal ionization were present. The influence of these ionization losses on the life of the bars was also studied. The tests of the foregoing paper are continued in the present paper, extending to a wider range of type of armature insulation. The general results are as follows: 1. The absolute values of loss due to internal ionization in well constructed armature bars are small compared with dielectric losses of other types. 2. The losses due to internal ionization do, however, cause a progressive deterioration of the insulation. This is shown by a gradual increase in the loss and power factor of the insulation. S. It is indicated that the principal function of mica in this type of insulation is in the reduction of the conductivity of the insulation and the withstanding of the action of internal ionization. The indications are that full mica folium content can be safely reduced only by use of the best grade of mica folium and the best conditions of application without variation.