Testing Electrical Insulation of Rotating Machinery with High-Voltage Direct Current [includes discussion]

Field test data are presented and a simplified high-voltage d-c method for determining the condition of the insulation in large electric apparatus is described. A graphical presentation is shown whereby the characteristic shape of the curve of the d-c microamperes versus d-c kilovolts is utilized as the indicator of the condition. This method has been under experimental observation for several years and the technique now has reached a stage where it has been adopted as the primary means for testing and determining the condition of the electrical insulation of large rotating machinery. All indications point out that predictions as to the life expectancy of electrical insulation, in large rotating machinery and other apparatus, may be possible after years of testing and recording the trend of the characteristic curves. This should aid very materially in setting up a program of planned maintenance and help to answer the question: when to rewind, repair, or replace old electric apparatus.