Maintenance testing of machine insulation using high-voltage dc is a widely accepted practice in the electric utility industry. Variations in measured current versus applied potential have proven useful in diagnosing certain insulation defects and modes of deterioration. A number of different methods are typically used for high-voltage dc testing. For example, the test voltage may be applied as a series of either uniform or graded steps. Alternatively, a linearly increasing ramped voltage may be used. This paper will describe the principles of de testing, with an emphasis on the measured stator insulation current response versus voltage. Advantages and disadvantages of the various voltage application methods will be outlined. Field measurements will be presented to demonstrate the superiority of the ramped dc test technique over the conventional stepped methods.
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