The use of time domain spectroscopy as a diagnostic tool for rotating machine windings

In order to assess stator winding insulation condition, a field instrument measuring the charge and the discharge current flowing through rotating machine stator winding groundwall insulation was developed at Hydro-Quebec. This instrument applies a constant DC-voltage of 1 kV during a certain time, usually 2000 s. Immediately following the charging, the stator winding is short-circuited during another period of time. The current is continuously recorded as a function of time during both steps. It is suggested that the magnitude and the shape of both curves may be indicative of the degree of aging of the insulating material. A number of measurements obtained either from generator windings at Hydro-Quebec, or from laboratory aged specimens were examined; spare bars or coils were also measured and used as reference. Measurements were conducted essentially on asphalt and epoxy bonded insulating material which are the two major stator groundwall insulating material for the 350 or so generators at Hydro-Quebec. The method was found particularly useful to detect delamination or leakage current due to moisture absorption or end-winding contamination.