High-voltage stator insulation assessment by measurement of loss tangent and capacitance

The measurement of the capacitance and associated loss tangent over a range of voltages is a common method of assessing insulation condition. The author considers the use of such techniques for establishing aging trends and detecting abnormalities in high-voltage rotating plant stator insulation. Whereas the inherent accuracy of the technique, as determined by laboratory measurements, is good, measurements on site are shown to be subject to large variations. It is concluded that the variations below discharge inception voltage are mainly due to environmental effects in the endwinding region, whereas discharge-related properties vary because of the time dependence of the discharge process. It is shown that, in some circumstances, environmental variability can be reduced by phase-to-phase comparisons