Detecting Thermomechanical Ageing of Rotating High-Voltage Machines: Investigating the Influence of the Time Between Mechanical Stress and Diagnostic Measurement

Thermal cycling tests of coils and bars according to standards [1], [2] suggest diagnostic partial discharge measurements to evaluate the condition of the insulation system. However, no recommendations are given concerning the time interval between the last cycling event and the diagnostic partial discharge measurements. During such cycling rotating high-voltage machines experience varying mechanical shear, tensile and compressive stress due to temperature changes. While epoxy resin-based insulation systems are thermosets and therefore mostly considered stiff, reality shows partially viscoelastic deformation in consequence of such stress. These viscoelastic deformations also affect voids in the insulation system. Accordingly, an influence on the results of diagnostic partial discharge measurements is expected. Since viscoelastic deformation recedes with time, the influence of the time interval between mechanical stress and diagnostic partial discharge measurements is investigated. The results show that viscoelastic deformation leads to varying partial discharge values. This time dependency should be considered when conducting partial discharge measurements to detect consequences of mechanical stress.