A new method of lifetime estimation for high-voltage insulating systems in rotating machines

The stator wedge supporting material is used for diagnosing the state of the main insulation in rotating machines. The stator wedge is placed in a stator slot, and the material is aged using the same degradation conditions experienced by the bar winding insulating system itself. The possibility of easy replacement of the supporting material without causing any damage to the whole insulating system during device shutoff is a major advantage of this approach. The samples were tested with differential scanning calorimetry (DSC) and were prepared from isolated bars of a real hydroelectric generator (after 57,984 operation hours). The bar insulating system material (glass-mica-based) and the stator wedge material (laminated glass-fabric and laminated cellulose paper (paper-phenolic laminates) were tested. Additional laboratory thermal aging of the samples from both the bar winding insulating system and the supporting material was conducted to verify the suitability of the DSC method. Based on previous experience and on the thermal class of the insulating system (F - 155 °C), the aging temperature was set to 180 °C. Enthalpies were measured after 52, 70, 100 and 200 hours of aging. The samples from three bars (phase, zero and changed bar) were analyzed. Powdered samples from compacted bar insulation and stator wedge material were measured using DSC in a pure air atmosphere. The area of the first thermal oxidative peak (enthalpy) was evaluated. The experiment showed good agreement with the enthalpy results for the insulating system and the supporting material.