Electrical machines in electrified heavy-duty vehicles are subjected to dynamic temperature loadings during normal operation due to the different driving conditions. The Electrical Insulation System (EIS) in a stator winding is aged as an effect of these dynamic thermal loads. The thermal loads are usually high constant temperatures and thermal cycling. The high average constant thermal load is well-known in the electrical machine industry but little is known about the effect of temperature cycling. In this project, the ageing of the EIS in stator windings due to temperature cycling is examined. In this project, computational simulations of different simplified models that represent the electrical insulation system are made to analyse the thermo-mechanical stresses that is induced due to thermal cycling. Furthermore, a test object was designed and simulated to replicate the stress levels obtained from the simulations. The test object is to ease the physical testing of electrical insulation system. Testing a complete stator takes time and has the disadvantage of having a high mass, therefore a test object is designed and a test method is provided. The results from the finite element analysis indicate that the mechanical stresses induced will affect the lifetime of the electrical insulation system. A sensitivity study of several thermal cycling parameters was performed, the stator core length, the cycle rate and the temperature cycle amplitude. The results obtained indicate that the stator core length is too short to have a significant effect on the thermo-mechanical stresses induced. The results of the sensitivity study of the temperature cycle rate and the temperature cycle amplitude showed that these parameters increase the thermo-mechanical stresses induced. The results from the simulations of the test object is similar to the results from the simulations of the stator windings, which means that the tests object is valid for testing. The test method that is most appropriate is the power cycling test method, because it replicates the actual application of stator windings. The thermally induced stresses exposing the slot insulation exceeds the yield strength of the material, therefore plastic deformation may occur only after one thermal cycle. The other components in the stator are exposed to stresses below the yield strength. The thermally induced stresses exposing the slot insulation are high enough to low cycle fatigue the electrical insulation system, thus thermo-mechanical fatigue is an ageing factor of the electrical insulation system.

Insulation in rotating machines is a common cause for failure. Therefore, research in the area is large and is important to further increase the knowledge of the phenomena related to insulation degradation. This thesis is a case study on 20 stator bars obtained from a generator that had been showing signs of insulation degradation while in operation. The insulation was investigated using a range of measurements to be able to evaluate the degradation of these bars. The measurements performed were electrical partial discharge measurements, acoustic partial discharge measurements, dielectric response measurements, and breakdown testing. Furthermore, a more scientific approach to the commonly used tap test in the industry was conducted, which has not been done before and the purpose was to validate this commonly used test method. It was concluded that the bars have a significant amount of partial discharges within them, and that the stress grading is likely to be blamed in many of the worst cases. Furthermore, the dielectric response measurements provided good information on the thermal stability of the insulation. The set-up of the tap test did not provide reasonable results, and the method approach as well as the experimental set-up was to blame for this. In future work, investigating this specific area could be beneficial. The electrical partial discharge measurements and the breakdown test provided unexpected results when compared, where the former indicated much degradation while the latter proved that the bars can withstand very high voltage levels. / Isolering i roterande maskiner är en vanlig orsak till fel. Forskningen på området är därför omfattande och viktig för att ytterligare öka kunskapen om fenomen som rör isoleringsnedbrytning. Denna avhandling är en fallstudie på 20 statorstänger som erhållits från en generator som hade visat tecken på isoleringsnedbrytning under drift. Isoleringen undersöktes med hjälp av en rad olika mätningar för att kunna utvärdera nedbrytningen av dessa stänger. De mätningar som utfördes var mätningar av elektriska partiella urladdningar, akustiska partiella urladdningar, mätningar av dielektrisk respons och genombrottstester. Dessutom genomfördes ett mer vetenskapligt tillvägagångssätt för det allmänt använda tapptestet i branschen, vilket inte har gjorts tidigare och syftet var att validera denna allmänt använda testmetod. Slutsatsen blev att stängerna har en betydande mängd partiella urladdningar i sig, och att spänningsgraderingen sannolikt får skulden i många av de värsta fallen. Dessutom gav mätningarna av den dielektriska responsen god information om isoleringens termiska stabilitet. Upplägget för tapptestet gav inte rimliga resultat, och detta berodde både på metodiken och på försöksupplägget. I framtida arbete kan det vara bra att undersöka detta specifika område. Mätningarna av den elektriska partiella urladdningen och genombrottstestet gav oväntade resultat när de jämfördes, där de förstnämnda visade på mycket försämring medan de sistnämnda bevisade att stängerna kan motstå mycket höga spänningsnivåer.