Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Limited research results are available on the aging impacts of surface discharges for High Voltage Direct Current (HVDC) excitation on practical insulators using the Tracking Wheel Tester (TWT) methodology. This thesis gives details of an experimental investigation to compare the aging performance of insulator samples using the TWT for High Voltage Alternating Current (HVAC) and positive and negative polarity HVDC excitation. Two series of tests were performed. The first series of tests evaluated the aging of six insulator rods, namely three Room Temperature Vulcanized Silicone Rubber (RTV SR) coated glass samples and three uncoated glass samples. Three creepage distances were used for each of the test materials, namely 277 mm, 346 mm and 433 mm. The tests were conducted with HVAC excitation and the test methodology described in the IEC 61302 standard. The second series of tests involved the testing of actual insulator samples. The test voltage and conductivity of the salt water solution were also adapted. Six insulators from different manufacturers, representing different materials and specific creepage distances, were tested with HVAC and positive and negative polarity HVDC excitation. The test samples consisted of Ethylene Propylene Diene Monomer (EPDM), High Temperature Vulcanized Silicone Rubber (HTV SR), porcelain and SR coated porcelain insulators. The aging performance of the different test samples for the three types of excitation are compared in terms of peak leakage current, visual observations of surface degradation and hydrophobicity properties. The leakage current data for HVAC excitation shows that the insulators from the different manufacturers perform differently, even for the same type of material. The results also indicate differences in the way the surfaces degrade, as well as the rate of degradation. For insulators representing the same specific creepage distance, but different materials, it has been shown that the material does influence the aging performance. Comparison of the leakage current data for the HTV SR insulators from the same manufacturer, but with different specific creepage distances, shows that the specific creepage distance affect the aging performance significantly. In general, the test insulators showed higher peak leakage currents with HVDC excitation compared to HVAC excitation. The results for positive polarity HVDC excitation show that the dry band arcing, as well as the discharges, has the same form for all six insulators. The colour of the dry band discharges ranged from a blue-ish orange to a dark yellow, depending on the intensity of the leakage currents. The hydrophobic insulators, namely the HTV SR, EPDM and RTV SR coated porcelain insulators, had lost hydrophobicity within the first week of testing. The results for negative polarity HVDC excitation show severe surface degradation compared to the results for HVAC excitation. The effect of positive polarity HVDC excitation, however, seems to be more severe in terms of leakage currents and aging compared to negative polarity HVDC excitation.

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.

Object of the master´s thesis is design of a high voltage power supply for testing insulation materials with output voltage 50 V - 2 kV at fixed frequency 50 Hz. The power supply is controlled by personal computer and measured data are sent back to the personal computer. The Universal Serial Bus (USB) is used for communication between the PC and the device.