High Voltage (HV) rotating machines are widely used in power generation and heavy industry companies. Silicon Carbide (SiC) material is used in the stress grading structure of HV electric machines because of its non-linear characteristic. In this thesis, the resistance characteristics of SiC-based stress grading tape in time domain and the dielectric parameters in frequency domain were measured. The measurement results of samples with axial current-direction in time domain and frequency domain are about ten times that of the samples with radial current-direction, which might be a feature of the tape construction although it also can depend on the measurement method. The finite element method (FEM) simulation of the coil insulation system was carried out by using the experimental parameters, and the results were compared with the actual measurement results. The calculation results show that the non-linearity of SiC-based material does not have much effect on the real part of frequency response of the whole coil. The relative difference of the real part of complex capacitance is less than 5%. However, the influence on the imaginary part is relatively large, the relative difference is about 4% - 60%. / Högspänningsmaskiner används ofta i kraftproduktion och tunga industriella anläggningar. Kiselkarbidmaterial (SiC) används i sådana maskiners potential-styrningssstruktur på grund av dess icke-linjära egenskaper. I denna avhandling mättes ledningsförmågan hos SiC-baserade fältstyrningstejp i tidsdomänen och de dielektriska parametrarna i frekvensdomänen. Resultaten med axiell strömriktning vid tiddomän och frekvensdomänmetoder tyder på omkring tio gånger så stor konduktivitet jämfört med den radiella ström-riktningen, vilken kan bero på tejpkonstruktionen fast även eventuellt på mätmetoden. Simuleringen av en statorhärvas isoleringssystem utfördes med finita element metoden (FEM) med parametrar från materialproven, och resultaten jämfördes med mätresultat från en sådan härva. Beräkningsresultaten visar att det SiC-baserade materialets ickelinjäritet inte har någon större effekt på hela spolens verkliga frekvensrespons. Den relativa skillnaden mellan den verkliga delen av komplex kapacitans är mindre än 5%. Påverkan på den imaginära delen är dock relativt stor, med en relativ skillnad på ca 4% - 60%.

High voltage rotating machines play a significant role in generation and use of electrical energy as the demand for power continues to increase. However, one of the main causes for down times in high voltage rotating machines is related to problems with the winding insulation. The utilities want to reduce costs through longer maintenance intervals and a higher lifetime of the machines. These demands create a challenge for the producers of winding insulations, the manufacturers of high voltage rotating machines and the utilities to develop new insulation materials which can improve the life of the equipment and reduce the maintenance cost. The advent of nanotechnology in recent times has heralded a new era in materials technology by creating opportunities to significantly enhance the properties of existing conventional materials. Polymer nanocomposites belong to one such class of materials that exhibit unique combinations of physical, mechanical and thermal properties which are advantageous as compared to the traditional polymers or their composites. Even though they show tremendous promise for dielectric/electrical insulation applications, there are no studies relating to the long term performance as well as life estimation of the nanocomposites. Considering this, an attempt is made to generate an understanding on the feasibility of these nanocomposites for electrical insulation applications. An epoxy based nanocomposite system is chosen for this study along with alumina (Al2O3) and silica (SiO2) as the nanofillers. The first and the foremost requirement for studies on polymer nanocomposites is to achieve a uniform dispersion of nanoparticles in the polymer matrix, as nanoparticles are known to agglomerate and form large particle sizes. A laboratory based direct dispersion method is used to process epoxy nanocomposites in order to get well dispersed samples. A detailed microscopy analysis of the filler dispersion using Scanning Electron Microscope (SEM) has been carried out to check the dispersion of the nanofiller in the polymer. An attempt is made to characterize and analyze the interaction dynamics at the interface regions in the epoxy nanocomposite by glass transition temperature (Tg) measurements and Fourier transform infrared (FTIR) spectroscopy studies. The values of Tg for the nanocomposites studied decreases at 0.1 wt% filler loading and then starts to increase gradually with increase in filler loading. This Tg variation suggests that there is certainly an interaction between the epoxy chains and the nanoparticles. Also no new chemical bonds were observed in the spectra of epoxy nanocomposite as compared to unfilled epoxy. But changes were observed in the peak intensity and width of the –OH band in the spectrum of epoxy nanocomposite. This change was due to the formation of the hydrogen bonding between the epoxy and the nanofiller. The thermal conductivity of the epoxy alumina and the epoxy silica nanocomposites increased even with the addition of 0.1 wt% of the filler. This increase in thermal conductivity is one of the factors that make these nanocomposites a better option for electrical insulation applications. The dielectric properties of epoxy nanocomposites obtained in this investigation also reveal few interesting behaviors which are found to be unique and advantageous as compared to similar properties of unfilled materials. It is observed that the addition of fillers of certain loadings of nanoparticles to epoxy results in the nanocomposite permittivity value to be lower than that of the unfilled epoxy over the entire range of frequencies [10-2-106 Hz] considered in this study. This reduction has been attributed to the inhibition of polymer chain mobility caused by the addition of the nanoparticles. The tan values are almost the same or lower as compared to the unfilled epoxy for the different filler loadings considered. This behavior is probably due to the influence of the interface as the strong bonding at the interface will make the interface very stable with fewer defects apart from acting as charge trapping centres. From a practical application point of view, the surface discharge resistant characteristics of the materials are very important and this property has also been evaluated. The resistance to surface discharge is measured in the form of roughness on the surface of the material caused by the discharges. A significant enhancement in the discharge resistance has been observed for nanocomposites as compared to unfilled epoxy/ microcomposites, especially at longer exposure durations. The partial discharge (PD) measurements were carried out at regular intervals of time and it is observed that the PD magnitude reduced with discharge duration in the case of epoxy alumina nanocomposites. An attempt was made to understand the chemical changes on the surface by conducting the FTIR studies on the aged surface. For all electrical insulation applications, materials having higher values of dielectric strengths are always desired and necessary. So AC breakdown studies have also been conducted. The AC breakdown strength shows a decreasing trend up to a certain filler loading and then an increase at 5 wt% filler loading for epoxy alumina nanocomposites. It has been also observed that the type of filler as well as the thickness of the filler influences the breakdown strength. The AC dielectric strength of microcomposites are observed to be lower than the nanocomposites. Extensive research by long term aging studies and life estimation are needed before these new nanocomposites can be put into useful service. So long term aging studies under combined electrical and thermal stresses have been carried out on unfilled epoxy and epoxy alumina nanocomposite samples of filler loading 5 wt%. The important dielectric parameters like pemittivity, tan  and volume resistivity were measured before and after aging to understand the performance of the material under study. The leakage current was measured at regular intervals and tan  values were calculated with duration of aging. It was observed that the tan  values increased drastically for unfilled epoxy for the aging duration considered as compared to epoxy alumina nanocomposites. The life estimation of unfilled epoxy as well as epoxy nanocomposites were also performed by subjecting the samples to different stress levels of 6 kV/mm, 7 kV/mm and 8 kV/mm at 60 oC. It is observed that the epoxy alumina nanocomposite has an enhanced life which is nine times the life of the unfilled epoxy. These results obtained for the nanocomposites enable us to design a better material with improved dielectric strength, dielectric properties, thermal conductivity, resistance to surface discharge degradation and enhanced life without sacrificing the flexibility in the end product and the ease of processing. Dry type transformers and stator winding insulation need to be cast with the above material developed and tested before practically implementing these in the actual application.

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.