The overall reliability of a power transformer depends to a great extent on the sound operation of the bushings thereof. In view of its overwhelming advantages, resin impregnated paper (RIP) is acquiring prominence over conventional oil impregnated paper (OIP) in transformer bushings. The main advantages of RIP bushings are low dielectric loss and capability of positioning them at any desired angle over the transformer. The RIP structure, being an all-solid system, is completely free from oil phase. The temperature rise in RIP bushings under normal operating conditions is seen to be a difficult parameter to control in view of the limited options for effective cooling. The degradation of dry-type insulation such as RIP is often due to thermal and electrical stresses. The long time performance thereof, depends strongly, on the maximum operating temperature. In order to be able to predict the regional temperature, it is necessary to consider the thermal and electrical parameters of insulation in question; and to identify and solve the governing equations under the relevant boundary conditions. Electrical failure of insulation is known to be an extremal random process wherein nominally identical specimens of equipment insulation, at constant stress fails at inordinately different times. In order to be able to estimate the life of power equipment like transformer bushing, it is necessary to run long duration ageing experiments under accelerated stresses, to acquire and analyze insulation specific failure data. The present work is an attempt to provide reliability and life estimation of High Voltage RIP bushing insulation. The literature survey carried out in this view indicate that investigation on thermal and electric field distribution and the models for failure under combined stress and analysis of the data so as to be able to estimate the possible life of RIP bushing is scanty. Having these aspects in focus, the scope of the present work is defined as: (i) Mapping of the temperature and electric field distribution in the body of 400kV RIP bushing (ii) Deduction of parameters of the probabilistic models for the failure under electrical and thermal ageing (iii) Estimation of life based on diagnostic testing using PD With this in view, the temperature distribution in the body of a 400kV RIP bushing is studied considering the heat generation both in central conductor and that in the insulation. Presence of multiple materials with non-confirming interfaces makes analytical solution rather difficult and hence numerical approach is adopted. In the present work, vertex-centered Finite Volume Method (FVM) is employed for both thermal and electrical analysis. The electric stress distribution is accurately evaluated considering both the non-zero conductivity of the RIP material and the presence of capacitive grading foils. These analysis has clearly shown that Stress grading foils uniforms the stress across the major portion of the bushing insulation Enhancement of the electric conductivity by the temperature is not found to be affective in changing the electric field distribution The temperature distribution is shown to have a maxima near the flange due to the influence of top oil temperature of the transformer Heat generated in the dielectric due to the prevailing electric stress is shown to be insignificant. This ruled out the possibility of thermal runaway and hence the dielectric temperature is within the safe working limits for the bushing considered. The deduction of physical models governing insulation failure depends on the nature of stress. In this work, the insulation failure at constant accelerated stress has been considered and the estimation of life is computed based on inverse power law coupled with Arrhenius law. A high degree of scatter is generic to the experimental data forming the ingredients to develop the models. In view of this, the concept of a random process is invoked. Probabilistic models for the failure of RIP bushing under synergy are adopted and an attempt is made to estimate the life. The well known Weibull distribution and probability plotting of life data is used in this endeavor. The maximum likelihood estimation is used to determine the scale and shape parameters of the Weibull distribution. In the diagnosis of the extent of degradation of insulation due to PD, under long duration electric stress, a non-conventional voltage application method called the classical stepped stress method is adopted. In this technique, the voltage is applied in pre-determined steps over predetermined duration of time. The magnitude of voltage steps is carefully computed based on Miners law and the end-of-life is computed using inverse power law. In summary, this thesis work has contributed to the thermal and electrical degradation of resin impregnated paper insulation for high voltage transformer bushing. The thermal and electrical field distributions computed in the body of bushing clearly shown that these stresses are well within the limit, thereby ruling out the possibility of a thermal runaway. Comparing the estimates of the most probable life of RIP, based on several methods appears to show that any of the method can be adopted. However, as matter of caution and safety, the lowest among them can be taken as a reasonable estimate.

Examensarbetet har genomförts på ABB Components i Ludvika. ABB Components är en affärsenhet inom ABB-koncernen som tillverkar nyckelkomponenter till transformatorer, bland annat genomföringar som kan ses som en isolerande hylsa vars uppgift är att föra högspänd ström genom jordade plan. Examensarbetets syfte är att utföra en förstudie som sedan ska fungera som underlag för ABB Components utvecklingsarbete vad gäller genomföringar av typen RIP (Resin Impregnated Paper). Bakgrunden till förstudien är ökade miljökrav världen över vilket innebär en marknad där miljöfrågor får allt större inverkan. Följden av marknadens utveckling är att ABB Components har insett att de har ett glapp i produktportföljen vad gäller genomföringar av typen RIP. För att skapa det underlag som är nödvändigt för att nå målet med förstudien har metoder som intervjuer, observationer och sökningar i databaser tillämpats. Målet med förstudien har sedan uppnåtts genom att generera lösningskoncept i form av CAD-modeller. Utifrån lösningskoncepten har sedan en identifiering av begränsningar i tillverkningsprocesser varit möjlig. Identifiering av begränsningar i tillverkningsprocesser har sedan konstaterat att ABB Components har kunskapen och de resurser som krävs för att tillfredsställa marknadens behov vad gäller genomföringar av typen RIP. / This thesis has been carried out at ABB Components in Ludvika. ABB Components is a business unit within ABB Group, which manufactures key components for transformers, including bushings that can be seen as an insulating sleeve whose function is to bring high-voltage current through a grounded plane. The thesis aims to carry out a pilot study that will serve as basis for ABB Components development of bushings of type RIP (Resin Impregnated Paper). The background to the pilot study is increased environmental requirements worldwide, which means a market where environmental issues are increasingly impact. The consequence of market developments is that ABB Components have recognized that they have a gap in the product portfolio in terms of bushings of type RIP. To create the information that is necessary to achieve the aim of the pilot study, methods such as interviews, observations, and searches in databases has been applied. The aim of the pilot study has been achieved by generating solution concepts in the form of CAD models. Based on solution concepts, an identification of limitations in manufacturing processes has been possible. Identification of limitations in manufacturing processes has determined that ABB Components have the knowledge and resources necessary to satisfy the market's needs in terms of bushings of type RIP.

High voltage bushings are the most critical components of power transformers. A common material used in bushings is resin impregnated paper (RIP). Hitachi Energy is investigating whether this can be replaced with a new material, resin impregnated non-woven (RIN). One of the main reasons is that non-woven is less prone to absorb moisture compared to paper. Thus, for design purposes the mechanical, thermal and absorption properties of RIN have been studied and compared to RIP. The mechanical properties were tested by tensile and bending tests at room temperature and 80 ℃, showing that RIN has a lower elastic modulus and tensile strength than RIP at both temperatures. However, it was demonstrated that RIN does not retain its elongation at break and elasticity properties at elevated temperatures. The bending test showed no significant differences in flexural properties for RIN between room and high temperature. The thermal properties were studied with the transient plane source method (TPS) showing that both RIN and RIP had a higher specific heat capacity than pure epoxy. The thermal conductivity of the materials will be measured and included later. Lastly, the water absorption test was performed in order to provide information about the suitability of the materials used in bushings. For this different methods were used; water immersion andwater vapor testing. The immersion test showed that non-woven is more water resistant than paper and that the composites only absorb a small amount of water. No useful information was achieved from the water vapor test due to limited testing time. The results demonstrate the promising potential of RIN in bushings.

The goal of the project was to create a model in COMSOL Multiphysics that can simulate thefilling of resin (epoxy) in Resin Impregnated Paper (RIP) bushings. Using the model new fillingrecipes that avoid problematic double-fronts and thereby avoid the creation of cavities in thebushings should be designed. Another goal was to use Thermocouples and Fiber optics tomeasure the temperature at different positions during the filling to get a better understanding ofthe filling. The first step was to design and determine functions, parameters (such as thepermeability), boundary conditions etc. for the simulation model in COMSOL Multiphysics and tocalibrate the model to align with measurements of fillings previously done. The second part wasto increase the filling time to minimize the maximum difference between filling fronts in thebushings and thereby decrease the risk of problematical double-fronts that create cavities,which decrease the bushing's dielectrical properties. The simulation model aligned somewhatwith the measured data but there is a lot of uncertainty both in the measurements and due toassumptions made in the simulation model. The measurements from Thermocouples and Fiberoptics where almost identical and they indicated that the filling of the bushing is symmetric.