Developmental Studies on Polymeric Nano/Micro Composite Insulation for Various High Voltage Power Applications

Abstract

The demand for electrical power is increasing day by day necessitating a higher voltage level for power transmission. Also the use of cast resin type dry transformers of compact size for use in confined areas and in certain critical applications is increasing since it is more reliable in extreme conditions and also they require less maintenance. All these applications demand for better insulating materials which can address all the above issues cost effectively. In recent years, the use of polymeric insulating materials in HV power apparatus is increasing. Hence this study focuses on the development of polymeric composite insulating material for various electrical power applications. Silicone rubber is a preferred material for use as weathershed material in outdoor polymeric insulators used in high voltage power transmission lines. The tracking & erosion on the insulator surface due to the electrical discharges and corona cutting of the insulator surface are the main issues related to outdoor polymeric insulators and these are addressed in this study. Tracking and erosion performance of silicone rubber filled with nano/micro fillers of different loadings is analysed using Inclined Plane Test (IPT) as per IEC 60587. A computational study on the behavior of the samples subjected to tracking is also done and the same is verified with the experimental results obtained in this work. Corona ageing studies are done by ageing the samples in a corona chamber for 25 hours. Hydrophobicity changes, crack width formation and erosion performance after corona ageing are evaluated. An effort is made to correlate the value of leakage current to the eroded mass and a reliable online condition monitoring tool is also developed as an offshoot of the present thesis work. Again, epoxy is extensively used in many electrical power apparatus and heat dissipation is an important area of concern when using epoxy as ground wall insulation in rotating machines and as an insulation in cast resin dry type transformer. The performance of epoxy filled with nano/ micron sized fillers are investigated in this study in terms of their heat removal capacity and at the same time retaining their dielectric properties. The improvement in thermal conductivity is correlated with the performance of various composites developed. The formation of track in the ground wall insulation and the failure of the machine is a major issue as far as rotating machines are considered. Hence the tracking time of various epoxy composites are observed and compared. The initiation of a faint track on the surface of the insulator is monitored with the help of the ratio of third harmonic component to the fundamental component. It has been in this thesis that this ratio can be used as an efficient condition monitoring tool for rotating machines by measuring the leakage current online. In the case of silicone rubber (SR) the tracking and erosion performance are studied for various composites by filling low weight percentage of micro boron nitride (uBN), nano boron nitride (nBN), micro aluminium nitride (uAlN), nano aluminium nitride (n AlN), and nano alumina trihydrate (nATH) and is compared with a high filler loading 0f 30 wt% for micro ATH, which is the conventional filler used for improving the tracking and erosion. Both nanocomposites with BN as well as ATH as fillers performed well in terms of the eroded mass. 2 wt% nano BN filled composite of SR performed better than the 30 wt% uATH filled composite. The nATH filled composite with 4 wt% filler loading gave a comparable performance with uATH composite. The corona ageing studies conducted also reveals that the performance of the nanocomposites of both BN and ATH filled samples are better than the uATH composite in terms of hydrophobicity loss and crack formation due to corona cutting. In the case of epoxy insulation, micro, nano, and hybrid (mixture of micro and nano) composites filled with BN are analysed for tracking time using the IPT procedure. The behaviour of epoxy samples tested for tracking and erosion under IPT is similar to the SR samples initially. But, as the arcing is confined to the ground electrode the tracking initiates as the back bone is carbon for epoxy whereas it is silicon and oxygen in the case of silicone rubber. Thus the arcing is sustained near the track and track starts propagating to the HV electrode finally causing the bridging of the electrode within few minutes. The tracking time is the time taken by the track propagation to reach the middle of the sample. The nanocomposite with a filler loading of 5 wt% gave the best result followed by 10 wt% uBN and 5 wt% uBN composites. The performance of hybrid composite is inferior to nano and micro composites. In summary, in this study polymeric composite insulating materials based on silicone rubber and epoxy with different fillers and loadings are developed and tested for electrical and thermal properties. The nanocomposites of BN and ATH with SR proved to be promising materials to be used as weathershed material in outdoor insulators. Micro composite of BN with a little higher filler loading as compared to nanocomposites also can be considered for cost-effectiveness. For epoxy both micro and nanocomposites of BN showed better performance as compared to the neat epoxy that is conventionally used for dry type transformer and high voltage rotating machine insulation.