Long Term Multistress Aging of High Voltage Nanocomposites

Abstract

Long Term Multistress Aging of High Voltage Nanocomposites Insulators are important elements of electrical power transmission and distribution systems, which not only provide separation between supply lines and towers but also provide mechanical support to the conductors. Therefore, for efficient and effective power transmission, insulators play a major role. Conventional porcelain and glass insulators were the sole insulators for a long time and then polymeric insulators were introduced which were recommended and installed in most of the advanced countries. Polymeric insulators have numerous compensations over ceramic insulators such as light weight, high dielectric strength and low cost etc. However, they have the limitation of aging due to electrical and environmental stresses. Since their development, different polymeric insulating materials have been researched in the field as well as in laboratory for the assessment of their durability against multiple stresses. Several polymeric materials are used in high voltage insulation applications and each has different strengths and weaknesses. Silicone rubber (SiR), Ethylene Propylene Diene Monomer (EPDM) and epoxy are some of the examples of prominent polymeric insulating materials. On the other hand, advancements in the field of nanotechnology helped in gaining polymeric insulators with improved characteristics. Consequently, polymeric insulating materials have been suggested to be replaced by their nanocomposites. However, polymeric nanocomposites lack long term history of performance. Therefore, they should be evaluated for adequate time with reference to neat polymeric materials to find the influence nano-fillers on their aging behavior. Type, size and loading of nano/micro fillers are the parameters that govern the properties in any composite. In wide range fillers, due to its high thermal stability and electrical resistivity silica is the one of the best choices in this field. In order to find out, the nanocomposite insulating materials with good aging resistance, careful preparation with uniformly dispersed nano-fillers and their comparative analyses are required. For precise comparative investigation of these insulators, they must be subjected to uniform aging/degradation environment having xi all probable stresses. Furthermore, keeping in view the dynamic aging behavior of polymeric insulators they should be investigated for long term. In natural environment multiple stresses are present, thus to obtain the laboratory results close to the natural environment, the insulators must be evaluated in a multistress environment. These stresses include ultraviolet (UV) light, heat, acid rain etc., periodic application of these stresses in an environmental chamber is called weathering cycles. At the end of a number of weathering cycles or aging period different analyses techniques can be used to analyze the condition of the samples or more accurately saying to check the effects of stresses on insulators which may affect electrical, physical and structural properties. A number of methods can be used for condition assessment of insulators; some of them are Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, Swedish Transmission Research Institute (STRI) hydrophobicity classification, leakage current (LC) measurement and visual inspection. Statistical tools should be applied along with these techniques to obtain reliable results. Furthermore, based on experimental data obtained shelf lives of the insulators can be evaluated using estimation statistics. These techniques are adequate to evaluate the important changes in any polymeric insulating material. This research work is aimed at detailed long-term multistress aging investigation of different silica based polymeric nanocomposites for high voltage applications through detail analysis techniques, statistical tools and modeling for life estimation. Different samples of neat, nanocomposites and hybrid nano-microcomposites of Silicone Rubber (SiR), Ethylene Propylene Diene Monomer (EPDM) and epoxy with different loadings of nano and micro-silica (SiO2) filler are prepared according to the standard procedures. These prepared samples are aged in an accelerated weathering environment as per Electric Power Research Institute‘s standard using actual service data. Aging and recovery of polymeric materials is unpredictable and their characterization is not reliable on the basis of short term testing. Therefore, duration of this multistress laboratory based aging is kept 9000 hours to explore these samples for long time and in detail. Leakage current (LC) measurement, STRI hydrophobicity classification method, detail Fourier Transform Infrared (FTIR) spectroscopy, visual inspection, Scanning Electron Microscopy (SEM) along with multiple statistical tools xii in SPSS software are used for condition assessment of the samples. Furthermore, life estimation through regression modeling was performed for HV nanocomposites in Origin8 software. Nanocomposites showed improved behavior over corresponding neat samples. After aging silicone rubber nanocomposite with 5% nanosilica (SNC-5) exhibited hydrophobicity class 1 (HC-1) and EPDM nanocomposite with 5% nano-silica (ENC) and epoxy nanocomposites with 5% nanosilica (EPNC) showed HC-2. Final leakage currents for SNC-5, ENC and EPNC were 0.6 μA and for EPNC it was 0.9 μA respectively. FTIR results showed negligible effect on Si-CH3 group of SNC-5. CH3 asymmetric group showed 33% and 44% reductions in EPDM and epoxy nanocomposites respectively. Both epoxy and EPDM nanocomposites revealed signs of oxidation by showing presence carbonyl groups, but this was much less in comparison to their neat samples. In SEM micrographs, after aging SNC-5 surface showed minor trails. While, cracks and loss of material were observed on ENC and EPNC surfaces, which were higher in the case of EPNC. Paired sample t tests and Wilcoxon signed rank tests showed that only 22% of aged nanocomposite samples were statistically different from their corresponding new samples. Using estimation statistics on leakage current data, SNC-5 exhibited longest shelf life of 19450 lab hours. While, ENC and EPNC showed life expectancies of 16600 lab hours and 11669 lab hours respectively. This is a unique work of its own nature, which presents detailed 9000 h multistress aging analyses and life estimation of silica based diversified high voltage polymeric nanocomposite insulating materials. Long term aging study through detailed analysis techniques and statistical tools, and life estimation of these nanocomposites has not been reported before, in any form. Several interesting results have been obtained from this research work, which are believed to be important contributions towards the field of high voltage outdoor insulation.