Search

Extruded polymeric insulation material has been widely used in high voltage alternating current (HVAC) power transmission system for a long time. Recent years, with the advent of renewable energy around world, more large energy capacity, long distance high voltage direct current (HVDC) transmission projects were developed. The quality of insulation materials of HVDC cables has become the most critical issue that restricts the development of HVDC transmission. Cross-linked polyethylene (XLPE) has shown superior electrical, thermal and mechanical properties due to its special cross-linked structure and has become the default material for HVDC cable insulation in the last few decades. But today, with the increasing concern of environmental protection and sustainable development, traditional XLPE cables are no longer the default for future HVDC applications due to its limited operating temperature, non-recyclable nature and high energy consumption in complex production process. Therefore, new thermoplastic polymeric insulation materials are being developed, which are recyclable and can be operated under higher temperature. However, these materials still have weak points like space charge accumulation. The incorporation of nanoparticles has been proven to be an effective method to overcome these drawbacks. In this study, thermoplastic polymers, polyethylene (PE) and polypropylene (PP), were selected as the potential matrix materials to study. Nano-alumina particles with and without surface modification were introduced into polymers at different filling contents to make nanocomposites. This thesis reports the comprehensive study on PE/nano-alumina composite systems and PP/nano-alumina composite systems. The surface chemistry of nano-alumina with and without surface modification was characterized by thermal-gravimetric analysis and Fourier-transform infrared spectroscopy. The morphological structure of nanocomposite systems was analyzed by using polarized optical microscopy and scanning electron microscopy. Electrical properties such as DC breakdown strength, DC conductivity, and space charge were investigated to evaluate the feasibility of the proposed nanocomposite materials as future HVDC cable insulation. A deep understanding of the effect of nanoparticles on the dielectric properties of polymeric nanocomposites was achieved. After comparing the performance of PE nanocomposites and PP nanocomposites, PP nanocomposites exhibit superior electrical properties and can be considered as the future alternative to XLPE. In conclusion, the experimental results and discussion presented in this project can contribute to the design and manufacture of recyclable polymeric nanocomposite materials for future HVDC applications.

Nowadays the crossing-linked polyethylene (XLPE) has been widely used as the cable insulation materials in the high voltage direct current (HVDC) application. It has some disadvantages including non-recyclable properties, pollutants in the crosslinking process and bad mechanical properties under high temperature. Therefore, it is a main research direction to develop the new generation of polymeric insulation materials with excellent electrical properties, recyclability and environmental-friendly characteristics for the HVDC cable insulation. Polypropylene (PP) is one of commercial thermoplastic polymers which can be recycled and used as a high voltage insulator under high temperatures. Additionally, the electrical performance of PP is better than XLPE in terms of breakdown strength, conductivity, and space charge characteristic. Based on the nanodielectrics theorem, the excellent electrical performance of polymers can be achieved through the nanocomposite process. Therefore, nanocomposites based on PP can have much higher electrical properties and higher thermal stability than XLPE, which provides a new idea for the development of new HVDC cable insulation technology. Over the past few decades, significant attention was paid to the traditional inorganic nanofillers, especially to nano-MgO and nano-Al2O3, in order to modify the properties of PP. However, there is not sufficient research information available in the literature on the effect of POSS on the properties of PP. The main aim of this PhD project is to develop new PP's nanocomposites with high DC electrical performance for HVDC underground power cable insulation. Firstly, nano-MgO, nano-Al2O3, octavinyl-POSS (OvPOSS) and octaphenyl-POSS (OpPOSS) are selected as nanofillers to manufacture the nanocomposites of PP by the solution method. Then, the electrical properties of PP and its nanocomposites were measured and the mechanism about the nanofiller effect on the electrical performance was explored though the micro observation and macro-tests of PP's nanocomposites, especially for OvPOSS and OpPOSS. The formulation and preparation of PP's nanocomposites were optimized through the comparison between the effect of different nanofillers on the electrical performance of PP. The result of electrical measurements shows the electrical performance of POSS/PP nanocomposites is better than MgO/PP and Al2O3/PP nanocomposites under the temperature of 30 ℃, which means that POSS/PP nanocomposites have higher potential to be the next generation of polymeric cable insulation materials with the compare of traditional PP's nanocomposites, including MgO/PP and Al2O3/PP nanocomposites. Secondly, the electrical properties of OvPOSS/PP and OpPOSS/PP nanocomposites at higher temperatures, including 50, 70 and 90℃, have been measured. Then, the electrical measurement revealed that the effect of temperatures changes on the breakdown strength, DC conductivity and space charge characteristics. The relationship between the conductive current and space charge suppression of PP nanocomposites under different temperatures was still discussed to show the mechanism about how the effect of POSS addition can affect the electrical performance of PP under high temperatures. Finally, OpPOSS/PP nanocomposites are selected to explore the electrical and mechanical properties of aged samples through the thermal-oxidation aging (TA) and the electro thermal aging (ETA) processes at the temperature of 110℃. Through the comparison between OpPOSS/PP and pure PP, the OpPOSS can still improve the electrical performance of PP, but it cannot suppress the aging process occurring in pure PP. Under the operation of HVDC cable, the addition of OpPOSS can reduce the electrical damage in the bulk of PP then enhance the lifespan of PP under the HVDC conditions. In this PhD research project the effects of OvPOSS and OpPOSS on the electrical properties of PP have been investigated and the potential of OvPOSS/PP and OpPOSS/PP nanocomposites for the HVDC cable insulation has been analysed. Compared with the traditional nanocomposites, this thesis showed that OvPOSS/PP and OpPOSS/PP nanocomposites have higher electrical properties at the temperature of 30 ℃, this thesis also analyses the reason why addition of OvPOSS and OpPOSS into PP can result in its higher electrical performance . By testing the electrical properties of OvPOSS/PP and OpPOSS/PP nanocomposites at high temperatures, the changes of the electrical properties of these nanocomposites at different temperatures were revealed. Based on the TSDC test results, the influence of nanocomposites on the high temperature electrical properties of PP was demonstrated, the physical mechanism and method of regulating the space charge accumulation in the bulk of PP under the action of electric field at elevated temperature were proposed and the concentrations of nanofillers were optimized. Finally, the experimental platforms for thermal-oxidation aging and electro-thermal aging processes were designed to carry out aging experiments on PP and its nanocomposites. Under different aging times, the effects of OpPOSS on the mechanical and electrical properties of PP were analysed by FTIR spectroscopy. Finally, at 110 ℃ and 60 kV/mm, the breakdown time of PP and OpPOSS/PP was measured, and the lifespan of PP and OpPOSS/PP nanocomposite insulation layer in the actual operation of HVDC cable was theoretically predicted, which indicated the potential of OpPOSS/PP nanocomposites to be the next generation of HVDC cable insulation materials with environmental-friendly properties. Key words: Polypropylene, nanocomposites, electrical properties, thermal-oxidation aging, electro-thermal aging.

In the high voltage industry one of the factors limiting the performance of a pulse power system is the build-up of charge on the surface of the insulators. In these systems solid insulators are used to support the conductors. When operating pulse power systems in repetitive mode, the accumulation of charge on the insulators after each pulse can promote 'surface flashover' and cannot be ignored as a limiting factor in the design of the system. The aim of this project was to enhance knowledge of the effect of accumulated charge on the surface conductivity, the surface charging/discharging behaviour and the surface charge decay behaviour of polymeric insulators. The project was carried out in 2 stages. Firstly surface charging/discharging transient currents were observed using a concentric cylindrical measurement system for uncharged PMMA and PMMA samples charged by exposure to corona. The charging/discharging transients were analysed to characterise possible conduction mechanisms and the DC steady state currents were used to calculate the surface conductivities. Charging the sample surfaces with corona led to an increase in the surface conductivities of the samples. The behaviour of the charging/discharging transients were complex and fitted well against 3 and 4 term exponential functions. Differences in behaviour were also observed due to sample thickness. In the second stage of the project surface charge decay measurements were performed on PMMA samples that were subjected to corona charging. This was done to determine the relative importance of surface conductivity and charge neutralisation to the decay of surface charge. A novel charge probe based on varactors was developed and calibrated to perform these measurements. This system was novel in its use on measurements of surface charge on polymeric materials. The measured surface potentials from the varactor probe were used to calculate the surface charge density distributions. It was observed that most of the charge accumulates near the electrodes. Analysis of the total surface charge showed that the rate of charge decay was higher when the decay occurs through the surface conduction mechanism as compared to conduction through gas neutralisation, where no significant charge decay was observed. The rates of charge decay for thin and thick samples, which were corona charged at a grid voltage of 1 kV, were observed to be 0.027 žµC/min and 0.048 žµC/min, respectively, for neutralisation through the gas; while the corresponding rates of charge decay through surface conduction were observed to be 0.096 žµC/min and 0.18 žµC/min, respectively. Evidence of the crossover effect has been observed in certain geometries. Correlations have been made between the charge decay measurements and the transient behaviours observed in the first part of the project. The charge decay behaviours were found to be in good correlation with the surface conductivities observed for corona treated samples where an external field was applied across the sample surface.