Performance of nanofillers embedded polymer for HVDC applications

  • Xiaosi Lin

Student thesis: Doctoral Thesis

Abstract

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.
Date of Award10 Jun 2022
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorWah Hoon Siew (Supervisor) & John Liggat (Supervisor)

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