Electric stress control in high voltage insulators using multi-layered functionally graded materials

The majority of transmission and distribution network owners adopt high voltage insulators as a means of insulating the live conductors in their overhead line systems. The performance of dielectric material, which is influenced by the high electric field strength, has become a major concern in recent years. Therefore, as shown in the literature, the electric field stress should be maintained below the maximum electric field level in order to avoid surface discharges in the high voltage insulators. Currently, the existing field stress control methods have achieved mixed results in mitigating the field enhancement on the surface of the insulator or inside the insulation materials. The advantages and disadvantages of the various existing field enhancement control methods have been discussed in the literature. Functionally graded materials (FGMs) are advanced composites resulting from the gradual modification of the structural and/or chemical arrangement of materials or elements to meet specific environmental requirements. They vary continuously as a function of spatial position and their application in electric field stress control in various high voltage applications has recently been proposed.<br><br> The main objective of this research is to carry out comprehensive theoretical and experimental studies involving the investigation of the dielectric properties of FGM in electrical insulation systems. In this work, silicone rubber is used as the base polymeric material and nano-sized Zinc Oxide (ZnO) and Barium Titanate (BaTiO3) are used as fillers. Detailed analysis will be carried out to obtain the electrical properties, such as the conductivity and permittivity, of the material as a function of the applied voltage. The introduction of ZnO and BaTiO3 nanofillers to a host matrix (silicone rubber) increased the permittivity of the dielectric materials by 5% and 14% respectively, compared to pure silicon rubber. Two groups of hybrid nanocomposite samples are fabricated by mixing both ZnO and BaTiO3 into pure silicon rubber. The electrical properties of the hybrid nanocomposite material are improved by 33% over those of the pure silicone rubber. The dielectric properties (permittivity and conductivity) of single-layered ZnO nanocomposite, BaTiO3 nanocomposite and hybrid nanocomposite are used to fabricate multi-layered functionally graded materials. In this study, two types of multi-layer functionally graded materials are synthesised: fully cured multi-layered FGM and half cured multi-layered FGM.<br><br> The electrical performance of multi-layered FGM and a selection of single-layered nanocomposite specimens are tested. Other characterisation methods such as the AC breakdown voltage measurement and partial discharge measurement, as well as the DMA methods are carried out. The results of this research show that half cured multi-layered FGM gives the best electrical performance among all of the specimens studied in this thesis.<br><br> The Finite Element Method (FEM) is used to compute the electric field stress and potential distributions within and along the insulating material. Finally, taking all these factors into consideration from both experimental results and simulation analysis, a polymeric insulator prototype with multi-layered FGM is fabricated and tested under various flashover voltage conditions, and compared to an insulator prototype without FGM. The results of AC and impulse flashover tests under a dry surface are indicated an enhancement of 17% and 25%, respectively. Under wet conditions, an improvement of the AC and impulse flashover withstand voltages of 11% and 13% is achieved. This proposed solution significantly reduces the electric stresses of polymer insulators and improved the flashover withstand voltage of the insulator prototype.