| dc.description.abstract | High-Voltage (HV) outdoor insulation systems play a critical role in ensuring the reliable transmission and distribution of electrical power. In recent decades, polymeric composite insulators have increasingly replaced conventional porcelain and glass insulators due to their lightweight structure, low cost, ease of installation, superior pollution performance, and inherent hydrophobicity. A typical polymeric outdoor insulator consists of a fibre-reinforced plastic (FRP) rod that provides mechanical strength, a silicone rubber (SiR) housing responsible for electrical insulation and surface performance, and metal end fittings for electrical and mechanical connection. Despite their advantages, the long-term reliability of polymeric insulators remains a major concern, particularly under harsh outdoor conditions involving combined electrical, environmental, thermal, and mechanical stresses. These stresses can significantly alter the properties of electrical materials, leading to aging, degradation, and premature failure.
With the growing deployment of high-voltage direct current (HVDC) transmission systems in addition to conventional high-voltage alternating current (HVAC) networks, polymeric insulators are now subjected to more severe and complex stress environments. HVDC operation introduces additional challenges such as greater pollutant accumulation, surface charge accumulation, polarity reversal, enhanced electric field distortion, and intensified surface discharges. Therefore, a comprehensive understanding of material degradation mechanisms, electric field stress distribution, and effective mitigation strategies is essential for improving the long-term performance and reliability of polymeric outdoor insulators.
The primary objective of this thesis is to investigate the properties and behavior of electrical materials used in high-voltage polymeric outdoor insulators, with particular emphasis on degradation mechanisms, electric field stress control, and surface engineering approaches for performance enhancement under both HVAC and HVDC operating conditions. The work integrates experimental investigations, physicochemical and mechanical characterization, and numerical modeling using the Finite Element Method (FEM).
The first part of the thesis focuses on the failure analysis of in-service failed 400 kV HVAC composite insulators. Detailed visual inspection and pollution severity assessment were carried out, followed by comprehensive physicochemical and mechanical characterization. Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDS), Fourier Transform Infrared Spectroscopy (FTIR), and Thermogravimetric Analysis (TGA) were employed to investigate surface morphology, elemental composition, chemical bond degradation, and thermal stability. Mechanical properties were evaluated using hardness and tensile strength measurements. The results revealed severe surface erosion, depletion of alumina trihydrate (ATH) fillers, oxidation and chain scission of silicone rubber, moisture ingress at the sheath-core interface, and degradation of the FRP core, ultimately leading to decay-like fractures. FEM simulations demonstrated significant electric field intensification at triple junctions, shed-shank interfaces, and defect regions such as cracks and voids. Based on these findings, an optimized corona ring design was proposed, which effectively reduced peak electric field stress and improved insulation reliability. The outcomes of this part of the work were published in IEEE Transactions on Dielectrics and Electrical Insulation and Electrical Engineering (Springer), as well as in two international IEEE conference proceedings.
The second part of the thesis investigates the long-term performance of polymeric insulators under HVDC stress, with special emphasis on polarity reversal effects. Accelerated aging experiments were conducted using rotating wheel and dip test facilities for durations up to 625 hours. Leakage current behavior, discharge activity, thermal response, and surface degradation were systematically monitored. The results showed that HVDC operation significantly exacerbates charge accumulation, dry-band formation, and localized surface discharges, leading to a substantial increase in leakage current by more than two orders of magnitude and failure of the samples. Physicochemical analyses using FTIR, SEM, EDS, and TGA confirmed severe chemical degradation, including breakdown of Si–O–Si and Si–CH₃ functional groups, oxidation, filler loss, micro-crack formation, and erosion along the leakage path. These observations clearly demonstrate that polymeric materials are more susceptible to electrochemical degradation under HVDC stress, particularly in polluted environments and during frequent polarity reversals. This work led to a journal publication in Engineering Failure Analysis (Elsevier) addressing failure mechanisms of polymeric insulators in HVDC converter stations.
To address electric field intensification issues, the third part of the thesis explores the application of nonlinear resistive field grading materials (FGMs) based on ZnO microvaristors. A novel mathematical formulation was developed to represent the field-dependent conductivity of FGMs, incorporating conductivity saturation effects at high electric field levels. FEM studies were conducted to analyze the influence of key material parameters, including switching threshold, nonlinearity coefficient, and initial conductivity, on electric field distribution. The results demonstrated that appropriately tailored FGMs can significantly suppress electric field peaks at critical locations under dry, wet, polluted, dry-band, and overvoltage conditions, thereby offering an effective and practical approach for electric field stress mitigation in high-voltage composite insulators. This work resulted in multiple journal and conference publications, including IEEE Transactions on Dielectrics and Electrical Insulation and the CIGRE India Journal, along with IEEE international conference contributions.
The final part of the thesis presents the development of a novel silica-based superhydrophobic coating for polymeric outdoor insulators. The coating was fabricated using silica nanoparticles and polydimethylsiloxane (PDMS) and systematically optimized in terms of nanoparticle concentration, PDMS content, and solvent ratio. Surface characterization confirmed the formation of a robust micro-hierarchical structure with exceptional hydrophobicity, exhibiting a static contact angle of approximately 160° and a sliding angle of about 7°. The coating demonstrated superior dielectric properties, excellent resistance to ultraviolet radiation, chemical exposure, and mechanical stress. Electrical performance evaluation through surface resistance measurements, leakage current analysis, and flashover voltage tests revealed significant performance enhancement under various operating conditions. Furthermore, long-term 1000-hour multi-stress aging tests under AC and DC voltages confirmed substantial reductions in leakage current, suppression of discharge activity, and excellent retention of hydrophobicity, while uncoated insulators exhibited severe degradation. This part produced three publications in IEEE Transactions on Dielectrics and Electrical Insulation and two international conference publications, including CIGRÉ 2024 and IEEE CATCON proceedings.
In summary, this thesis provides a comprehensive understanding of the characteristics of electrical materials governing the performance of high-voltage polymeric outdoor insulators. By integrating detailed failure analysis, electric field stress mitigation using optimized corona rings and nonlinear FGMs, and advanced surface engineering through durable superhydrophobic coatings, the work establishes scientifically robust strategies for enhancing the long-term reliability and durability of polymeric insulation systems used in modern HVAC and HVDC transmission networks. | en_US |
