Thermal ageing study on palm oil methyl ester-impregnated thermally upgraded kraft paper insulation for reliable application in HVDC converter transformers

Exploration of sustainable insulation solutions for HVDC converter transformers has prompted the investigation of palm methyl ester (PME) as a viable alternative to traditional mineral oil. This study presents a detailed thermal ageing analysis of PME-impregnated thermally upgraded kraft (TUK) paper, focusing on critical elements of the oil–paper insulation system. The research emphasizes the importance of interactions between the oil and paper components, particularly in influencing long-term insulation performance and reliability under HVDC stress. Acidity measurements confirmed PME’s chemical stability over time, while scanning electron microscopy (SEM) provided insights into ageing-induced morphological changes in TUK paper, affecting its mechanical integrity and dielectric strength. The degree of polymerization was evaluated to assess the extent of chemical degradation, crucial for predicting the insulation system's lifespan. Furthermore, the study examined key electrical properties, including DC breakdown voltage, quasi-DC conductivity using the polarization depolarization current method, and space charge dynamics analysed with pulsed electro-acoustic techniques. Results showed that PME-impregnated paper offered superior performance over mineral oil-impregnated paper, with a higher DC breakdown voltage, lower quasi-DC conductivity, more stable charge distribution, and a significantly reduced electric field distortion rate (28% vs. 16%). These findings underscore PME’s potential as a viable, eco-friendly insulation solution for HVDC applications. The study advocates for further investigation into PME in oil–paper systems, emphasizing the pivotal role of these specific interactions and properties in enhancing the development and efficiency of HVDC technologies.