Impact of moisture and thermal ageing on space charge characteristics in natural ester liquid-impregnated paper insulation system

Abstract

The HVDC technology has facilitated the long-distance transmission of large amounts of power. This benefit of HVDC technology allows for the decentralisation of energy resources, enabling the utilisation of remote renewable resources such as offshore wind farms. To keep the HVDC power transformer operating reliably, it is crucial to design the insulation system effectively. In HVDC power transformers, the predominant dielectric materials are cellulose and mineral oil. Mineral oil is still utilised in HVDC power transformers due to its strong dielectric strength and cooling function. However, the limitations of mineral oil, such as poor biodegradability, high fire risk, and scarcity, have motivated the development of an alternative. As a consequence of its high biodegradability, low fire risk, and renewability, natural ester liquid has been seen as a promising dielectric liquid to replace mineral oil. Despite this, more research is being conducted to determine the viability of using natural ester liquid in the HVDC power transformer. Therefore, a comparative study between natural ester liquid and mineral oil is critical. The HVDC power transformer, unlike the AC system, is subjected to AC, DC, and AC-DC combined stresses. Under DC conditions, space charge accumulation is a common occurrence in dielectric materials. Space charge accumulation is undesired in the oil-paper insulation system because it enhances the electric field in certain areas, which accelerates the insulation's degradation and may even cause it to fail prematurely. To study the space charge characteristics, the pulsed electroacoustic (PEA) technique was applied. One of the most unwelcome by-products of oil-paper insulation systems is moisture, which aids the penetration of space charges deeper into the oil-impregnated paper. Natural ester liquid is hydrophilic and may absorb water actively from cellulose. As a result, investigating space charge characteristics in natural ester liquid-impregnated paper and mineral oil-impregnated paper under moisture impact circumstances is critical. During the HVDC power transformer operation, hot spots might form close to the windings. Therefore, the long-term operation of the HVDC power transformer results in the thermal ageing of oil-paper insulating systems. The space charge characteristics of oil-impregnated papers can be influenced by a number of variables, including physical and chemical impacts under thermal ageing conditions. Comparing natural ester liquid with mineral oil under thermal ageing circumstances can provide additional data for determining if natural ester liquid can be employed as the dielectric liquid for HVDC power transformers. This thesis investigated the effect of moisture and thermal ageing on natural ester liquid-impregnated papers by comparing it with mineral oil-impregnated paper. Moisture and thermal ageing are considered major factors in deteriorating insulating materials. Natural ester liquid and mineral oil have different chemical structures, which react differently with moisture and thermal ageing. Especially under different moisture conditions, natural ester liquid delayed the accumulation of heterocharges compared to mineral oil. This indicates that natural ester liquid can behave as an inhibitor of heterocharge accumulation in the oil-paper system. Natural ester liquid has ester linkages in its chemical structure, making it more polar than mineral oil. Hence, natural ester liquids can dissolve more by-products like water and acids because of their greater polarity, contributing to improved cellulose conditions during thermal ageing. Regarding the space charge decay, natural ester liquid showed a faster charge dissipation rate than mineral oil. When the polarity is reversed, faster charge dissipation of natural ester oil can help mitigate the negative effect of polarity reversal. From a dielectric point of view, natural ester liquid can provide advantages such as faster charge dissipation, an inhibitor for heterocharge accumulation, and higher breakdown strength. However, additional research is still required to accept natural ester liquid for HVDC converter transformers fully.

More information

Identifiers

ORCID for Hyungjin Yoon: orcid.org/0000-0002-6097-8487

Catalogue record

Export record