Abstract:Space charge has close relation with the trap distribution in the insulation material. The phenomenon of charges trapping and detrapping has attracted significant attention in recent years. Space charge and trap parameters are effective parameters for assessing the ageing condition of the insulation material qualitatively. In this paper, a new method for calculating trap distribution based on the double exponential fitting analysis of charge decay process and its application on characterizing the trap distribution of oil impregnated insulation paper was investigated. When compared with the common first order exponential fitting analysis method, the improved dual-level trap method could obtain the energy level range and density of both shallow traps and deep traps, simultaneously. Space charge decay process analysis of the insulation paper immersed with new oil and aged oil shows that the improved trap distribution calculation method can distinguish the physical defects and chemical defects. The trap density shows an increasing trend with the oil ageing, especially for the deep traps mainly related to chemical defects. The greater the energy could be filled by the traps, the larger amount of charges could be trapped, especially under higher electric field strength. The deep trap energy level and trap density could be used to characterize ageing. When one evaluates the ageing condition of oil-paper insulation using trap distribution parameters, the influence of oil performance should not be ignored.
In oil-paper insulation systems, it is easy to accumulate space/interface charge under a direct current (DC) electrical field. At present, direct measurement of space/interface charge for a thick multi-layer insulation system is not possible. It is necessary to study the multi-layer oil-paper insulation system via simulation method. In this paper, the space/interface charge simulation based on the bipolar charge transport model and a simulation parameter using FEM for the multi-layer oil–paper insulation system was proposed. The influence of electrical field strength, temperature, and the combined influence of the electrical field strength and temperature on the space/interface charge behaviors were analyzed, respectively. A new method for calculating the space/interface charge density and the total charge quantity of the multi-layer oil-paper insulation under the combined action of electrical field strength and temperature was presented. Results show that the interface charge density absolute value and the total charge quantity at steady state both increases with the electrical field strength and temperature in an exponential way, respectively. Besides, temperature has a more significant influence on the charge density and the total charge quantity than the electrical field strength. The electrical field strength–temperature shifting factor αT’ was introduced for the translation of the charge density curves or the total charge quantity curves to construct the charge density main curve or the total charge quantity main curve under the combined action of electrical field strength and temperature. The equations for calculating the charge density or the total charge quantity of the multi-layer oil-paper insulation was provided, which could be used to calculate the charge density or the total charge quantity under the combined action of electrical field strength and temperature.
Considering that the working temperature is much higher in high-voltage direct-current (HVDC) convert transformers than in conventional AC transformers, the combination of thermally upgraded paper and natural ester might be a better choice to withstand high temperature in long-term operation. This study compares the chemical structure, DC breakdown voltage, space charge and trap distribution of thermally upgraded paper aged in natural esters and mineral oil. The relationship between DC breakdown and accumulated charges was analyzed. Results show that the thermally upgraded paper aged in natural ester (NEI-TUP) has a higher volume resistivity and DC breakdown voltage than that aged in mineral oil (MOI-TUP). The NEI-TUP group has a larger deep trap density than the MOI-TUP group due to the chemical structure of natural ester. The larger deep trap density could contribute to the lower charge accumulation quantity and higher DC breakdown voltage. The thermally upgraded paper immersed in natural ester presents better insulation capability under thermal and DC electrical stress than that immersed in conventional mineral oil. This research indicated that the combination of thermally upgraded paper and natural ester might be a potential alternative for the mineral oil immersed paper in the future.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.