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.
Highlights
Oil-paper insulation has been widely applied in power transformers
The trap distribution of oil impregnated insulation paper based on the first-order exponential fitting analysis impregnated insulation paper based on the first-order exponential fitting analysis and the double exponential fitting analysis is compared
An improved method for calculating trap distribution based on the double exponential fitting
Summary
Oil-paper insulation has been widely applied in power transformers. It is reported that the space charge has great effect on the insulation property of the oil-paper insulation in convert transformer [1,2,3,4,5].Space charge means the excess electric charge over a region of space, rather than distinct point-like charges [6,7]. Space charge could severely distort the local electrical field, which causes the material degradation or leads to the insulation breakdown [1,2,3]. Oil-paper insulation gradually degrades under the combined stresses of thermal, electrical, and chemical during transformer routine operation [8,9,10]. It is found that the space charge injection and accumulation in oil-paper insulation is strongly dependent on its ageing condition [11,12,13,14,15]. Space charge may be an ageing condition indicator for the oil-paper insulation [1,8,9,10]. Ageing could introduce defects (physical and chemical) and lead to the rise of localised states in the material. Space charge are determined by the trap distribution [1,16,17,18,19], which is related
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