Abstract
The water molecule migration and aggregation behaviors in oil-impregnated pressboard are investigated by molecular dynamics simulations in combination with Monte Carlo molecular simulation technique. The free energy and phase diagram of H2O-dodecylbenzene (DDB) and H2O-cellulose mixtures are calculated by Monte Carlo technique combined with the modified Flory-Huggins model, demonstrating that H2O molecules can hardly dissolved with infinitesimal content in cellulose system at temperature lower than 650 K, based on which the oil/cellulose layered structure with water impurity representing three-phase coexistence in oil-impregnated pressboard are modeled and performed for molecular dynamics. The molecular dynamics of H2O/DDB/cellulose three-phase mixture simulating oil-paper insulating system with H2O impurity indicates that DDB molecules can thermally intrude into the cellulose-water interface so as to separate the water phase and cellulose fiber. The first-principles electronic structure calculations for local region of H2O/DDB interface show that H2O molecules can introduce bound states to trap electrons and acquire negative charges, so that they will obtain sufficient energy from applied electric field to break DDB molecular chain by collision, which are verified by subsequent molecular dynamics simulations of H2O−/DDB interface model. The electric breakdown mechanism under higher than 100 kV/m electric field is presented based on the further first-principles calculations of the produced carbonized fragments being dissolved and diffusing in DDB phase. The resulted broken DDB fragments will introduce impurity band between valence and conduction bands of DDB system, evidently decreasing bandgap as to that of conducting materials in their existence space. The conductance channel of these carbonized DDB fragments will eventually be formed to initiate the avalanche breakdown process by the cycle-feedback of injected charge carriers with carbonized channels.
Highlights
Synthetic liquid insulating material such as synthetic oil is a mixture of many hydrocarbons, the purification and preparation process of which is complex
H2O/DDB/cellulose three‐phase coexistence in oil‐impregnated pressboard with water impurity are in high voltage transformers
Monte Carlo molecular simulations indicate that H2 O molecules cannot dissolved into cellulose system at temperature lower than 650 K, attributed to which the layered three-phase coexistence system of oil/cellulose interface structure with water impurity are modeled with molecular dynamics
Summary
Synthetic liquid insulating material such as synthetic oil is a mixture of many hydrocarbons, the purification and preparation process of which is complex. Because the general insulating oil is easy to burn and has low heat resistance and dielectric constant, it is necessary to study and develop novel kinds of liquid insulating materials with excellent properties [1]. The formation mechanism of dielectric properties of liquid insulating materials should be adequately understood. Under the condition of transformer operation, the insulating oil is inevitably dissolved with trace impurities (such as water), which determine the breakdown characteristics of the insulating oil by the distribution and state of these impurities [2]. The breakdown characteristics of oil insulation mainly depend on the impurity state and content in microsoluble insulating oil [3]. Transformer oil is a product of petroleum fractionation, the main components of which are alkanes, naphthenic saturated hydrocarbons, aromatic unsaturated hydrocarbons, and other compounds
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