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The present work aims at investigating the single excessive hydrogen (SEH) mechanism in the current transformers (CTs) by reactive force field molecular dynamics (ReaxFF MD) simulations. The chemical components in the CTs, i.e. mineral insulating oil, insulating paper, nickel crystal and moisture were established. Temporal evolutions of dissolved-gas-analysis (DGA) characteristic gases were captured reasonably in the mono-component, dual-component and multi-component simulation systems under electrical stress. And the sources of H2 molecules were traced as well as their dehydrogenation reactions. In addition, the oil chromatogram and dismantle experiments were performed to verify the dynamics simulations. The relative contents of the gas productions in the all-component composite simulation system were well agreed to that from oil chromatogram measurement. The results identified that the nickel-catalyse dehydrogenation from insulating oil played the dominant role in SEH phenomenon. The Nickel promoted the generation of H2 by seizing H atoms from oil molecules under the imposed electric field. In the process of dehydrogenation, the insulating oil molecules reacted to produce macromolecule wax-like solid unsaturated hydrocarbons, which were found in the insulating paper of a dismantled CT. Keywords: Single hydrogen excessive, Electrical stress, ReaxFF molecular dynamics simulation, Nickel-catalysed dehydrogenation, Dissolved gas analysis