Molecular Dynamics Simulation on High Temperature Pyrolysis of Transformer Insulating Oil under Thermal Stress

In the long-term operation of the transformers, the electrical and mechanical properties of its insulating materials gradually deteriorate due to the influence of thermal stress and other factors. Transformer insulating oil will decompose to generate a variety of small molecular substances at high temperatures. These small molecular substances will form bubbles in the insulating oil, and small gas bridges will be formed in severe cases, resulting in transformer insulation discharge. Therefore, the researches on the pyrolysis laws of insulating oil under thermal stress are helpful for early detection of latent thermal faults in power transformers, thereby reducing the probability of insulation faults in power transformers. In this paper, the molecular dynamics method based on Reactive Force Field (ReaxFF) is used to establish the models of mineral oil containing three typical hydrocarbon molecules, and the constructed molecular models are subjected to high-temperature pyrolysis studies at four different temperatures of 2200K, 2400K, 2600K and 2800K. In computer simulations, the main pyrolysis pathways of transformer insulating oil are summarized by using element tracing technology and simulation images. The main conclusions are as follows: (1) The three hydrocarbons are all cracked under high temperature, and the order of pyrolysis is paraffin, naphthenic, and aromatic hydrocarbons; the reaction pathways of the insulating oil pyrolysis are roughly divided into two stages: 1) In the initial stage of pyrolysis, three hydrocarbons mainly undergo dehydrogenation and chain scission reactions, and a large amount of small molecular chain hydrocarbons are generated. 2) In the late stage of pyrolysis, the secondary pyrolysis occur in small molecule chain hydrocarbons, the ring-opening and pyrolysis occur in cycloalkanes and aromatic hydrocarbons. (2) With the temperature increase, the number of small molecule gases generated by pyrolysis gradually increases; C20 is first decomposed into C10~C19, and then C10~C19is decomposed into smaller molecules C1~C9; most of the carbon atoms in the system are collected in C1~C3at the end of pyrolysis, which proves that Cl~C3are the final products of insulating oil pyrolysis.