Effects of electric arc on discharge inception, combustible gases and chemical structure of transformer oils

Transformer oil is an important part in a high voltage transformer. Under excessive condition, electric arc may occur in the transformer oil. The electric arc may reduce the performance of the transformer oil. This paper explains the experimental results on the effects of electric arc on the partial discharge (PD) inception, the dissolved combustible gases and chemical characteristics of mineral transformer oil. In the experiment, electric arc was generated under AC voltage using needle-pale electrode system to produce highly divergent electric field. The arc treatment was adjusted by the distance between electrode and the duration of arc application. The experimental results showed that arc treatment significantly reduced the PD inception voltage. The electric arc also intensively produced combustible gas such as methane (CH₄), hydrogen (H₂), ethane (C₂H₆), ethylene (C₂H₄) and acethylene(C₂H₂). The concentration of released combustible gases from the lowest to the highest is C₂H₆ → CH₄ → H₂ → C₂H₄ → C₂H₂. The concentrations of the last 2 combustible gases are much higher than the others. The results indicated that ethylene (C₂H₄) and acethylene(C₂H₂) are the finger prints of the arc in transformer oil. From FTIR spectroscopy it was found that electric arc significantly reduced the aliphatic (C-H) content indicated by the increase of transmittance at wave number of 3000-2850 cm^-1. The experimental results also clearly indicated the strong correlation between the reduction of aliphatic (C-H) content and the total combustible gases (TCG).