The effects of insulator structure and insulating oil to charge accumulation in oil-immersed transformers

We directly measured the electric field in transformer oil/pressboard (PB) system under dc voltage using an electro-optic method by Kerr effect. We measured time variations and spatial distributions of electric field in the insulating oil. We measured them in both fresh oil/PB and degraded oil/PB composite systems to clarify the effect of material characteristics. Additionally, we changed the PB arrangement to investigate the effect of the PB insulator structure. We obtained the time decaying characteristics in the electric field distribution. From the results, we discussed physical mechanism of the charge accumulation.