A modified X-model of the oil-impregnated bushing including non-uniform thermal aging of cellulose insulation

The frequency domain spectroscopy (FDS) technique is widely used for the aging condition assessment of cellulose insulation on the bushing core. Generally, these assessment activities could be completed by the analysis of the FDS data between the laboratory sample and field bushing with the help of the traditional X-model. However, the aging condition of cellulose insulation on the bushing core is non-uniform due to the existence of temperature gradient. In this case, the traditional X-model is considered inappropriate since it ignored the effect of non-uniform thermal aging. In view of this issue, a modified X-model is reported in this work. Relying on the theoretical analysis of the modified X-model, both the mathematical expression and the equivalent circuit is derived naturally. The reliability of the modified X-model is confirmed by the experimental researches. In addition, the effect of aluminum foil in the modified X-model on FDS data was also studied. Findings reveal that the addition of aluminum foil would cause a slight deviation of the FDS curves. Therefore, the modified X-model might utilize to study the information of the non-uniform thermal aging. In that respect, the contribution of this work is in the exploration of the modified X-model as a potential tool for connecting FDS data corresponding to the lab condition and field condition including non-uniform thermal aging of cellulose insulation on the bushing core.