Moisture and aging identification of oil-paper insulated bushing based on frequency domain spectroscopy

Frequency domain spectroscopy (FDS) has attracted great attention in assessing the insulation state of oil-immersed power equipment. However, it is difficult to identify the moisture and aging degree of oil-paper insulation, because of behaving similar FDS results. This paper studies the dielectric response characteristics of oil-paper insulation with different degrees of moisture and aging under AC voltage with sweeping frequency from 1 mHz to 5 kHz, exhibiting nonlinear FDS phenomenon. It seems to be caused by the cellulose barrier effect and the Poole-Frenkel effect. Some characteristic parameters are extracted to distinguish between moisture and aging. Based on the difference of FDS results, the related moisture and aging degrees are identified via the nonlinear loss peak value. Finally, an evaluation model is established to diagnose the insulation condition of oil-paper insulated bushing.