Moisture and Aging Degree Evaluation Method for Oil-Paper Insulation Based on HV-FDS Characteristics Analysis

Oil-paper insulation is the “heart” of power transformer, and frequency domain spectroscopy (FDS) is an efficient method to evaluate the insulation state of it. Nevertheless, traditional FDS test voltage is still at 200 V, the insulation characteristics of FDS at high voltage levels are worth exploring. Therefore, this paper studies the FDS characteristics of oil-paper insulation at various test voltage and proposed an evaluation method of moisture and aging degree of oil-immersed pressboard based on high voltage FDS. The results show with the increase of test voltage, the C ’ of mineral oil increases from 1 mHz to 10 Hz while of oil-immersed pressboard and “liquid-solid” oil-paper insulation decreases below 10 Hz; tan δ of all samples increases significantly with the increase of test voltage. The tan δ at 0.1, 1 and 10 Hz is selected as characteristic parameter to evaluate the moisture content of oil-immersed pressboard, tan δ at high voltage is more sensitive to moisture content compared with that at low voltage, tan δ at 0.1 Hz and 7000 V increases 1130 times compared 75 times at 0.1 Hz and 200 V. The integral of tan δ, S _tanδ , from 0.1 Hz to 10 Hz is selected as parameters to evaluate the aging degree. The oil-immersed pressboard and distribution transformer are tested and evaluated by above evaluation method, and the results show that the error of moisture content and aging degree evaluation result is less than 10%.