A Novel Curve of Moisture Equilibrium of Oil Paper Based on the Effect of Electric Field

Moisture equilibrium curve of oil paper (MEC-OP) is an important and commonly used foundational curve in transformer design and condition assessment. For AC transformers, the MEC-OP is weakly affected by the electric field, so only considering the temperature. However, for the converter transformer, the DC electric field present inside has not yet been considered. Therefore, Based on a new method of moisture distribution measurement by terahertz spectroscopy, a novel curve of MEC-OP considering the effect of DC electric field is obtained. The results show that the moisture in oil and paper increases as the field strength grows, the reason leading to the rise of moisture may be the effect of electric field force and dielectrophoretic force, while molecular simulation calculation is employed to support in explaining the experimental phenomena. Then, the coefficient of the field strength affecting the moisture content is introduced to correct the MEC-OP, and the moisture of cardboard calculated at 25 kV/mm under 40 °C, 60 °C, and 80 °C increases by 15.65%, 19.68%, and 32.92%, respectively. This research fills a gap in the effect of DC electric field on the MEC-OP under the converter transformer and provides support for the accurate calculation of the moisture in insulating paper.