Synergistic enhancement effect of moisture and aging on frequency dielectric response of oil-immersed cellulose insulation and its degree of polymerization evaluation using dielectric modulus

Oil-immersed cellulose insulation is the heart of the power transformer, which will will gradually deteriorate in the operation process of transformer. Therefore, accurately evaluating the aging condition of the oil-immersed cellulose insulation is important for maintaining safe and stable operation of transformer. In this paper, the synergistic effect of moisture and aging on frequency dielectric spectroscopy (FDS) of oil-immersed cellulose insulation was analyzed in depth using the modified cole–cole model. A novel degree of polymerization (DP) evaluation method for oil-immersed cellulose insulation based on the dielectric modulus analysis and its curve translation method was proposed. Results show that the higher the moisture content and the lower the DP, the greater the frequency dielectric parameters, the smaller the α relaxation time, the greater the DC conductivity and hopping conductivity of the oil-immersed cellulose insulation. The novel DP evaluation method for oil-immersed cellulose insulation based on calculating the integral area of the imaginary part of dielectric modulus A (DP–mc%) and translating the A (DP–mc%) curve was proposed, which could simultaneously consider the synergistic effect of moisture and aging on FDS. The novel DP evaluation method for the oil-immersed cellulose insulation is suitable for moisture range is 0.5–5%, and the DP range is 250–1300. The verified effectiveness of the novel DP evaluation method presents that it is ideal for transformer aging condition assessment, which is superior to the existing evaluation methods. Graphic abstract