Coupling influence of moisture and electric field on oil-impregnated pressboards in power transformers: High-field dielectric analysis and conductivity modeling

High-field dielectric response technique is a promising approach to explore mechanisms and assess the insulation states for power assets. Nevertheless, the coupling stress effect of moisture and applied field strength on the high-field dielectric dynamics for oil-impregnated pressboards (OIP) remains ambiguous. This article, therefore, offers a comprehensive investigation and modeling analysis of the high-field broadband dielectric dynamics for OIPs in the presence of diverse coupling stresses of moisture (0.81 % to 4.24 %) and field strength (0.2 kV/mm to 7 kV/mm that increases by 35 times than 0.2 kV/mm). Sensitives of dielectric dynamic behaviors (complex capacitance, dielectric loss tangent, and conductivity) to various moistures, field strengths, and frequencies are also studied. Moreover, linear regression models considering confidence and prediction bands are established to estimate quasi-DC conductivity under coupling effects. Experimental results show that nonlinear behaviors of dielectric dynamics for OIPs become more noticeable as a function of coupling stresses. The charge storage capacity of OIPs decreases while the contribution of conductance losses increases. The linear regression model can achieve high accuracy in estimating quasi-DC conductivity under coupling effects. The research provides a comprehensive insight into reliability-oriented insulation analysis for oil-immersed high-voltage power transformers.