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This paper describes the unique characteristics of the oil-paper insulation system utilized in power transformers, their influence on the dielectric response and means for proper modeling of these characteristics. Dielectric response measurement between windings of a power transformer is used to estimate the water content of the solid insulation. By measuring the dielectric response over a wide frequency range different properties of the insulation materials can be determined. These properties can be modeled in a laboratory so that measured response can be compared to modeled response with known properties. This comparison enables the estimation of properties of the materials such as water content and conductivity. The influences of temperature, aging and insulation geometry are described. Case studies are presented which demonstrate the validity of the test method. The influence of frequency range, temperature, aging byproducts, oil conductivity and insulation material type on the accuracy of the moisture estimation is discussed.