Aim of the work 1. To analyze the methods used for the quality and resource assessment of complex insulation; 2. To develop the models for the assessment and analysis of polarization features, dielectric parameters and moisture content in oil- paper insulation; 3. To develop the method for the assessment of moisture quantity by measuring the polarization characteristics; 4. To analyze the impact of water quantity on dielectric parameters of transformer insulation; 5. To classify the quality criteria of oil-paper insulation; 6. To identify the tendencies in variation of insulation’s dielectric parameters and moisture quantity; 7. To implement the developed models and methods in the electricity industry of Lithuania for the assessment of the transformers’ insulation quality and resource.