Mechanism of assessing moisture in transformer insulation by return voltage method

Water of oil paper insulation in transformer will accelerate its insulation ageing, which is the key factor for its failures. Therefore, it is necessary to estimate water content of solid insulation for guaranteeing a safety and reliable operation of power transformers. The microstructure of insulation molecules can be varied by water, which affects the conductivity and polarization properties. Return voltage measurement based on dielectric response theory can detect variation of dielectric properties of insulation material. The paper analyze dielectric response properties of oil-paper transformer in detail, and describes the relationship between water content and polarization characteristic quantities, finally, reveals the mechanism of diagnosing moisture content of solid insulation in transformer using return voltage measurement. The moisture content have a great influence on polarization characteristic quantities, especially, the relaxation time is an important characteristic quantities for assessing solid insulation condition.