Experience and applications of the currently available methods for on-line monitoring and diagnosis of high voltage power apparatus are briefly reviewed. Based on the dielectric physics

a new definition of dielectric loss factor with harmonics feature is proposed to evaluate the operational conditions of power apparatus on a more reasonable basis

along with corresponding digitised algorithms. Two approaches

namely the decoupling algorithm and the approximating algorithm

are presented to implement the methodologies with the new definition of dielectric loss factor. The impacts of background noise and power frequency fluctuation on estimation accuracy are analysed by computer simulations showing better application qualities of the approximating algorithm than the decoupling one. A comprehensive strategy of simultaneous monitoring of sum current

dielectric loss factor

resistive current and capacitive current is recommended to give an effective assessment and diagnosis.