The principle of on-line monitoring on dielectric loss tangent of capacitive equipment is introduced as well as the source of high-frequency pulse and random signals which affect the measurement result.Two preprocess methods based on statistical principle

which can eliminate the affection to the measurement result from high-frequency pulse and random signals

are mentioned.As a result of the analysis and comparison of various methods

it can be seen that it is not good for disposing the monitoring data with Fourier transform barely.And if the data is preprocessed with the two methods based on statistical principle before distilling the fundamental wave phase angles of current and voltage signals with Fourier transform

the numerical value is obtained which is very close to the real value of tanδ

and its variance is smaller

no singular point appeared.Therefore

this method is more exact and reliable than other methods mentioned.