In order to obtain a law that the leakage current of long string insulators changes with salt deposit density（SDD） and altitude

we carried out the artificial contamination tests at different altitudes and different SDDs

and obtained the waveform data of leakage current. Then we changed the leakage current from the time domain to the frequency domain through Fourier transform. The analysis results of the leakage current amplitude-frequency and phase characteristics show that

with the increase of leakage current amplitude

the content of third harmonic decreases

the current waveform more approaches sine wave

and current and voltage phase difference is gradually reduced. When the leakage current amplitude is 54.23 m A

101.50 m A

and 355.28 m A

the third harmonic content is 47%

38.6%

and 20.6%

the current and voltage phase difference is –18.04°

–17.17°

and –13.68°

respectively. Through analyzing the insulator strings’ leakage current peak in different SDDs

it can be found that insulators’ contamination degree can be characterized by leakage current peak. Through analyzing the leakage current peak at different altitudes

it can be found that leakage current peak and altitude satisfy a certain formula. The results show that the insulator contamination flashover leakage current warning threshold should differentiate at different altitudes

the rule has a certain significance for safe and stable operation of insulators in high altitude areas.