Time delay estimation is the basis of the localization algorithm for partial discharge（PD）in GIS based on time difference

and it is the key to the accuracy of localization.In this paper

the theoretical basis of employing fractional Hilbert transform for time delay estimation is described

an algorithm for estimating the time delay of ultra high frequency partial discharge in GIS is proposed based on fractional Hilbert transform

and corresponding numerical implementation steps of the algorithm are given.According to the abnormal PD signals found in the switch air chamber during an ultra high frequency PD inspection of a certain 220 kV GIS equipment

high speed oscilloscope is adopted to collect high frequency electromagnetic wave signal at pouring holes of the disk-type insulator on the two sides of the switch air chamber.According to the time delay of the signal

the space position of the PD source is located 1.25 meters away from the disk-type insulator pouring hole.After disintegration of the switch air chamber

the distance is measured as 1.3 meters.The location error of PD source based on fractional Hilbert algorithm is significantly lower than that based on the bi-spectrum time delay estimation algorithm and the interpolation cross-relation time delay estimation algorithm

respectively.