An ultrasonic location method of partial discharge（PD）in transformer based on the improved threeplatform direction-finding principle is proposed. The method takes it into consideration that azimuth and elevation error will enlarge positioning error with the increase of the distance between PD source and platform. The distance between the auxiliary anchor point and the direction finding line is proportional to the average of the maximum peaks of received signals. The average of the maximum peaks of received signals from different sensors in same array is measured through laboratory simulation experiment

and the experiment needs to be conducted for several times if the distance between PD source and platform is different. When the average of the maximum peaks is obtained

the distance between PD source and platform can be easily achieved on the basis of the relationship curve. Considering that the position of auxiliary anchor point is on the common perpendicular and the distance between auxiliary anchor point and direction finding line is proportional to the average of the maximum peaks

a plane circle can be determined by three auxiliary anchor points following the three-platform direction-finding principle

and the center of the circle is taken as the position of PD source. Experiment verifies that the proposed method can improve the location success rate and accuracy of transformer PD position.