The flashover criteria of the insulators are an important part of the lightning protection analysis methods. The leader propagation model recommended by CIGRE is widely used. However

this model hasn’t considered the high altitude

the average electrical field of the leader El and the height of the final jump hf. Thus

50% disruptive-discharge voltage U50% and volt/time curve for ±800 k V V-string and I-string composite insulators were measured under positive lightning impulses at the National Engineering Laboratory for UHV Engineering Technology（Kunming）. The experimental results show that the U50% of the V-string is lower than that of I-string

and the volt/time curve of V-string is steeper. An impr

oved leader propagation model is proposed based on the analysis of the flash over/discharge process of the insulator/gap

and the average electrical field of the leader Ul and the height of the final jump hf are considered in the improved leader propagation model. Based on the volt/time curve experimental results

the coefficient k and E0 for I-string in the positive improved leader propagation models are modified to 0.73′10

-6

m

2

/（V

2

·s） and 495 k V/m respectively and for V-string are modified to 0.53′10

-6

m

2

/（V

2

·s） and 440 k V/m respectively. The improved leader propagation models were proposed for the flash over criteria of I-string and V-string at high altitude.