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Leader Propagation Models of Ultrahigh-voltage Insulator Strings based on Voltage/time Curves under Negative Lightning Impulses at High Altitude I. INTRODUCTION LIGHTNING strikes are a major cause of outages of transmission lines . It is thus important to improve the performance of lightning protection of transmission lines and thus ensure the safety and stability of the power grid. The lightning withstanding level and lightning outage rate are two parameters used to estimate the performance of lightning protection of transmission lines. The parameters are calculated by analyzing the lightning overvoltage. II. EXPERIMENTS 2.1 TEST EQUIPMENT Tests were carried out in the outdoor experiment field of the National Engineering Laboratory (Kunming) for Ultrahigh-voltage Engineering Technology at an altitude of 2100 m. The dimensions of the field were 197 m × 103 m. Impulse, power frequency and DC test equipment were set up at three separate, circumjacent points. In the middle was a door tower with height clearance of 70 m and width of 70 m. A ±800 kV simulation tower header was hung from the door tower. Here, a six bundled conductor line with a length of 25 m was suspended and adjusted within a height range from 45 to 60 m. The distance between sub-conductor lines was 550 mm. Figure 2 shows that there were two kinds of lightning impulse discharge paths for the I-string. One had a downward leader and an upward leader, and the other had two downward leaders and two upward leaders. In both cases, the attachment part of the attachment process was located in the middle of the gap. There was 50% probability of generating one leader in discharge in this experiment. IV. THE FORM OF THE FLOW CHART The template of the flow chart shows below. Table 1. Values of k and E0 recommended by CIGRE   Configuration Polarity k(m2v-2s-1) E0(kVm-1) Air gaps, post and longrod insulators + 0.8×10-6 600 - 1.0×10-6 670 Cap and pin insulators 1.2×10-6 520 1.3×10-6 Fig. 1 Diagrams and photographs of the experimental arrangement 2.2 TESTED SAMPLES The tested objects were composite insulators, set with grading rings. The insulators were arranged in V-type and I-type layouts, as shown in Figure 1. The dry arcing distance was 7.1 m for both the V-string and I-string. The surfaces of tested insulators were clear (without any contaminants). The insulator type and configuration were similar to those of the ±800 kV Yunnan-to-Guangdong UHVDC power transmission lines. V. CONCLUSION In this paper, the flashover test of the composite insulator used in ±800kV UHVDC transmission lines was carried out in an area of high altitude of 2100 m. U50, the voltage/time curve and constants of the leader propagation model of the I-string and V-string composite insulators were proposed, and the following conclusions were drawn. The negative U50 of the V-string is 7% higher than that of the I-string composite insulator for the same insulation length. The voltage/time curves of I-string and V-string composite insulators were obtained. The test results show that the voltage/time curves of the I-string and V-string are different even for the same insulation lengths, with the latter being above the former. These results do not agree with the constants of the leader propagation model listed in Table 1. III. Results (c) (a) (b) Fig. 2 . Typical lightning impulse discharge processes of the I-string. a, attachment process in the case of one upward leader and one down leader under 4229 kV. b1, leader process, and b2, attachment process in the case of two upward leaders and two downward leaders under 3391 kV. The time interval between b1 and b2 was 7 μs. Tel： E-Mail：