Experimental Study Of Breakdown Characteristics Of Long Air Gaps And Their Applications In Lightning Protection

In order to resolve the imbalance of energy source and power demand in our country, The UHV transmission line has been the emphasis of in our country’s power system development strategy to meet the needs of interconnection of power system and large capacity power transportation. The operation experiences at home and abroad indicate that the trip outs are mainly caused by lightning with the increase of the voltage class. The TEPCO observed its common-tower double-transmission lines of HUV for ten years and found that almost all the lightning strip-out are caused by shielding failure. At present, countries all over the world and some international organizations such as the IEC and IEEE use the EGM to calculate lightning shielding performance of transmission line. However, the result calculated by TEPOC with EGM is far from the real situation, which indicate that the striking distance formula deduced from the results of1～4m air gaps discharge experiments may not be suitable to the UHV transmission line with large scale. Therefore as the transmission line developed from HV, EHV to UHV, Some parameters in the original EGM are no longer accurate and can not be used to design the lightning shield performance of new lines. So it’s necessary to modify some parameters in EGM according to the operating experience and test data of longer air gap to improve the applicability of EGM in engineering. The main research work in this paper are as follows:(1) In this paper test study on the lightning shielding performance of small-scaled500kV AC transmission line is carried out.500kV AC transmission line as object of study, a series of simulation experiments are carried out for studying the impacts such as the direction of the electrode along the span placement, the incident angle of lightning leader, line protection angle and ground tilt angle. Results of nearly100thousand times of discharge show that:the shielding failure rate initially increase and then decrease with the distance between the tower head and the span central. The best electrode location that reflect the average shielding failure rate is found, and the regular pattern is got that the shielding failure rate increases with the tilt angle of the electrode and the line protection angle, which also increases initially and then decreases with the ground tilt angle. The residue upward leader incepted from the surface of conductors which fails to intercept the downward leader and the primary discharge can be observed at the same time. (2) An experiment on characteristics about the shielding failure spatial distribution in the500kV AC transmission line is conducted. Scaling down the500kV AC transmission line based on the ZB6T tower with the ratio of1:80,1:40and1:25respectively, this paper conducted a scale of discharge test on these three models by means of standard lightning surge and standard switching surge separately. Then the shielding failure spatial distribution is drawn, and the experimenting phenomenon is consistent with the shielding failure spatial distribution law which is calculated and analyzed using the classic EGM theory. Assuming that lightning leader is distributed evenly in space, this paper calculates the shielding failure ratewith the area of the spatial distribution. According to the calculation results and theoretical analysis, it’s recommended that for the small-sized lightning shielding model tests which has short discharging gap, we’d better use the negative polarity standard lightning surge to do the experiment. In this way, the test result is lessaffected by the model size with a good linearity.(3) The negative switching surge discharging tests of three kinds of typical electrode are carried out. For the three typical rod-rod, rod-plane and rod-conductor gaps, we use the-20/2500μs and-80/2500μs operating impulse wave to have the discharging tests with the largest air gap of10meters. And we apply the up and down method to get the50%flashover voltage of the gaps. The three typical gaps’curves of negative polarity50%flashover voltage and gap distance are drawn, and it’s found that the gap’s negative polarity flashover characteristics have an obvious tendency to level off. When the gap distance is larger than4meters, for the same gap distance-80/2500μs operating impulse surge makes a lower50%flashover voltage than that of-20/2500μs operating impulse surge. The flashover characteristic curves of the rod-rod and rod-plane gaps turn to reverse when the gap distance is4～5meters around. The grounding of the conductor electrode obviously reduces the negative polarity flashover voltage. The stepped discharging process occurs in the tests and the gap’s mean strength of breakdown electric field decreases as the gap size grows. According to the T-test results, with the change of gap distance, how the height of the lower rod electrode impacts on the rod-rod gap’s negative polarity flashover characteristics doesn’t show much regularity.(4) In this paper the breakdown time of negative polarity of rod to rod air gaps were counted statistically, and compared to the pictures of negative flashover taken simutanously by a high speed camera. The time of negative flashover and the step numbers in this progress increased with the front time. Based on the definition of striking distance, when to modify the striking distance formula with the results of long air gap tests, the most ideal condition is a discharge without stepped leader. So, it’s better to use short front switching impulse tests to simulate the last step of the lightning, and in this paper, we recommend-20/2500μs switching impulse test to simulate the lightning discharge.(5) The striking distance formula was modified according to the results of rod to rod and rod to plane air gaps switching impulse tests. The negative discharge characteristic of rod to rod air gaps (h=4.5m) under-20/2500μs switching impulse test is U50%=0.9667×d0.614. According to the lightning leader potential formula V5=60(I/v1)×ln(2r’/d’), which is proposed by Wagner, and the probability distribution of the return strike velocity in lightning channels proposed by Idone, the author gets the modified striking distance formula of UHV transmission line with large scale: rc=(0.286/v1)1.63I0.63.(6) According to the-20/2500μs switching impulse discharge test results of the rod-rod gap and rod-plane gap, the existing striking distance factor is fixed. And this paper establishes the relationship between50%negative breakdown voltage of rod-rod gap (h=4.5m) and of rod-plane gap under-20/2500μs switching impulse voltage and gap distance d, in addition to the relationship between50%negative breakdown voltage of rod-rod gap (h=4.5m) and rod-rod gap (h=7.3m) and gap distance d. Assuming lightning strike distance is100m, earth striking distance factor is1.25, and lightning conductor correction factor is1.0.(7) The calculation of lightning shielding performance for double circuits transmission lines of UHV on the same tower. The lightning shielding properties of Japanese UHV and double circuite transmission lines of500kV on the same tower are calculated by A-W classical model, modified A-W classical model in paper[8] and modified EMG model in this paper, and the results are compared to the real situation. Though all the results are close to the reality, the distributions of the shielding failure rate and trip out rate of three phase calculated by the modified EMG model in this paper are more accurate than A-W classical model, which reflect the superiority of the modified EMG model in this paper for large scale transmission lines. With this model we calculate the shielding performance of the SZT1common-tower double-transmission lines of UHV in our country and the results show that:when the two transmission lines have the same shielding angle, the shielding failure rate in our country is much smaller than that in Japanese UHV transmission lines and the lightning protection design in our country is superior to Japanesedue to a shorter tower and higher insulation level.