Study On Lightning Stroke Characteristics Based On Negative Impulse Discharge Characteristics Of Long Air Gap

Ultra-high voltage (UHV) transmission lines are generally built across the quite open and spacious wilderness areas spanning various complicated landscape such as high mountains, great rivers and even deserts. And they stretch over hundreds of thousands of miles to transfer power. Consequently, the shielding failure probability of UHV transmission lines is increased, which threats the stability and safety of the power system's normal operation. There is not enough operation experience of UHV transmission line on the world today. The number of lightning stroke on the voltage-reduced UHV transmission line in Japan and Russian is obviously much higher than expected. It indicates that the operation experience of lower voltage transmission lines can not give perfect guidance to the shielding system designation of UHV transmission line. Negative impulse discharge characteristics of long air gap is a long-term concerned basic issue of lightning shielding field in high voltage power transmission engineering. And most studies on lightning stroke characteristics of UHV transmission lines nowadays are based on long air gap discharge test data whose length is shorter than 4m. However, the practical gap distance of lightning final stroke is much longer, and its discharge characteristics are much different from the test data. Therefore, lots of long air gap discharge tests have been conducted in this paper to get plenty of reliable discharge data and to summarize their discharge rules. The present study can provide basic data for the correction on theory model calculating lightning shielding failure rate of UHV transmission lines. The main research work in this paper is as follows:(1) Experiments have been conducted to study on the negative impulse discharge characteristics of three typical gaps of 1m-10m. Two negative switching impulses of 20/2500?s and 80/2500?s were applied to rod-rod gaps, rod-plane gaps and rod-conductor gaps. The saturation discharge characteristics of three typical long air gaps were all systematically studied. The statistical discharge characteristic of long air gaps was also recorded. The optical observation results show that the length proportion of last step streamer in entire gap decreases when gap distance is increased, resulting in the saturation phenomenon of negative switching impulse discharge voltage of long air gap. The 50% breakdown voltage relation between rod-rod gaps, rod-conductor gaps and rod-plane gaps reverses when gap distance d=4m. The gap factor between rod-rod gap and rod-conductor gap tends to be 1.05 with increasing gap distance d. Parameters such as probability distribution function of breakdown voltage and volt-second characteristic of rod-rod gaps and rod-plane gaps have also been obtained by statistical methods.(2) Test study have been carried out to research the effect of conductor electrode arrangement on 50% negative switching impulse breakdown voltage of rod-conductor gaps. The 50% breakdown voltage of rod-conductor gap decreased with increased conductor radius, but remained unchanged with changed conductor height. The 50% breakdown voltage of rod-ungrounded conductor gap is higher than that of rod-grounded conductor gap. The induced voltage of ungrounded conductor calculated by Finite Element Method shows that with the increasing gap distance, the induced voltage increases firstly and then decreases. Discharge process observation results of 0.8m rod-conductor gap by high speed photography indicate that the streamer discharge process of rod-ungrounded gap is the same as that of rod-grounded conductor gaps.(3) Experimental work has been done to get negative impulse discharge characteristics of 0.4m-1.2m rod-rod gap and rod-double rods combined gap. The 50% breakdown voltage has been achieved with negative switching impulse voltage of 250/2500?s. The results show that little influence of grounded electrode shape with semi-spherical or conical tip has on the 50% breakdown voltage of rod-rod gap. When gap distance d is less than 1.2m,50% breakdown voltage of rod-double rods combined gap is less than that of rod-rod gap. The T-test results show that the grounded electrode head shape has no significant effect on the 50% breakdown voltage of rod-rod gap, but there's significant difference of 50% breakdown voltage between rod-rod gap and rod-double rods combined gap.(4) Development characteristics of discharge path have been analyzed with parameters calculated by combinational algorithm and discharge graphics recorded by high-speed camera. With increasing gap distance, the HV rod discharge inception angle was increased, resulting in enhanced stochastic development of HV rod initial discharge direction. But for the ascending streamer occurred with increasing gap distance, the grounded rod head shape had significant effect on the discharge path selection characteristics in rod-double rods combined gap. All parameters have increasing statistical range with the increased amplitude of applied impulse voltage, including angle range of the HV rod discharge direction and vertical direction, discharge path length, diameter and area. It all indicates an increase on stochastic development of HV rod initial discharge direction and the dispersibility of discharge path probability distribution.(5) Comparative experiments have been conducted to research on influencing factors of lightning stroke performance of large scaled 1000kV AC transmission line. A series of simulation experiments have been carried out for studying the impacts such as grounding way, operating voltage, shielding angle and impulse voltage waveforms with scaled UHV transmission lines model with the ratio of 1:12.5. Two types of negative switching impulse voltage were applied with 20/2500?s and 80/2500?s. Experimental results show that there's a significant effect of conductor grounding way on the discharge path probability and ungrounded conductor is recommended for simulating practical phase lines. Three-phase voltage of+72 KV,-36kV and-36kV has been proved to have no effect on breakdown voltage and discharge time of rod-transmission line combination gap, which can affect the discharge probability of phase line. No obvious influence is revealed between impulse voltage wave time and discharge probability. Under negative switching impulse of 20/2500?s, the dispersion of discharge path probability distribution increases with impulse voltage amplitude and the discharge probability to phase line is reduced, which explains that shielding failure rate decreases with lightning current amplitude. Moreover, the smaller the protection angle of transmission line is, the lower discharge probability to three-phase conductor is.(6) Lightning stroke performance of 1000kV UHV transmission lines has been studied by simulation tests in plain and mountain areas. Negative switching impulse of 80/2500?s has been employed. The gap distance in tests was 5m-12m. The shielding failure spatial region has been drawn by interpolation and fitting method, and equiprobability curve of the region shows an inclined parabola curve. The upper and lower spatial boundary of shielding failure area tends to rise up with increasing gap distance. There's little effect of terrain on the average breakdown voltage and average breakdown time of rod-plane gap. But test results show that the slope changed the discharge path resulting in increased discharge dispersibility of rod-plane air gap. The calculation results by finite element method show that surface electric field of transmission lines in plain is smaller than that in mountainous area, and the electric field bias enlarged with increasing gap distance. It indicates that upward leader will be generated more easily on the surface of lines and ground wires in mountainous area than that in plain, which increases the shielding failure probability of UHV transmission line in mountainous area.(7) The negative impulse discharge characteristics of long air gap is applied to the modification of striking distance coefficient. Test results show that the probability distribution of discharge path is related to both the 50% breakdown voltage of air gap and the low voltage electrode shape. On the assumption that the 50% negative breakdown voltage of long air gaps in equals to the lightning leader tip potential, striking distance coefficient kg of UHV transmission lines is deduced to be 1.3. The test data in was used to verify the rationality of the striking distance coefficient, and the striking distance coefficient of ground wire could be 1.0.