Research On The Characteristics Of Long Air Gap Positive Streamer Discharge Under Impulse Voltage

The characteristics of long air gap discharge are the basis of the external insulation and lightning protection of the transmission lines. Due to the large dispersion, complex electromagnetic environment and high cost, the characteristic parameters of long air gap discharge lack accurate measurement and in-depth study. With the development of photovoltaic technology, manufacturing measurement system for getting the higher time resolution, more accurate and more kinds of the characteristic parameters maybe successful. In this paper, the process of long air gap positive sreamer discharge as the study object. The long air gap discharge measurement system and experimental study have been adoptted. The characteristics of streamer initiation, streamer branches and streamer propagation have been studied.The experimental observation platform has been founded. It contains optical image observation system, discharge current measurement system, transit electric field measurement system and synchronous acquisition system. Based on this platform and artifical climate chamber, the lm rod-plane experiments under impulse volateges have been done. The influences of humility, electric field distribution and viriation rate on streamer initiation have been studied. The variations of streamer initiation delay and minimum intiation voltage have been obtained. Then, the traditional streamer initiation criterion under DC voltage has been modified to be suitable for impulse voltages. The computing method of electrons detachment rate has been optimized in critical volume model. This made the computing rate of streamer initiation delay faster. The humility coefficient has also been modified in this model, and the computing accuracy has been impoved.Based on the features of long air gap positive streamer discharge, the axis-radial orthogonal observation method has been proposed, and the more recognizable images of streamer branches have been obtained. According to the branch morghology and the differences of the ogriginal and the new streamer bifurcations, the Monopodial and Dichotomous streamer branch types have been proposed and defined. The influences of electric field distribution and change rate on these two types of branches have been studied. It concluded that the branch length, the bifurcation angle and channel diameter decrease with the streamer propagtion and the decreasing of geometrical electric field. The influence of viriation rate of electric field on the branches is so weak that it can be neglected. Comparing with the two types branches, the distributions of bifurcation angles and relationship among the new branches have been obtained. This concluded that the generation propotion of the Monopodial branches increases with the electric filed increasing. Based on the theory of streamer photoionization, the formation mechanism of the two types of branches has been obtained.The electric step phenomenon and electric viriations of the streamer region and beyond have been measured by the transit electric field measurement meter. The velocities of early and post streamer propagation have been estimated. The influences of electric distribution and variation rate on the rise time and drop time of streamer discharge currents have been counted. Related to the process of streamer propagation, the explanation that the rise time of discharge current is the process of streamer propagation in the critical volume has been proposed. Then, the computing method of the streamer propagation velocity in long air gaps has been derived, and the single branch streamer propagation model has been improved. From the influence of electric field distribution and variation rate of electric field on the streamer charges, the influences of number of branches have been obtained, and the empirical formula of computing the number of equivalent streamer branches has been derived. In summary, the computing method of streamer propagation parameters considering the electric filed distribution, the rising rate of impulse volatge and humility has been established. This method has been verified by the experiments. The influence of streamer discharge on the leader inception has been discussed, and the inference that the charge of single branch streamer may be the leader inception criterion has been proposed. At last, the implication of this study to the external insulation and lightning protection of the transmission lines has been discussed.