A Physical Model For The Simulation Of Positive Discharge Development In Long Air Gaps

Insulation characteristics of air gap are of great importance for the design of UHV transmission system. At present for external insulation design of the UHV transmission system, insulation characteristics of long air gaps are obtained on the basis of experiments, which are affected by many factors and at high cost. Furthermore, the experimental values has big dispersion and cannot give more help for further optimization of external insulation design of the UHV transmission system, since which is lack of the support of theoretical model and analysis method. This paper mainly focuses on long air gap discharge process under positive impulse, the modeling of long gap discharge physical model has been performed and carried out based on deeply and meticulous research.Using the comprehensive simultaneous observation platform for the positive long gap discharges, the inception and development features of the streamer under lightning impulse voltage are studied. It was obtained that the relationship between the initial time delay of the streamer and the electrode curvature radius and the voltage increasing rate, and the symmetrical hypothesis about the electric field distribution of the streamer was verified for the existing models. Based on the theory of critical volume, the mathematical expression of the effective free electron generating frequency was derived and a more perfect streamer initiation calculation model was proposed by considering the effect of the electrode curvature radius and the rate of voltage rise on the generating probability of initial free electrons. Moreover, it was founded that the streamer inception time delay meets the Rayleigh distribution. The corona space charge calculation model was proposed based on the charge simulation method which can consider the actual shape of the streamer region and the influence of the existing space charge. At last, the proposed model was validated by the test results and other models.The critical charge, dark period, voltage increase and inception voltage of the leader under switching impulse voltage and the relationship between the first corona charge and them were analyzed and obtained with the observation platform. Based on the thermal ionization theory of streamer to leader transition in the root of the streamer, the positive leader inception model is proposed with the charge simulation method, which can take into account the process of the second corona inception and the electric field shielding effect near the initial leader of the corona space charge. Furthermore, the model was validated by the measured results.Using the observation platform, experiments of the positive leader discontinuous development were carried out under switching impulse voltage of low voltage rise rate, and two types of Restrike were obtained. Meanwhile, the charge per unit length for leader channel was obtained during the stage of the leader discontinuous development. The mechanism for Restrike which is composed of three physical processes was proposed. By introducing a deionization parameter to describe the evolution of the electric field of the leader channel, based on the mechanism and mathematical description for the Restrike process proposed in this paper, the leader development model considering Restrike process was proposed, and the rationality of the model was verified by experimental results.A physical model including corona inception, leader initiation, streamer-leader process and the final jump for the positive long gap discharge was carried out based on the charge simulation method (CSM), furthermore, on the basis of the full model, with reasonable simplification of space charge calculation, considering the randomness of the leader path, a simplified simulation model was established. Moreover, the models were validated by the experimental values. Finally, using the model proposed by the paper, discharge characteristics for the configurations of the rod-plane, rod-rod, and conductor-tower window air gaps were studied, and the effect laws of the electrode curvature radius and the wave front time on breakdown characteristics for the typical air gaps were obtained.