| Because of its high aptitude and steep slope, Very Fast Transient Overvoltage (VFTO) caused by operating of the Disconnecting Switch (DS) in Gas Insulated Switchgear (GIS) can potentially jeopardize the equipment insulation. As simulation can greatly save time and expense, it’s an important method in study of Very Fast Transient phenomenon. And yet the modeling of the spark channel between the DS contacts is the bottleneck of simulation since the existing models such as fixed resister model, exponential resistance model are inaccurate and hard to extend. The basic reason is that these models cannot depict the physical process in the microscopic view. Particle in Cell (PIC) is a method can simulate charged particles’motion in electromagnetic field. It’s widely used in simulation of the vacuum microwave device, and is a potential simulation method for the spark model in DS. However, existing PIC cannot directly applied for the spark model. It has to overcome some problems such as collision, photoionization and photoemission on the cathode under a very high gas pressure. Therefore, we have to develop the PIC method more deeply, and on that basis build a more accurate VFTO simulation method.The existing PIC are commonly applied in very low pressure even high vacuum circumstances. Yet the SF6 spark in DS is of high pressure and contains very large quantity of particles. The existing PIC applies super particle model to solve the particle quantity problem. However, super particle model is a collideless method, thus it cannot be applied to the SF6 spark. To solve this problem, we developed sample particle model. Sample particles are relevant small quantity of particles that follow the same velocity distribute as all particles. And their velocity distribute can stay the same with all particles’via account-resampling. Sample particle model makes it possible to simulate the collisions among particles and decreases amount of calculation.The existing PIC lack the algorithm of photoionization and photoemission on the cathode, which play important roles in the SF6 spark. We developed an algorithm of photoionization and photoemission. This algorithm is based on Monte Carlo Collision (MCC). But since it exploits the invariability of the light speed and the collision cross section, its calculation efficiency is greatly promoted.Since there is not enough basic experimental data of the SF6 spark, and in order to validate above model and algorithm, we employed argon discharge data in literature to compare with that in argon discharge simulation. The comparison of electron drift speed and impact ionization factor between simulation and experiment confirmed the validity of sample particle model. The electron proliferation processes are observed with and without the algorithm of photoionization and photoemission. And that confirmed the validity of the algorithm, indirectly. Besides, the discharge channel’s voltage-current characteristic is obtained, which can well coincide with the experimental one.The attachment and recombination algorithm in SF6 spark is built in light of the electronegativity of SF6. A coupling method between PIC and circuit simulation is designed. And a DS transient spark model based on PIC is formed. On this basis, a 252 kV voltage class VFTO model is built. The simulation waveforms are analyzed. Comparing with the exponential resistance model, the error of VFTO aptitude with our model is 13% smaller, and the damping of the high frequency oscillation is more proximal. Finally, a DS entire operation simulation model in light of the randomness is built and the statistical characteristic of VFTO waveform in simulation is obtained, which also coincide with the experimental one. The modelling method in this paper is valuable for the study of the VFTO characteristic in extra high voltage. |
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