Yuan Tao

Local electron mean energy profile of positive primary streamer discharge with pin-plate electrodes in oxygen?nitrogen mixtures

Local electron mean energy (LEME) has a direct effect on the rates of collisional ionization of molecules and atoms by electrons. Electron-impact ionization plays an important role and is the main process for the production of charged particles in a primary streamer discharge. Detailed research on the LEME profile in a primary streamer discharge is extremely important for a comprehensive understanding of the local physical mechanism of a streamer. In this study, the LEME profile of the primary streamer discharge in oxygen-nitrogen mixtures with a pin-plate gap of 0.5 cm under an impulse voltage is investigated using a fluid model. The fluid model includes the electron mean energy density equation, as well as continuity equations for electrons and ions and Poissons electric field equation. The study finds that, except in the initial stage of the primary streamer, the LEME in the primary streamer tip tends to increase as the oxygen-nitrogen mole ratio increases and the pressure decreases. When the primary streamer bridges the gap, the LEME in the primary streamer channel is smaller than the first ionization energies of oxygen and nitrogen. The LEME in the primary streamer channel then decreases as the oxygen-nitrogen mole ratio increases and the pressure increases. The LEME in the primary streamer tip is primarily dependent on the reduced electric field with mole ratios of oxygen-nitrogen given in the oxygen-nitrogen mixtures.

New optimization method on electric field distribution of composite insulator

Due to the geometry configure of composite insulator, the electric field strength near the two ends of composite insulator is always strong. The high electric field strength can cause the partial discharge on the insulation material and the surface of the metal electrodes, and bring premature aging of the insulator. In this paper, a new optimization method on the electric field distribution of composite insulator is put forward, which is the combined use of composite insulators and several units of glass insulator. Based on the finite element method, a three-dimensional electric field calculation model of insulators in the transmission line is established. Then the optimization conditions of surface potential and electric field distribution of composite insulator with glass insulator installed in the high voltage end of composite insulator at different levels of voltage is studied. The results show that by this method, the electric field distribution near the ends of composite insulators is significantly reduced, which can prevent the partial discharge and the aging of the composite material. Finally, this paper recommends the number of glass insulators which should be installed at the high voltage end of composite insulator at different voltage levels.

Identification of induced lightning and direct striking based on wavelet transform

Based on the different discharge processes of lightning striking, lightning overvoltage can be divided into two types: induced lightning overvoltage and direct lightning striking overvoltage (including back-flash and shielding failure). The characteristics of these overvoltages are investigated in this paper and it is concluded that induced lightning overvoltage exists simultaneously in three phases of the transmission line and similarity of its inter-phase is more significant than that of direct striking lightning overvoltage. Therefore the energy matrix similarity dependent method with the implement of wavelet transform can be applied to identify them. Mass simulation with consideration for many different fault conditions has been computed to prove its effectiveness based on ATP.

Analysis on Shielding Effect of the Grounding Electrodes under Impulse-Current

Generally, the flow of a lightning impulse current from a grounding electrode into ground is a very complicated process determined by many factors. In order to analyze the mechanism of the impulse current dissipating in the earth from grounding electrode, the experiments that had carried out by other authors almost used a single horizontal grounding wire or vertical grounding rod for sake of simplicity. However, in practical conditions, most of the grounding systems are constructed of grounding electrodes with branches in different directions. In this paper, basing on the principle of dimensional similarity, impulse simulation experiments are performed on the common ground electrodes with conductor branches. We focus on analyzing the impulse current dispersal regularity of different branches when injecting at one point. Comparing with the leakage current distribution of a single ground electrode, it is found that the leakage currents along the branches increase with the distance to the current feed point, and the more conductors near the injection point, the more uneven the leakage current distribution is. This work indicates that shielding effect should be taken into account when analyzing the impulse characteristics of grounding electrodes.

Lightning back-flash performance of 220kV AC quadruple-circuit transmission lines on the same tower

In this paper, an insulation flashover criterion model, in which the change of multi-conductor system coupling coefficient induced by impulsive corona is considered, is constructed by the electromagnetic transient analysis software ATP-EMTP. The model proposed, having taken the effect of the impulsive corona into account, is able to present the influence of conductor corona characteristics on potential difference of two ends of the insulator when an impulse flashover on insulator string occurs. A model of 220kV multi-circuit transmission line, which adopts insulation tower model with multi-wave impedance, is constructed as well. Thereby, it is feasible to study lightning back-flash proof levels (LBPL) and its influencing factors. The simulation result shows that the tower height plays a predominant role in deciding the performance of the counterattack lightning level of transmission lines. Moreover, the back-flash level of quadruple-circuit lines is remarkably lower than that of double and single circuit lines. It is suggested that, firstly the tower should be much lower if it is permitted; secondly the conductors at different heights in tower should adopt different insulation levels.

Combination Operation for neutral point of 35kV grids in mountainous area

Based on a lot of measurement and analysis, This paper find that the unbalanced nature of 35 kV grids due to unsymmetrical capacity values gives difficulty in neutral point operation in mountainous area of west China. Improving compensation of suppressing coil can rectify imbalance of voltage but bring up new problem that suppressing coil can not extinguish grounding arc effectively in fault. To put down contradiction mentioned above, This paper proposes a new neutral point operation method as neutral point insulated in routine operation and suppressing coil grounding in fault, then EMTP simulation acts. Simulation indicates that new neutral operation method can improve lightning withstand level and decrease trip-out rate of grids.