Research On Motion Characteristics And Dynamic Modeling Of Secondary Arcs With Long-distance Transmission Lines

With the development of Global Energy Interconnection, the ultra-long distance transmission technology, especially the half-wavelength transmission, has nowadays attracted extensive and worldwide attention. With unique features of ultra-high voltage, long distance, large capacity and special circuitry structure, the secondary arc issue becomes prominent with the transmission lines. If the secondary arc cannot be extinguished in a designated time scale, there would exist an arcing ground fault, consequently resulting in prompt failure of the single-phase auto-reclosing operation. Therefore, it is an urgent task to carry out research on the inherent physical mechanism and motion characteristics of the secondary arcs with long-distance transmission lines, and explore the dynamic modelling methodology as to establish an accurate calculation method to predict the arcing time, and thereby present indispensable theoretical basis in developing cost-effective and reliable arc suppression measures, all the above of which are of theoretical significance and application potentials.In this thesis, both experimental research and theoretical analysis were implemented. Well-planned studies were carried out on a physical simulation platform as to explore the macroscopic motion characteristics and the microcosmic physical nature of the secondary arcs with long-distance transmission lines. Firstly, experiments were conducted to account for the internal mechanism and the key influential factors on the formation, dynamic characteristics and the motion evolution of the secondary arcs. Secondly, a multi-field coupling dynamic model was established as to simulate the motion trajectory of the secondary arcs. Further, based on the balanced input and output energy evolving process of the secondary arcs, endeavors were made to set up an accurate arcing time calculation method. The proposed research aims to create a foundation for developing effective and reliable single-phase auto-reclosing technologies for the long-distance transmission lines.Low-voltage simulation experiment is a cost-effective way to study the physical characteristics of the secondary arcs. Based on a distributed parameter model, an equivalent single-phase physical simulation circuitry for the long-distance transmission lines was proposed and a test platform was specifically designed for initializing secondary arcs. The experiments showed that the motion characteristics of the cathode and the anode arc roots rendered an obvious polarity effect. There was a jumping phenomenon at the anode arc root, and secondary excitation phenomenon normally occurred at the cathode arc root. Elucidations were given to account for the motion characteristics mentioned above as well as the microscopic mechanisms. However, the motion of the arc column was much more complicated, manifesting an upward motion tendency driven by the thermal buoyancy force and often concurring with evident short-circuit and partial extinction phenomena. There were two typical short-circuit phenomena existing in the motion process, one is short-circuit between the arc column and the metal electrode, and the other is short-circuit among the inner parts of the arc column itself. Experiment also showed that there existed different extinction characteristics with different in-series compensation schemes. By appropriate control of the compensation scheme, the secondary arc extinction time with the half-wavelength transmission line could be considerably shortened. With installed compensation circuitry, the secondary current before extinction was much lower than before, and the rise rate of the recovery voltage could also be reduced, which may significantly decrease the number of arc re-ignitions.An arc chain model incorporating multi-field coupling dynamics was established to integrate the imposed stresses of electromagnetic force, thermal buoyancy force, wind fluid force and the air resistance. The arc root model was also included in the integrated simulation methodology after a depth analysis of the arc root motion mechanism. The established dynamic model could produce an effective simulation of the motion trajectory of the secondary arcs. Based on analysis of the transition process from the primary arcing to the secondary arcing, a stochastic simulation model regarding random initial arcing positions was proposed. The quantitative correlation between the arc conductivity and the temperature was acquired and the radial conductivity distribution was also obtained. Then the probability of each possible arc developing direction was achieved. The stochastic model to account for the initial arcing position of the secondary arcs was further embedded into the multi-field-coupled dynamic model as to calculate the stochastic motion trajectory of the secondary arcs. The simulation results were compared with the experimental ones, both of which coincided well and indicated effectiveness of the proposed stochastic model.Through concrete analysis on the energy evolution process of the secondary arcs, the balanced energy equations between the input power and the output power were established. Being integrated into the multi-field-coupled dynamic arc model, a new method for calculating the arcing time was then proposed. Low-voltage simulation experiments were conducted as to compare with the computational results for verifications. The results showed that the secondary arcing time could be calculated more accurately by the new proposed energy balance scheme, especially under windy conditions. The above scheme presented a precise method for predicting the secondary arcing time that is useful as to achieve reliable single-phase auto-reclosing operations.Based on the established dynamic simulation model, the physical mechanism of the arc root jumping phenomenon was analyzed and also verified by the simulation analysis. There were two main reasons that caused the jumping phenomenon of the arc root. One was the thermal buoyancy force that drove the arc column to keep stretching outwards, which facilitated short-circuit between the arc column and the metal electrode. The other one was due to the electrodes structure. A downwardly inclined electrodes structure rendered much easiness in causing short-circuit phenomenon between the upper arc root and the arc column. During the motion process the arc length kept increasing, however, the arc length may become temporarily shortened due to the short-circuit phenomenon. There was often a sharp rise phenomenon of the arc length at the instant before the secondary arc extinction due to sudden increase of the secondary current. In addition, different wire configuration executed sensible impact on the arc motion behavior. If the two separated wires were perpendicularly intercrossed to each other, the arc motion would be much more complex and the arc length increased faster that the paralleled wire structure, which implied in this case the secondary arc developed intensively and might achieve easy extinction.The motion characteristics of the secondary arcs with different secondary current and wind speed were also studied through simulations. Also, the influencing mechanism of the multi-field coupling forces was elucidated through quantitative calculations. The results showed that, with increased secondary current, the spinning phenomenon of the secondary arc became even more prominent and the arcing time was elongated accordingly. There existed an approximately linear relationship between the two parameters. Without any wind stress, the arc motion was co-determined by the electromagnetic force and the thermal buoyancy force. The arc root motion was mainly determined by the electromagnetic force while the arc column motion stretched upwards due to the effect of the thermal buoyancy force. Under windy conditions, if the secondary current was small, the arc root motion was co-determined by the electromagnetic force and the wind stress while the arc column motion was principally determined by the wind stress. With increase of the secondary current, the arc root motion was gradually determined by the electromagnetic force while the arc column motion was still governed by the wind stress.The proposed research work as well as the achievements in this thesis can further enrich the theoretical basis and analytical methodologies for the secondary arc studies with long-distance transmission lines, especially presenting fundamental data and theoretical reference for future development of the half-wavelength transmission technologies.