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A Simulation analysis to improve the dielectric strength inside High Voltage Vacuum Interrupters
(2015)
Vacuum circuit breakers are expected to be one of the possible alternatives for SF6 circuit breakers in transmission voltages up to 230kV because of the excellent insulation as well as environmental friendly characteristics of vacuum. But for higher voltages, maintaining the electrical insulation inside and outside the interrupter tube is very important and becoming a challenge for the design engineers. Normally a vacuum interrupter consists of metal shields sandwiched between the ceramic insulator blocks inside the tube. The primary purpose of these metal shields is to protect the insulator walls by avoiding metal vapor deposition during the arcing process. On the other hand, these metal shields also influence the electric field distribution inside the interrupter tube. The presence of the metal shields may reduce the dielectric strength of the interrupter tube if proper measures are not taken.
This research is devoted to provide the information about the possible areas inside and outside the interrupter tube that are considered as critical in terms of dielectric strength because of the presence of metal shields. Possible solutions are also given in this thesis to overcome the high field stress in these critical areas with the help of 2D simulations that are simulated in ANSYS Maxwell.
The critical areas and their respective solutions presented in this work are (1) unidentified edges outside the interrupter tube which are formed by the metal shields that are inserted between the ceramic blocks. These edges, at high field stress, may act as a source of discharges between the interrupter tube and the outer insulator. This problem can be reduced by the combination of using a pressurized insulating gas (which is in this case N2) between interrupter tube and outer insulator and by extending the unidentified edges and covering them with field grading rings which are conductive in nature. (2) Triple junctions (Vacuum-Ceramic-Metal shield) are the sources of high field stress inside the interrupter tube and are considered as a primary source of Secondary Electron Emission Avalanche that takes place on the (3) ceramic surface. The triple junction emissions can be avoided by properly designing the insulator geometry at point of contact with the metal shield. In addition, inserting metal parts of certain depth at the both ends of the ceramic insulators can also reduce the field stress at the triple junctions and avoid surface flashovers on the ceramic surface. (4) The gap between the metal shield and the contact rod is also considered to be a critical area which can be highly stressed (field) if the geometry of metal shield curvature is not properly designed. Various metal shield curvatures are proposed and simulated and an optimum geometry is suggested that reduces the electric field stress between the metal shield and contact rod. Using this optimized metal shield curvature, the diameter of the interrupter tube can be reduced considerably which in turn reduces the size of the interrupter tube.
This PhD project started from one basic question: whether vacuum technology can be applied to 145kV electrical power system networks as a potential substitution to SF6 technology which has been utilised for decades of practice, due to environment and economic concerns. Possible threats and challenges, which might cause problems for the proposed replacement, are identified mainly in three areas: (1) small inductive current switching, (2) capacitive load current switching and (3) short-line fault switching. Three circuit-breaker programming models, therefore, have been developed based on statistic data provided by breaker manufactures: (1) a maximum di/dt fixed model which has been utilised for small inductive current switching tests and capacitive load current de-energising tests; (2) a dynamic di/dt model adopting from Mayr’s classic arc model for SF6 circuit-breakers which has been utilised for short-line fault tests; and (3) a current making model for capacitive load current energising tests. In a general conclusion, vacuum technology shows its superiority in most of the switching duties although in some rare cases, SF6 technology still stands a chance to break it even. But if we take the environment and economic factors into consideration, vacuum is definitely worth investigating in the future market.
For several decades HVDC technology was used primarily for point-to-point bulk power transmission. Although multiterminal HVDC systems are very few in number, it is expected that multiple HVDC links may be interconnected into HVDC transmission grid. Since every meshed grid requires reliable protection solution, an HVDC circuit breaker must be developed and introduced. This thesis focuses on the performance evaluation of different HVDC circuit breaker concepts. The performance evaluation study is carried out by means of numerical simulations. The obtained quantitative results describe capabilities and limitations of different HVDC circuit breaker concepts as well as the reaction of HVDC network on switching operations. Different converter protection measures and in particular application of fault current limiter were considered, too. Based on the research findings, a number of recommendations for HVDC circuit breaker development are given and suggestions for further research activities are presented.
With the recent developments in electrical transmission system, HVDC transmission for long distances has become feasible. With this development, many insulators are being used in HVDC system. Different kinds of insulators are situated at different places (example: desert, near to sea, agriculture area, etc.) so they will get expose to different types of pollution. Pollution affects the behavior of insulation in terms of breakdown and withstand capability. The application experience of insulators under HVDC conditions is limited. There is a necessity to understand the flashover performance and to recognize key parameters in the design and dimensioning of insulators used under HVDC conditions. This dissertation presents the difference between the analysis of partial breakdowns at AC and DC.
The dissertation explains the behavior of a water drop on insulator shed surface energized with DC. It deals with the moving water drop and hanging drop at the edge of the shed.
There are many situations where insulator structure should be in parallel. For example, the structure of insulators in vertical disconnector equipment often is parallel. If insulators are arranged in parallel, then the behavior of the electric field is totally different. It is important to know the behavior of these insulators used in HVDC system. This report also explains the pollution and non-pollution behavior of parallel insulators energized with DC.
The results can be a source of information to optimize the design and dimensioning of HVDC insulators, especially in pollution conditions.