| Due to the inverse distribution between energy resources and load demands in China, the strategic objective of developing UHV transmission backbone coordinated with multi-level voltage grid was put forward by State Grid Corporation of China. The Jindongnan-Nanyang-Jingmen UHVAC demonstration project was officially put into operation on January6th,2009, which means significant breakthroughs in core technology of UHVAC transmission and equipment domesticization. According to the planning, UHV grid composed of1000kV UHVAC,±1100kV and±800kV UHVDC transmission lines will be constructed by2020, to realize long-distance, large-scale and highly-efficient transmission, and promote the intensive development of coal power, hydropower, nuclear power and renewable energy power.According to laws of development, it would be a high-risk period in the primary construction of higher voltage grid. During the development of UHV grid, many new technology problems closely related to the structure of transmission backbone may appear, typically including multi-level electromagnetic loop, short circuit current exceeding, multi-infeed DC interaction and integration of large-scale renewable energy. Potential dangers would threaten secure and stable performance of UHV grid if these problems could not be solved properly. In order to escape failures caused by unreasonable structures, it’s theoretically and practically necessary to make further study on the structure of UHV transmission backbone.Based on current researches, a detailed study on the structure of transmission backbone has been carried out in this dissertation, which includes transmission backbone evaluation model, multi-level electromagnetic loop partitioning, current limiting schemes optimization and the vision of renewable energy power grid. The whole research introduces many theorys, such as multi-attribute decision making, fuzzy theory, complex network theory and multi-objective optimization, and refers to many specific aspects, such as mathematical models, solving algorithms, checking constraints and planning strategies. Main contributions and innovations are described as follows:1) Considering the restrictive relation between improving system stability and limiting short circuit current, a transmission backbone evaluation model for operation planning is built. The fuzzy comprehensive evaluation method executes a quantitative assessment of different transmission backbone schemes, with the introduction of branch overload index, bus voltage violation index, power angle difference index, voltage security margin index, short-circuit current exceeding index, short-circuit capacity balance index and active power loss index. Some theory analysis on multi-level electromagnetic loop is given from two sides involving power transfer and short circuit current exceeding. Multi-level electromagnetic loop of North and Central China UHV grid is simulated to point out the drawbacks of this operating mode, and the evaluation model is applied to assess both the opened and closed schemes of electromagnetic loop. Case studies performed on the actual power system demonstrate the effectiveness of this proposed model and method.2) A new method for electromagnetic loop partitioning based on quantitative analysis of network community structure characteristics is presented. Considering line length and parameter characteristics of power grid, admittance modulus is chosen as edge weight, then the weighted edge betweenness and the weighted modularity index are defined on that basis. The traditional Folyd-Warshall algorithm is improved to be suitable for the calculation of edge betweenness in that network, which has multiple shortest paths of the same length. According to the distribution of centre substation and system topology structure, several parts are separated from the low voltage level network using GN dividing algorithm. The partition scheme set could be determined depending on the sequence that every part separated from the network in the dividing process. The weighted modularity index is employed to evaluate the rationality of the dividing results. Case studies demonstrate that this proposed model and method can effectively provide theory basis for electromagnetic loop partitioning.3) A multi-objective optimization model of current limiting schemes is built to coordinate the problems of short circuit current exceeding and multi-infeed DC interaction. The influence on impedance matrix caused by each current limiting measure is analyzed, and a variable selection strategy considering sensitivity relation between current limiting measure and node self-impedance is proposed. The relation between multi-infeed short-circuit ratio and network structure is discussed, which brings a conclusion that current limiting measures may lead to a increase in multi-infeed short-circuit ratio, also may make it smaller. Opening lines and adding current-limiting reactors are chosen as decision variables in this model. To minimize total investments, to minimize short-circuit capacity margin and to maximize weighted multi-infeed short-circuit ratio, nondominated sorting genetic algorithm Ⅱ is used to find the Pareto optimal solutions. Case studies performed on the actual power system demonstrate the effectiveness of this proposed model and method.4) After introducing EU "Super Grid2050" transmission backbone, the vision of renewable energy power grid in Northwest, North and Northeast China is proposed. Considering renewable energy power base planning in this region, network structures of this vision in the year of2020,2030and2050are given. The technical feasibility of renewable energy power grid is demonstrated from the comparison of AC and DC transmission mode, technical features of new type DC transmission and the development of multi-terminal DC transmisson. This vision is presented under the background of strategic adjustments of energy structure in our country. The complementarity of renewable energy in the wide area, in some ways, can balance their fluctuation and uncertainty, accordingly to reduce the investment of energy storage system and provide stable and reliable power. |
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