Research On The Construction And Optimization Of The Complex DC Power Grid

Comparing with the AC synchronous power grid, the DC power grid has many advantages, such as higher control flexibility, higher capability of restricting AC synchronous power grid’s coverage, reducing the power grid’s complexity, lowering power system loss. The DC power grid can achieve direct interconnection between different DC lines, improve transmission capacity of DC system, and lower the transmission losses. Meanwhile, the DC power grid technologies are greatly promoted by progresses in plenty of aspects, such as the key equipments’technical breakthroughs, the increasingly high cost efficient HVDC projects, the steadily increasing transmission capacity, voltage level and project complexity, together with the growing research and engineering experiences.DC power grid is huge, complicated, while it has plenty of variables and uncertainties.However, decision making and their implementation require timely response to system real-time changes and it is possible to results in curse of dimensionality in the process of system modeling, incredible magnitude of computation in the process of simulation and optimization, as well as huge data exchange in the communication systems.Hierarchical control theory of the large scale system is an effective way to get rid of the dilemma in the process of decision-making and optimization.The basic idea of the hierarchical control theory is to adopt the variable scale, together with the decomposition-coordination principle to decompose the complex system and their control questions, and result in improving efficiency, together with reduced complexity of decision-making. On base of energy strategy and technological development trend, as well as full consideration of compatibility with the existing grid technological management system, the DC grid’s key technologies are studied. The first one isadaptability of the large-scale system hierarchical control theory being applied to DC power grid research, the second one is the structure and implementations of the DC grid power control,the third one is the meshed network mode and distribution technology of the DC power, and the fourth one is topology structure design of the DC power grid structure. All the above-mentioned work will speed up construction and exploration of DC power grid which will be reliable, efficient, sufficient, flexible and clean.(1) Research on adaptability of the large-scale system hierarchical control theory being applied to both construction and optimization of the DC power gridWith the typical features of the large-scale system, the DC power grid faces plenty of similar puzzles in process of optimization and decision-making. As an effective tool of dealing with coordination among various control functions, the large-scale system hierarchical control theory is beneficial to both improve efficiency and to reduce complexity in process of decision-making. It is pointed out that the mature layered frequency control technology can be extended to the DC power grid, which can improve compatibility with both the technological and management system of the present grid, will reduce the generalization obstacles in process of the DC power grid research and project construction. It shows that the DC power grid will be characteristiced by the clear structure, the transparent open platform, the flexible interaction capability on condition that it can be constructed according to the hierarchical control theory. The above-mentioned research results lay solid theoretical foundations for optimization of both the future grid’s control and the structure.(2) Research on the structure and implementations of the DC grid power controlOn base of analyses of the similarities and differences between the AC and DC power grid, the layered power control system schema and each layer’s functionsare defined. The control strategies, the operation modes and the technical routes of each layer are investigated. The key parameters and coordination skills among different layers are studied. The proposed scheme has lots of advantages, such as utilizing the existing AC technical and managerial experiences, being compatible to the present technological and managerial structure, improving the research and speeding up construction steps of the DC power grid.(3) Distribution optimization strategy of the DC power grid’s transmission power in the meshed network modeIt is described that the meshed network mode, insteading of the widespread end-to-end mode, would become the mainstream of the power transmission of the DC power grid, due to being in line with the technological trendy of the power grid. It was pointed out that the power transmission optimization strategy of the DC power grid, aiming to meet requirements of security, economic, market and environment, and so on, had solid theoretical foundation together with potential of implementation in projects. Aiming to realization of harmonization between accuracy and complexity of mathematical models and their solution algorithm, the proposed technological roadmap includes adopting the network flow analysis method, introducing the principle of user-optimized equilibrium (UE), transferring the constraint conditions into mathematical equations and inequalities, utilizing reasonable assumptions, establishing Beckmann optimization distribution mathematical model, utilizing Frank-Wolfe algorithm to obtain the global unique optimal solution, verifying the strategy by an example.(4) The layered and partitioned DC power grid structure based on the hierarchical and differential principleBased on direction and requirements of the DC power grid, according to the hierarchical and differential strategy and principle, the layered and partitioned DC power grid structureis consistent with the DC power grid technological characteristics, adapts to the creative trendy of the conception, technology and management. Firstly, the grid structure is layered, according to the voltage grade, function and power supply area. Secondly, the independent power supply districts are constructed which are hearted by the receiving power grid, and have capabilities of maintaining the basic balance between power supply and demand. Thirdly, there are strong internal connections inside the regional grid, while there are weak inter-area connections, which realize partitioning power grid.The proposed scheme in this paper has lots of advantages, such as utilizing the existing AC technical and managerial experiences, being compatible to the present technological and managerial structure, improving the research level and speeding up construction steps of the DC power grid.