| The distribution of energy resources and power loads are reverse in China, the power transmission with long distance and large capacity is necessary. More and more HVDC lines will be applied as an economical transmission technology. The ultra high voltage direct current transmmision technology was overcome in the first decade in 21th century. The HVDC system with highest voltage and largest norminal capacity is in operation in China.The 500kV power grids in main power receiving are mature, some problems such as high short circuit current and shortage of orridor and land resources for electrical projects are becoming stand out. The electric power should be transmitted to the central areas by HVDCs, but the locations of convertor station are restricted, then the typical multi-infeed DC system is formed.In order to keep the HVDC in normal operation status, enough reactive power should be supllied by AC systems. In multi-infeed DC system, more reactive power is needed than the amount calculated respectively because interaction effect of HVDC systems. More interaction effect results in shorter electrical distance. So, losing voltage stability will be the main security risk in receiving end power grid, while the main risk is losing angle stability or losing frequency stability in the past. For keeping the power systems stability, the capacities of power transmitted by HVDCs is restricted.Since entering 21th Century, the HVDC technology in China entered the stage of rapid development. The State Grid Corporation of China plans to build over 20 HVDC projects in 2016-2020. According to the operations status of present power gird, the systems stability is getting worse with more and more HVDC projects putting into operation. The study for multi-infeed HVDC problems has been extended to the power grid planning stage. To ensure the security and stability of planning power grid, on the one hand, we have to deside the HVDC terminals’location, the power grid structure and the dynamic var compensations’location. Based on these, the maximum multi-infeed HVDCs’ capacity to receiving end power grid can be studied. So, it is very valuable for power grid planning to research the maximum power to receiving end power grid by HVDC lines.Based on the voltage stability theory and its evaluation indexes for multi-infeed HVDC systems, the effect factors of maximum power by HVDC lines are gotten. Then the new evaluation indexe for the voltage support ability is put forward, the optimization methods for locations of HVDCs’convertor stations, dynamic var compensations are created. The optimization method for power grid structure is improved. The process for getting the maximum power by HVDC lines is formed and its feasiblity is proved by applied to the Yangtze River delta planning power grid. The research contents and achievements are in this doctoral thesis are summarized as follows.(1) After analyzing the formula of multi-infeed short circuit ratio (MISCR), locations of HVDC s convertor station, the power grid structure and the dynamic var compensations are regarded as determinations of maximum power by HVDC lines. We realized the maximum power can be gained by multi-objective optimization. In order to simplify the problem and avoid the curse of dimensionality, the problem is simplified by using the constraint method in the classical multi-objective optimization method.The HVDC terminals’ location optimization, the power grid structure optimization and the dynamic var compensations optimization should be carried out respectively. Then the heuristic method is applied to organize the optimizations as a complete method for maximum power to receiving end power grid by HVDC lines.(2) Based on the equivalent impedance and HVDC’s rated capacity, the HVDC’s weighting coefficient which reflects its influence is defined. Based on MISCR, the definition and calculation method of evaluation indicators for terminal location schemes are given. Based on multi-infeed interaction factors, the screening strategy for HVDCs which are included in the indicators calculation is put forward. The process of HVDC terminal location selection covers preliminary screening, ranking of the location schemes according to their indicators and security and stability analysis. This method reflects the characteristics of multi-DC interactions, provides a fast, straightforward analysis tool for determine the HVDCs’ terminal location in power grid planning. The method is applied to thg Yangtze River delta planning power grid, proposed HVDC scheme ranking and security stability simulation calculation results highly consisten.(3) The effects caused by grid structure change to short circuit current, HVDCs’ interaction and MISCR are analysed. The result shows the relationship between the grid structure and MISCR is not monotonic. So, the optimization of the grid structure is possible. Based on the optimization of power grid structure which gives consideration to short circuit current control, the community structure theory is used to getting the subarea according the HVDC groups. The lines between the subareas are composed as the decision variable set which is used to optimize the power grid structure.The method improves the efficiency of power grid structure optimization. And it can be used to optimize the power grids which have reasonable short circuit currents. The method is applied in the south of Jiangsu power grid structure optimization; the grid structure optimization scheme is feasible.(4) According to the fact that the voltage sunken areas exist in multi-infeed DC systems, the dynamic voltage geography grid diagram is gotten based on time-domain simulation results, to determine the installation area of dynamic var compensation which can improve the efficiency of site selection. According to the theory of pilot bus, the voltage sensitivity to assess the impact of dynamic var compensation is defined and its calculation method is put forward. The weight of DC used to calculate the voltage sensitivity is also defined. Then, the process of site selection method for dynamic var compensation in multi-infeed DC power grid is determined, the process includes the voltage sensitivity for all sites in the installation area are calculated and sorted according to their values from large to small. The calculation and sorting is cycled. The number of cycles equal to the number of required sites. The application of the method in the Yangtze River delta planning power grid, the dynamic var compensation seheme is proved feasible.(5) Based on the fundamental definition of MISCR, a new definition and calculation is deduced using Jacoby matrix. The new definition of MISCR can take all kinds of dynamic component model and HVDC control strategies into account. It eliminates the defects of original MISCR’s formula in which only impedance and capacity are considered. The method is used to calculate the MISCR in Yangtze River delta planning power grid with different HVDC control strategies, the results and DC characteristics are coincide.The index is also used to create an evaluate index for effection of dynamic var compensations. The typical genetic algorithm is used to search the best dynamic var equipment configuration whose index value is maximum. The method is applied in the south of Jiangsu power grid structure optimization; the grid structure optimization scheme is feasible.(6) Based on power grid structure optimization, the HVDC terminals’location optimization of and dynamic var compensations optimization, the heuristic method is applied to get the maximum multi-infeed HVDCs’ capacity to receiving end power grid. The three optimization methods are applied to Yangtze River delta planning power grid, the results show the stability and supporting ability for HVDCs are improved. The maximum capacity is 71800MW by 12 HVDC lines with several sets of dynamic reactive power compensation devices, 63800MW by 5 HVDC lines without dynamic reactive power compensation devices. |
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