| The HVDC is getting more and more applications with the expansion of the socialdemand and the requirement of becoming greater capacity and more closely related in thepower grid scale development. The VSC-HVDC, which is advanced in its flexible control,high applicability, and many other advantages, has a wider application prospect compared tothe Line Commutated Converter-HVDC. However, as long as the research in VSC-HVDC isconcerned, our technology is lagging behind the advanced technological levels in the world.In the same time, VSC-HVDC has a short age of the engineering practice and is lack ofengineering operating experience. It implies us that the problem how to make full use ofadvantage of the power transmission of VSC-HVDC system to improve the operating safetyand voltage stability of the power grid is worth to research.This thesis firstly summarizes the main influence in the operation of power grid after theVSC-HVDC access to it. By means of reviewing the existing problems in the AC/DC parallelsystems with VSC-HVDC at home and abroad, it points out the transmission power allocationproblem in AC/VSC-HVDC system, which will be mainly discussed in this thesis. Based onthe detailed analysis of the mechanism and the control strategies of VSC-HVDC system, itpoints out the key of improvement of transmission power allocation in AC/VSC-HVDC is toset up the orders from the system-level controllers of VSC-HVDC reasonably.The objective functions are established with considering the constraints of multipleobjectives such as line loss, equipment failure rate, and power system stability in this thesis.Based on the fuzzy theory, the objective functions are specified to their correspondingmembership functions, and then the optimal assigned value of active power of the AC/DCparallel transmission system can be figured out. Comparing to the method of allocating activepower flow in the AC/DC parallel system based on operating mode and operating experience,the multi-objective optimization method is much more scientific and reasonable withtheoretical basis.To give full play to the power control of technology advantages of VSC-HVDC, and thereduce pressure on ac/dc system of power grid voltage regulation, as well as to improve thevoltage level, the thesis studies to consider VSC-HVDC as one means of voltage regulation in the volt/var control optimal objective function. Under the current constraints of VSC-HVDCin different control modes, it puts forward a reactive power optimization strategy which isbeneficial to improve the efficiency and operation stability of the power grid.The power optimization control method proposed in this thesis, can provides technicalreference for VSC-HVDC engineering application to making reasonable power allocationschemes. It can also enhance the stability of power grid and improve its economy, and has thevital significance in giving full play to the advantages of flexible control of VSC-HVDC. |
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