| Offshore wind energy is inexhaustible,clean and green,and the development of offshore wind power technology will bring profound changes to social progress.The access of offshore wind farms to main grid will change the power flow distribution,and influence the scheduling,loss and voltage distribution of power system.For the system of offshore wind farm with connection to the grid,under the condition of meeting the security constraints of the power system,the study of the optimal power flow is helpful to explore the economic operation rule of the power system with the offshore wind farm connected to the grid.In addition,it is of great significance to determine the optimal access point of offshore wind farms and the optimal installation capacity of offshore wind farms.It is of great importance to ensure the stable operation and improve the economic efficiency of the power system.Firstly,the grid connected mode of offshore wind farm is studied,and the transmission structure and principle of three transmission technologies including high voltage AC transmission,traditional HVDC transmission and VSC-HVDC transmission is analyzed in this paper.The work characteristics of the three kinds of grid connected mode is compared.The voltage source converter VSC is more advantageous than the others in technology.Focusing on the VSC-HVDC technology,the basic characteristics of VSC is analyzed and the engineering application of VSCHVDC link is introduced.On the basis of a large number of literature,this paper analyzes the safe operation limit of the doubly fed induction generator in the wind farm,derives the power flow equation of the VSC-HVDC system.Based on the maximum valve current and the maximum DC voltage of VSC,the capacity constraints of the VSC is given.In order to minimize the total generation cost of power plant in AC grid,the optimal power flow model of the power system with connection to the offshore wind farm by VSC-HVDC link is established,and the nonlinear programming solver based on GAMS software is used to solve the model.A case study is carried out to verify the correctness and effectiveness of the optimal power flow model on IEEE 14 bus test system and IEEE 118 bus test system,resepectively.In order to reduce the influence of reactive power distribution and voltage distribution of the system with connection to the offshore wind farm,the planning of the reactive power compensator is considered while planning the offshore wind farm with integration into the grid.Then,the installation,investment,operation and maintenance costs of offshore wind farms,VSC-HVDC systems and shunt capacitors is taken into account,with consideration of the discrete characteristics of shunt capacitor banks during the planning period.So as to maximum the total benefit of the power system investment operators,an integrated planning model for offshore wind farms based VSC-HVDC and shunt capacitors is proposed in this paper.In essence,the proposed model belongs to a complex mixed integer nonlinear programming problem.Thus,a corresponding decoupling method is proposed in this paper,and the correctness and effectiveness of the model and the decoupling method are verified on the modified IEEE 118 bus system.At last,with comparation to the planning models before and after decoupling,it is found that the optimal solution of the decoupled planning model can be obtained on the GAMS platform.In addition,according to the simulation results before and after installing the capacitors,it is found that the integrated planning model of offshore wind farm planning with consideration of the shunt capacitor banks is more economical,and the total active power loss of the system is lower,too. |
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