| The ever-increasing scale of grid connection of renewable energy generation has caused the grid to pose new challenges to its transmission methods.Flexible DC transmission has many advantages such as supplying power to passive networks,decoupling active and reactive power,and has gradually attracted widespread attention in the industry.Although it has certain advantages in transmitting renewable energy,it still cannot fundamentally solve the problems of fluctuation,intermittentness and randomness of renewable energy.Consider connecting variable-speed pumped storage units in the straight system as a tool to stabilize the fluctuation of renewable energy output and peak-cutting and valley-filling of the power system to ensure the stability of the transmitted power.In this paper,the basic theory and coordinated operation strategy of the joint system of flexible DC transmission system and variable-speed pumped storage unit including wind turbine are analyzed and simulated,and the static and dynamic factors affecting the active power regulation capacity of the joint system are evaluated quantitatively.Firstly,this paper introduces the structure and operation principle of each component of the combined system,constructs the mathematical model of variable speed pumped storage unit,wind turbine unit and flexible DC transmission system,and studies the control strategies of the converter on the generator side and grid side of the doubly fed machine,and the converter on the sending side and receiving side of the flexible DC system.Based on the MATLAB/Simulink simulation platform,the dynamic simulation analysis of the active power regulation capacity of the variable-speed pumped storage unit under the wind power output fluctuation is carried out when the variable-speed pumped storage unit operates in the two working conditions of power generation and electric power respectively.The results show that the proposed mathematical model and control strategy are correct and effective.When the joint system supplies power to the load on the receiving side,various factors will affect the active power transmission capacity of the joint system.In this paper,the static influencing factors of variable speed pumped storage unit side and wind turbine side are listed through mathematical coupling relationship,and the dynamic control parameters are analyzed through simulation model.Finally,based on the fuzzy analytic hierarchy process,the quantitative evaluation system of the active power regulation ability of the joint system is established.The importance of each influencing factor is ranked by the fuzzy judgment matrix.The membership function in the fuzzy theory is used to convert the weight value into the intuitive evaluation value,and the strong and weak influencing factors related to the active power regulation ability of the joint system are obtained.The research results of this paper can provide theoretical guidance and technical basis for the coordinated operation of the joint system and the optimal output of active power.It can realize the flexible operation of the DC power grid and the fast and reliable consumption of large-scale renewable energy access,thus ensuring the power balance of the joint system and the safety and reliability of the grid operation. |
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