Coordinated Optimal Strategy For MTDC System With Large-scale Offshore Wind Farms

From the Industrial Revolution to the present,fossil energy has been the source of power for social development and progress.With the continuous exploitation and use of fossil energy,fossil energy is facing exhaustion,and environmental pollution are increasing.The development of clean energy to replace fossil energy and to change the existing energy structure has become the primary task of all countries in the world.Among the clean energy,wind power generation is one of the most mature,lowest operating cost,and most commercially-applied power generation methods.In recent years,offshore wind power generation has been favored by countries all over the world with its huge potential.Many countries have begun to develop and deploy offshore wind power generation.The high-voltage direct current(HVDC)transmission system is used to connect offshore wind farms(WFs)and the power system,which provide electrical energy to the grid.In recent years,many multi-terminal direct current transmission(MTDC)projects connecting offshore WFs and power systems have been established in the world.But the safe and stable operation of the MTDC system with large-scale offshore WFs has faced many economic and technical challenges.The safe,stable,and economic operation of the MTDC system with large-scale offshore WFs is very important to the grid.The coordination and optimization strategy of the MTDC system with large-scale offshore WFs is an effective means to achieve this goal.The coordination and optimization strategy of the MTDC system with large-scale offshore WFs has become one of the main factors restricting the economic benefits of the MTDC system with large-scale offshore WFs.In particular,the global coordination and optimization strategy suitable for the MTDC system with a large number of ports and large-scale offshore WFs remains to be studied.This paper focuses on the coordination and optimization strategy of the MTDC system with large-scale offshore WFs,aiming to make the voltage of the entire system including WFs and MTDC in a reasonable range,and the entire system is in an economic operation state.This paper expands from the common two-layer coordination and optimization strategy to the centralized coordination and optimization strategy,and proposes a more flexible and robust distributed coordination and optimization strategy in combination with distributed algorithms.The main research work and contributions of this paper are as follows:(1)The mathematical model of the MTDC system with large-scale offshore WFs is established in this paper,including the mathematical model of the output active and reactive power dynamic response process of the WT,and the mathematical model of the wind farm side voltage source converter(WFVSC),the mathematical model of the grid-side voltage source converter(GSVSC),and the mathematical model of the DC system.To coordinate the internal voltage of WFs,this paper introduces a calculation method of voltage sensitivity coefficient suitable for the radially distributed distribution network.Based on the voltage sensitivity coefficient,the calculation method of the sensitivity coefficient of the active power loss is derived.(2)In this paper,for the MTDC system with large-scale offshore WFs,the WF optimization model is established to minimize the internal voltage fluctuation,the active power loss,and the fairly output active power of WTs in consideration of the constraint of the WFVSC terminal voltage,the output active and reactive power of WTs,and the output power of the WF;the MTDC optimization model is built to minimize active loss of the MTDC in consideration of the constraint of the current limit of the VSC DC side bus,the current limit of the DC cable connected to the VSC,and the DC side voltage limit of the VSC of the MTDC system and the output power of the GSVSC.Based on the constructed the WF and MTDC optimization model,this paper proposes a centralized model predictive control(MPC)coordination and optimization strategy suitable for the global optimization of the MTDC system with large-scale offshore WFs based on the two-layer coordination and optimization strategy proposed by previous scholars to independently optimize the WF and MTDC.The centralized MPC coordination and optimization strategy suitable for the MTDC system with large-scale offshore WFs has a large amount of calculation and a heavy burden on the central controller.Based on alternating direction multiplier method(ADMM),this paper proposes an ADMM-based distributed MPC coordination and optimization strategy suitable for the MTDC system with large-scale offshore WFs.The distributed MPC coordination and optimization strategy based on ADMM could reduce the heavy burden of calculation on the central controller.The distributed MPC coordination and optimization strategy based on ADMM proposed in this paper has few iteration steps,fast convergence speed,and real-time nature,which is suitable for engineering applications.(3)The proportional-integral(PI)controller be introduced at the output power of GSVSC to enhance the tracking performance of GSVSC on dispatching demand in this paper.The introduction of the PI controller can eliminate the static errors caused by the inaccurate model and interference of the entire system.While ensuring the tracking accuracy of the GSVSC output power,it can also improve the dynamic and static performance of the system.(4)This paper builds a 5-port MTDC based on MATLAB/Simulink,including 3WFVSCs and 2 GSVSCs.Each WFVSC is connected to an offshore WF.Based on this model,the three control strategies involved are verified in the simulation.