Research On Control Strategy Of PMSG-Based Offshore Wind Farms Connected To The Grid Via HVDC Transmission

The offshore wind farms with huge exploitation potential are gradually becoming an important part along with the further deployment of the renewable energy field,which is the strategic driving force for accelerating the realization of China’s lowcarbon energy and power development.MMC-HVDC transmission technology has gradually become the best choice for large-scale offshore wind power generation system of long-distance and large-capacity centralized transmission.However,with the increasing penetration rate of wind farms integrated to the power grid,the occurrence of wind turbine off-grid accidents by the grid voltage and frequency oscillations has become more and more serious in recent years.For these reasons,the research of gridconnected capacity of wind turbines and improving the fault ride-through capability of MMC-HVDC integrated offshore wind farms is the key to establishing the steady-state operation characteristics of the grid-friendly system and an important guarantee for realizing the dual-carbon goals.In this paper,the following research work has been carried out:(1)Aiming at the low voltage fault ride-through problem of the permanent magnet direct-driven wind turbines(PMSG)under voltage dips transient conditions,a voltage control strategy considering supercapacitor and dynamic reactive power support is proposed in this paper.The active current command of the current inner-loop controller can quickly suppress the active power input for the machine side VSC,and the DC side of wind turbines absorbs the unbalanced energy by using the supercapacitor.The grid side VSC simultaneously adopts reactive power priority control to provide maximum reactive power support to the system grid in conditions of system fault.The simulation research results show that the proposed method can suppress the transient fluctuation of the DC link voltage during the grid faults,and improve the operation characteristics of wind turbines.(2)Aiming at the problem that the PMSG does not have the ability to dynamically regulate the frequency in the maximum power point tracking mode and has the problem of frequency secondary drop,a coordinated frequency control strategy based on rotor kinetic energy and supercapacitor energy storage is proposed.The coordinated use of rotor kinetic energy and supercapacitor energy storage can fully exploit the hidden inertia response capability of wind turbines under the system frequency fluctuation conditions.And while simultaneously using supercapacitor to participate in the whole process of system frequency regulation,it can improve the problem of frequency secondary drop.The simulation research results indicate that the proposed strategy can enhance the frequency regulation capability of wind turbines under full working conditions while further improving the economy and adaptability of the PMSGs.(3)Aiming at the fault conditions of the grid-connection point of MMC-HVDC integrated offshore wind farms,a coordinated optimization control strategy is proposed to alleviate the system stability problem.The proposed method solves the power surplus problem on the DC grid side of the MMC-HVDC system from two perspectives:voltage reduction coordinated control and frequency coordinated control,respectively.First,the topology structure and control method of MMC-HVDC integrated offshore wind farms are analyzed.Then,the MMC and the active power control capability of the PMSGs are used to maintain the stable operation of the DC side voltage of the MMC-HVDC integrated offshore wind farms.The simulation results show that the proposed coordinated control method can effectively achieve AC systems fault ridethrough under different voltage fault transient conditions.In summary,this paper focuses on the voltage ride-through modeling and control,the coordinated frequency control strategy of PMSG-based wind farms,and the coordinated optimization control scheme of the MMC-HVDC integrated offshore wind farms.The effectiveness of the proposed control strategies is verified in this paper by simulation system analysis,so as to improve the overall operation stability and better control performance of large scale wind farms.