Research On Frequency Support Control And Fault Ride-through Strategy Of Offshore Wind Farms Connected To The Grid Through MMC-HVDC Transmission

The symmetrical bipolar DC system based on modular multilevel converter(MMC)has the characteristics of flexibility,controllability,and efficiency,making it an effective solution for achieving offshore wind power aggregation and reliable transmission.Due to the decoupling between the AC frequency on the offshore wind farm side and the frequency of the receiving-end grid,the wind farm cannot actively provide frequency support for the receiving-end grid,resulting in a gradual decrease in system inertia.At the same time,faults in the receiving-end AC grid can easily cause power surplus in the flexible DC grid,resulting in DC overvoltage and posing a serious threat to the safe and stable operation of the system.Therefore,it is urgent to carry out research on frequency support control and fault ride-through strategies of offshore wind farms connected to the grid through MMC-HVDC transmission,in order to improve system safety and stability.Based on the analysis of the frequency support capability and the characteristic of receiving-end AC fault,this paper proposes a multi time scale frequency regulation control method and a coordinated control strategy for AC fault ride-through in the offshore wind farms connected to the grid through MMC-HVDC transmission.The effectiveness of the proposed method is verified through real-time digital simulation.The main research work is as follows:Aiming at the problems of reduced inertia and reduced frequency regulation capability of receiving-end AC grid caused by large-scale offshore wind power connected to the grid through MMC-HVDC transmission,a coordinated control strategy of offshore wind power and MMC-HVDC is proposed to support system frequency,which makes them show the characteristics of the dominant power supply urgently needed by the new power system.In terms of inertia support,by analogy with the inertia response characteristics of synchronous generator,using the energy of DC capacitor to actively support the system inertia,and establishing the coupling relationship between fan speed and frequency through DC voltage,a coordinated control strategy of wind farm is proposed to support inertia,which based on differential rotor kinetic energy regulation,improve the inertia level of receiving-end grid.In terms of frequency deviation regulation,according to the local DC voltage deviation,the primary frequency regulation strategy of wind farm based on fan speed control and pitch angle control is proposed.Combined with the gradual scarcity of traditional frequency modulation resources,the secondary frequency regulation strategy of wind farm based on additional pitch angle control is designed to fully tap the frequency modulation potential of offshore wind farm,to improve the frequency stability of system.Aiming at the DC overvoltage problem of the MMC-HVDC system caused by the AC fault of the receiving-end grid,based on the analysis of the mechanism of DC overvoltage caused by the fault of the receiving-end AC grid,the coordinated suppression strategies of monopolar and bipolar DC overvoltage are proposed to achieve the receiving-end AC fault ride-through.For the monopolar DC overvoltage,by reasonably switching the control mode of bipolar MMC,the power transfer capability of the non-fault pole MMC can be improved while maintaining DC voltage stability,and the precise load-shedding control strategy of wind farm is designed to ensure the full load operation of non-fault pole MMC,so as to reduce the impact of monopolar MMC outage on the receiving-end system.For the bipolar DC overvoltage,a wind farm load-shedding control strategy based on local DC voltage measurement information is designed.This can actively reduce the output power of the wind farm according to the DC voltage change rate and deviation,and effectively suppress the rise rate and amplitude of DC voltage.Then additional pitch angle control and its parameter selection principle are proposed to make the wind farm and the capacitors in each converter station jointly maintain the stability of DC voltage,so as to improve the fault ride-through capability of the system.