Research On Low Voltage Ride-through Technology Under Unbalanced Grid Situations For Integrating Offshore Wind Farms Via MMC-HVDC

Nowadays,offshore wind power generation is a new direction for the development of wind power industry,achieving more and more attentions over the world.Many countries have invested massive manpower and material resources in researching related technology of offshore wind power generation.Grid-connection technology of the offshore wind farms based on HVDC flexible is the hotspot and front in offshore wind power generation research field.With continuous expansion of offshore wind farms' scale,more and more strict offshore wind farm Integration requirements are formulated that the low voltage ride-through problem for integrating offshore wind farms via HVDC-Flexible power transmission becomes an unavoidable problem.MMC-HVDC is a new type of HVDC topology,which has lots of advantages,such as excellent expansibility,wonderful reliability,high output voltage,low harmonic content and so on.It is recognized to be the best Grid connection scheme for offshore wind farms.However,the study on the low voltage ride through technology of offshore wind farm connected to grid system via MMC-HVDC is still in the initial stage.In view of this,this article mainly researchs on the low voltage ride through problem of the integrating offshore wind farm based on MMC-HVDC,especially focusing on low voltage ride through technology under asymmetrical grid faults situation.Firstly,this paper analyzes the DFIG operation principle,mathematical model and control strategy.Based on this,30 MW simulation model of doubly fed offshore wind farm is builded up in Matlab/Simulink.Secondly,the mathematical model of MMC is established.According to 7level MMC,the inner current loop based on internal model controller is designed.Then the the control scheme of MMC-HVDC system is put forword.Based on it,the steady state simulation model of 30 MW integrating offshore wind farm via MMC-HVDC is established in Matlab/Simulink.Thirdly,the mathematical model of MMC under unbalanced grid is analysed.In order to suppress the negative sequence current and the DC bus voltage fluctuation,this paper proposes two control schemes based on both PI controller and internal model controller.Simulation results shows that compared with PI control scheme,internal model control scheme can achieve better control effect.After analysing the infulance of asymmetrical grid faults on system operation and the fault characteristics of the integration system,the System level control scheme of the grid connected offshore wind farm based on MMC-HVDC system under unbalanced grid situations is established.With two control targets of suppressing negative sequence current and reducing DC bus voltage fluctuation,the internal loop controller and DC energy releasing circuit is designed in the control system.At last the steady-state and dynamic performance of the proposed controllers and control scheme under two kinds of unbalance operation conditions are shown by the simulation of the 30 MW integrating offshore wind farms via MMC-HVDC in Matlab/Simulink software.The simulation results show that,the control strategy proposed in this paper can ensure the offshore wind farm based on MMC-HVDC can keep on working while the grid volatage is in asymmetric situation and effectively improve the low voltage ride through ablilty of the offshore wind farm.