Research On Key Technologies Of Grid Connection Control And Protection Of VSC-HVDC System With Wind Farm Integration

With the increase of the transmission distance of wind farms,the charging effect of the submarine cable makes the shortcomings of traditional AC transmission methods,e.g.,low efficiency and voltage stability,become increasingly obvious.Therefore,the flexible direct current transmission(voltage source converter based high voltage direct current,VSC-HVDC),is widely regarded as the future trend of the grid integration of large-scale wind farm clusters.It has relatively low long-distance transmission cost,flexible active and reactive power control,no charging current,and the capability to integrate to islanded grids.With the construction of related practical projects,the control and protection technology for VSC-HVDC systems with wind farm integration has become the research focus.Based on the VSC-HVDC systems with wind farm integration,this paper aims at solving two main challenges:the frequency stability problem of the power grid with high renewable energy penetration and the fault safety of large-scale VSC-HVDC system with wind power integration.The research is carried out in three levels:modeling,grid frequency support and fault response.The main research contents and innovation results of this paper are as follows:1)Autonomous grid-synchronizing control of VSC-HVDC with real-time frequency mirroring capability for wind farm integration.Inertial synchronizing control is proposed and adopted in the receiving end converter(REC)station.The inherent dynamic characteristics of the DC bus capacitor is utilized in the REC to synchronize with the grid autonomously without using a Phase-Locked-Loop(PLL).With this control strategy,the grid frequency variation can be reflected on the DC bus voltage.A real-time link is estabilished between the DC voltage and the AC grid frequency,thus realizing an autonomous sensing of the grid frequency.Also,the influence of wind power fluctuations and grid voltage fluctuations on the grid frequency sensing is analyzed,and a method to suppress these disturbances is proposed.The frequency mirroring control is proposed for the sending end converter(SEC)station.With this control strategy,the grid frequency can be mirrored to the AC collection bus of wind farms to facilitate the inertial response of wind turbines.Then the active power of the inertial response is extracted to the AC grid through the REC,which can damp the frequency fluctuation of the AC grid.2)Autonomous frequency response control of multi-terminal DC systems with wind farm integration while connected to multiple AC gridsFor the scenario where the wind farms are integrated into multiple AC grids through the VSC-HVDC system,the SECs will maintain the frequency mirroring control.In RECs,the P-U_dc droop control is introduced into the inertial synchronizing control,so that the RECs can automatically share the power flow without communication,and provide autonomous inertial response.In addition,the AC grids connected by the HVDC system are capable to provide primary frequency regulations to each other.3)Control and protection of VSC-HVDC system with wind farm integration under faultFirstly,the system's fault control and protection method is analyzed under different system(point-to-point DC link and multi-terminal DC grid)and different fault types(AC main network fault,wind farm side fault,DC fault).Following the deloading requirements under different fault ride-through process,several deloading schemes are compared,and the optimal deloading scheme is given under different system structures.A focused research is made on the VSC-HVDC system with offshore wind farm integration.4)Coordinated low voltage ride-through strategy between the VSC-HVDC system and wind farmsA coordinated low-voltage fault ride-through strategy between the DC-side deloading devices and wind turbine deloading devices is proposed in this paper.This control strategy includes:the method to transmit the AC grid fault to the wind farm side rapidly;the coordinated switching sequence for DC-side deloading devices and deloading devices in wind turbines.The operating time of DC side deloading devices is reduced to about 100ms.Moreover,the deloading resistors are distributed into the submodules of the receiving end modular multilevel converter.This method controls the deloading power more accurately and reduces the cost of the deloading devices.With this coordinated strategy,the low-cost fault ride-through for VSC-HVDC systems and wind farms is realized.