Coordinated Ac Fault-ride Through Control Schemes Of Offshore Wind Farms Integrated To Grid Via MMC-HVDC

Offshore wind energy has advantages of rich resources and concentrated distribution,and the MMC-HVDC technology is considered as an ideal solution for offshore wind power integration.However,since the MMC-HVDC decouples offshore wind farms and onshore power grid,the unbalanced power during grid fault would lead to serious dc-line overvoltage that endangers the secure operation of the equipment.Therefore,the fault ride-through capability of the system remains a major challenge.Among the recent fault ride through technologies,the voltage drop control method can effectively compromise the technology and economy,while there are still some key problems to be solved: at the equipment level,the interaction between the wind farm and MMC-HVDC may lead to the dc line voltage out of limit and the wind farms over-current;At the grid level,the integration of large-scale wind farms reduces the equivalent inertia of power system,thus grid fault induced wind power delayed recovery would aggravate grid frequency excursion.In order to improve the engineering application value of the voltage drop method,this paper makes profound studies on the key issues such as simplified model combinations of MMCs and WTs under voltage drop control,grid fault induced dc-line overvoltage and grid frequency dip related to post-fault active power recovery.Modular multilevel converter stations and offshore WTs are both composed of a large number of power electronic switches and complex multi-order control systems,and detailed models in electromagnetic transient simulation would seriously reduce the calculation efficiency.Therefore,appropriate model simplification is very important for fast simulation on dynamic interactions between onshore MMC and WTs after onshore grid fault.Firstly,basic simplified models are provided and the coordinated voltage drop scheme are utilized for the MMC and WTs respectively.Then,the transfer function models are derived to identify the impact of WT’s model simplification on the response accuracy of the transient output current.Meanwhile,the instantaneous power in each phase of MMC is calculated,and the impact of MMC’s model simplification on the response accuracy of dc line voltage is analyzed.Finally,the applicability of different simplified model combinations for MMC and WTs on simulating voltage drop control behaviors are further clarified through simulation comparative studies.MMC-HVDC connected PMSG-based wind farms could suffer serious challenges: dc-line overvoltage during a fault and post-fault dc-line voltage restoration.To mitigate dc-line overvoltage,a two-stage voltage drop control scheme is proposed for offshore MMC to realize a fast active power reduction and prevent the synchronization instability,by coordinating with a voltage-dependent active current control of individual wind turbines.The voltage drop depths are designed by considering the offshore de-loading efficiency and the dynamic stability limit of WTs.To improve the post-fault dc-line voltage recovery performance,an adaptive voltage rise control scheme is proposed to coordinate the recovery of onshore and offshore active power.The recovery trajectory of dc-line voltage is formulated and further employed to design the onshore active power ramp rate for uninterrupted operation of WTs.Case studies have been conducted on two test systems to validate the proposed method.Since DFIG-based WTs are more sensitive to AC voltage variation,the voltage drop at wind farm terminal may lead to the dc voltage oscillation instability in MMC-HVDC.A stepwise voltage drop control is proposed for the offshore MMC to de-load WFs and demagnetize DFIGs.Meanwhile,a feedforward stator current control structure is designed for the rotor-side converter of the DFIG to eliminate dc component in the stator current.The combination of the two strategies can effectively suppress the dc-line voltage oscillation.The eigenvalue method is used to analyze the impact of voltage drop depth at offshore terminal and the RCC’s bandwidth on WFs’ dynamic stability,by which the parameters of the stepwise voltage drop control are designed.The proposed method is validated by comparing with typical methods through time-domain simulations.When large-scale wind farms are integrated into a low inertia system by the MMC-HVDC,ac grid fault induced delayed recovery of wind power may lead to serious frequency excursion.By deriving the analytical transient model of AC/DC system,the impact of the active power ramping rate in onshore MMC and the transient frequency excursion in power grid are analyzed,and the relationship between wind power delayed recovery behavior and dc-line voltage overshoot constraints is described.An adaptive dc-line voltage control strategy is proposed for onshore MMC,and an adaptive two-stage active current recovery scheme is designed for WTs.The combination of the two schemes can not only improve the transient performance of dc-line voltage,but provides more inertial power to ac grid during post fault recovery period,such that the fault induced frequency dip can be mitigated effectively.The proposed method is validated by time-domain simulations.