Research On Fault Ride Through Strategy Of Offshore Wind Farm Connected To Flexible DC Grid

The exhaustion of fossil energy is uncertain,and environmental problems are becoming increasingly severe.In the future,the proportion of renewable and clean energy integrate to the grid is bound to increase significantly.As a typical and abundant renewable clean energy,offshore wind power will also develop rapidly.At present,flexible DC technology and its derived DC grid technology have incomparable advantages in terms of transmission distance,anti-interference ability and control response of offshore wind power integration.The largescale offshore wind farm through the dc grid interconnection,and remote to the load center project has not yet existed,its system anti-interference characteristics and ac and dc fault ride through strategy are lacking.To this end,this paper has carried out the following work:(1)Based on the research of offshore wind power generation system and flexible dc transmission technology,the electromagnetic transient simulation model of offshore wind farm connected to flexible DC grid is established based on PSCAD/EMTDC,and the dynamic and transient response characteristics of the model are analyzed.The results show that the steadystate operation and disturbed response characteristics of the model are consistent with the theoretical analysis,which lays a foundation for further research.(2)The dc side fault ride through strategy based on hybrid dc circuit breaker is presented.The response characteristics of wind farm and dc power grid during the process of dc fault ride through are studied respectively.In the process of fault ride through,the power of the dc side of the system fluctuates greatly,however,the fluctuations are smaller at point common coupling(PCC)of wind farm.On this basis,the methods to improve the system's dc fault crossing ability are discussed.Aiming at the possibility that the wind farm crossing fails due to temporary blocking in the converter station,a dc current limiting measure is proposed.The current limiter has low on-state loss,fast operation speed,and does not affect the steady-state response characteristics of the system.Simulation results show that this measure improves the fault ride through capability of the dc side of the wind farm dc power grid system and enables the system to cross the most serious dc line fault.(3)Under different voltage sags in AC power grid,the response characteristics of wind power and DC power grid side and the dynamic process of control system are studied.The simulation results show that the system can pass through slight AC faults under the coordination and buffering of each control.However,under more serious AC faults,the voltage on the wind farm and DC grid side exceeds the limit and oscillates,and can not be restored after the fault is eliminated.Closely around the relevant technical regulations of wind power and flexible DC,the main contradiction of AC fault traversing in the system,power surplus,is determined.The adaptability of two typical traversing strategies is analyzed.A Fault ride through strategy based on AC phase-to-phase energy dissipation resistance is proposed.The cost of switching devices required in this strategy is low,and no additional offshore grounding devices are needed.The simulation verifies the feasibility of the strategy.