DC Fault Ride-through Coordinated Control Strategies For Wind Power Connected Bipolar MMC-MTDC System

With the shortage of traditional energy and the improvement of environmental protection awareness,renewable energy represented by wind power has been vigorously developed in recent years.High voltage direct currect transmission technology based on voltage source converter（VSC-HVDC）has become an effective way for long-distance transmission of large-scale wind power due to its unique advantages.Considering that the DC system usually uses overhead lines for DC transmission,the failure rate is high.When a unipolar grounding fault occurs on the DC side and the DC circuit breaker is isolated,it is easy to produce a large amount of unbalanced power,which seriously endangers the safe operation of the system.In order to absorb the unbalanced power in the DC system and finally realize DC fault ride-through,this paper proposes a coordinated control strategy between converter stations and a coordinated control strategy between converter stations and energy dissipation resistance,and designs the energy dissipation resistance control criterion and wind farm load-shedding control strategy.The effectiveness of the proposed control strategy is verified by PSCAD simulation platform.Firstly,the components of wind power access to DC system are analyzed.The mathematical model of permanent magnet synchronous generator（PMSG）under normal operation is established.The modular multilevel converter（MMC）is analyzed from the perspective of system level control,station level control and valve level control.The influence of wind farm on DC system under wind speed fluctuation is simulated,which lays a foundation for the subsequent research on the control strategy of converter stations and energy dissipation resistance of multi-terminal HVDC based on modular multilevel converter（MMC-MTDC）and the load-shedding of wind farms.Then,according to the relationship between the unbalanced power in the system and the power margin of the converter stations,the coordinated control strategy between the converter stations and the coordinated control strategy between the converter stations and the energy dissipation resistance are adopted respectively.The coordinated control strategy between the converter stations are to allow the non-fault pole converter station at the sending end to independently absorb the fault pole unbalanced power of the system through the P-f droop control.The coordinated control between the converter stations and the energy dissipation resistance are designed by designing the energy dissipation resistance input and removal control criteria,and cooperates with the power transfer of the converter stations in the early stage of the fault.The above coordinated control strategy can effectively reduce the unnecessary power loss of the system and ensure the safe operation of the DC system during fault ride-through.For the receiving-end converter stations,the P-U_dc droop control can improve the dynamic adjustment ability of the DC voltage during the fault ride-through period,and the normal transmission of power can be guaranteed by adjusting the droop coefficient before and after the fault ride-through.The effectiveness of the above coordinated control strategy is verified by simulation.Finally,the overspeed and variable pitch angle control of wind farms are introduced,and the control criteria of power transfer and energy dissipation resistance of converter stations are improved.In the case of self-consumption,the fault poles and non-fault poles of the two converter stations carry out power transfer separately or jointly.Aiming at the situation of coordinated consumption,the converter station is used to cooperate with the corresponding energy dissipation resistance.When the permanent fault of the system is determined,the wind farms will jointly carry out load-shedding to reduce the probability of wind farm tripping,and effectively improve the safety of wind power access to the DC system during fault ride-through.The feasibility and effectiveness of the coordinated control strategy are verified by simulation.