Control Strategy Of MMC-MTDC Transmission System With Windfarms

Modular multilevel converter(MMC)is a new type of voltage source converter,which has the advantages of high voltage quality,low switching loss and easy expansion.Compared with the two-terminal HVDC transmission system,the multi-terminal HVDC(MTDC)transmission system is more flexible and reliable,which is the foundation of the DC power grid construction in the future.The multi-terminal HVDC transmission system with modular multi-level converters can not only take the advantages of multi-terminal HVDC transmission into account,but also make full use of the unique technical advantages of MMC,showing a strong application prospect in regional grid interconnection and grid connection of large wind farm.In this thesis,the modeling and control of MMC converter station,the coordinated control of MMC-MTDC system with wind farm,and the power transfer control of MTDC system when DC fault occurs are mainly studied.Fast and accurate electromagnetic transient simulation is the basis for the study of MMC and MTDC control strategies.Firstly,the operation characteristics and basic control strategies of MMC are detailed introduced in this thesis.On this basis,the controller suitable for unbalanced grid voltage and the third harmonic modulation technology are applied.Based on the above,the speed-up method of simulation model is studied.The quicksort algorithm is applied to accelerate the sorting speed of capacitor voltage.The fast electromagnetic transient simulation algorithm is applied to MMC.Considering the normal and blocking operation of converter,the simulation speed of MMC is greatly improved by simplify the SM and arm by means of Thevenin equivalence.The overall modeling of MTDC with windfarms is the basis of studying its operation characteristics and control strategy.In this thesis,the wind power system based on DFIG is introduced.A single equivalent windfarm model is established and verified by simulation.Based on which,the control strategies of MTDC transmission system is studied.Based on Zhangbei flexible DC grid,a four-terminal flexible HVDC transmission model with single-machine equivalent windfarms is established in PSCAD/EMTDC.In order to coordinated control the multi-terminal DC system,DC voltage coordination controller is configured in the grid side converter which reference to the margin control idea.Considering the isolated operation mode of windfarm,AC voltage droop controller is applied in windfarm side converter.The simulation results verify the correctness of the model and the controller.Finally,a power transfer control strategy for MTDC transmission system is proposed to solve the problem of power over-limit in DC network after the DC line cut-off.Considering the power margin of the converter and the capability of the DC circuit breaker,the boundary conditions for the safe operation of the system are determined.The isolated wind farm is regarded as an uncontrollable power source.The power transfer control between the poles of the wind farm side converter is realized by considering the translation of the droop curve and the cut-off control of the windfarms.Which is used for limit the DC line current.The simulation results shows that the control strategy can improve the power transmission capability of MTDC during the faults,realize the safe operation goal in "DC line N-1"status,which improve the reliability of the system.