Research On Operation Analysis Of Hybrid Multi-terminal Hvdc Transmission System

With the development of power electronics technology,hybrid DC transmission combined with line commutated converter(LCC)and voltage source converter(VSC)is considered to be an effective solution to solve new energy integration and constitute a multi-terminal topology.In consideration of large capacity and low losses,the traditional DC transmission is fit for large-capacity long-distance transmission.The control of hybrid DC transmission is flexible and convenient to form multi-terminal DC transmission,which is suitable for new energy integration and power supply to weak AC system.Hybrid multi-terminal DC transmission combines the advantages of the two,which can realize multi-power supply and multi-point power supply,providing a more flexible and fast power transmission method.Also,it has unique characteristics in the grid connection scheme of long-distance offshore wind power and photovoltaic power generation.Some projects have presented the prototype and plan ning of hybrid multi-terminal topology,so it is necessary to carry out research on hybrid DC technology.The main work of this paper includes the following parts:(1)Based on the basic composition of hybrid DC transmission,the basic operating principles of conventional DC transmission technology and flexible DC transmission technology are explained,and the mathematical models of the two are established.In this way,the basic control methods of conventional converters and flexible converters are clarified,laying a theoretical foundation for subsequent research.(2)The topology of the hybrid multi-terminal HVDC transmission system is described.On this basis,a four-terminal hybrid HVDC transmission system is set up.Then,the topology,basic principles and especially the control strategy are studied.The PSCAD simulation model is used for verification.(3)The control strategy applicable to hybrid multi-terminal HVDC transmission system is studied.Due to the fixed droop coefficient of the traditional fixed-slope droop control strategy,some converter stations may run at full load.Therefore,this chapter adopts a nonlinear droop control strategy suitable for hybrid multi-terminal DC transmission systems.Then,the corresponding mathematical model is established and the control method is analyzed.The simulation model established by PSCAD is compared with the simulation model of the fixed-slope droop control to verify the effectiveness of the nonlinear droop control strategy.(4)A control strategy for hybrid multi-terminal HVDC transmission system based on cyber-physical fusion is proposed.First,the cyber-physical model is introduced and a hybrid multi-terminal HVDC transmission model with cyber-physical fusion is established.Secondly,considering the risk of commutation failure caused by LCC converter on the inverter side,the probability of commutation failure is reduced by adding voltage dependent power order limiter control strategy to the host scheduling.Finally,considering the complicated information exchange between converter stations,control strategies for steady-state and transient-state are proposed to solve the problem of coordinated control between stations.The proposed coordinated control strategy is proved to be effective through simulations in various scenarios.