Research On Modeling And Control Strategy Of LCC-VSC Hybrid DC Transmission System

Hybrid DC transmission combines the advantages of conventional HVDC transmission and flexible HVDC transmission.It has outstanding advantages in large capacity transmission,power supply to passive network and new energy export,and has become a research hotspot of HVDC transmission.This paper analyzes the point-to-point hybrid HVDC and hybrid multi terminal HVDC systems.The main research contents are as follows.Firstly,this paper analyzes the advantages and disadvantages of conventional HVDC and flexible HVDC,constructs a point-to-point current source converter modular multilevel converter(lcc-mmc)transmission structure,and constructs a multi terminal hybrid HVDC system based on the invariable voltage polarity of VSC.In order to fully highlight the advantages of the Hybrid DC transmission structure,this paper analyzes the point-to-point lcc-mmc transmission system in three typical conditions,namely,the transmission condition with active network at both ends,the power supply condition with passive network at one end,and the condition applied to offshore wind power transmission.This paper analyzes the mathematical model of Hybrid DC transmission under different working conditions,constructs different control strategies according to three different working conditions,and builds simulation models under different working conditions for simulation verification.In order to suppress the inter phase circulating current,a circulating current suppression strategy of MMC is proposed based on negative sequence decoupling.The innovation of this paper is to study the start-up control of lcc-mmc transmission system supplying power to passive network.Because there are few literatures on Hybrid DC start-up control,this paper compares the start-up process of transmission system with MMC at both ends,The differences in the start-up process of hybrid HVDC system and the instantaneous current fluctuation after MMC pre charging and unlocking are analyzed.In order to solve the problems of excessive AC current impulse and large DC voltage fluctuation at the moment of MMC unlocking in the inverter side of hybrid HVDC transmission system,a constant DC voltage control strategy with slope is proposed in the rectifier side,With the bridge arm energy sharing control and sub module capacitor voltage regulation control,the sub module voltage is charged to the rated value.A control group experiment was designed and a simulation model was established to verify the proposed improvement strategy.Transient analysis plays a key role in the system.Based on the background of Hybrid DC transmission system supplying power to passive network,the transient simulation analysis of Hybrid DC transmission system is carried out.The single-phase ground fault at the end of the AC line at the inverter side is set,and the fault duration is 50 ms.The waveform changes of the important parameters of the system before and after the fault are compared to observe whether the system can recover after the fault.Because the voltage polarity of VSC is not changed,it is more convenient to build a multi terminal network.Different from the conventional multi terminal network,the hybrid converter structure is adopted in this paper,which is similar to Wudongde super transmission project.The sending terminal includes a conventional station,LCC converter,a wind power transmission station,MMC structure,and an active network through MMC outgoing station;The two receivers are MMC flexible stations connected to the active network.Aiming at the hybrid five terminal HVDC transmission structure,the coordinated control among stations is studied,and the active power balance strategy is designed when both receiving converter stations are out of operation.