Research On Modeling And Control Strategy Of LCC-MMC Hybrid Hvdc Transmission System

Large-scale long-distance transmission from the west to the east is an important trend in the development of China's power grids,and the DC transmission system plays an important role.At present,China's large-scale long-distance power transmission has the characteristics of a single trend and dense dc landing point of the receiving end system,while hybrid HVDC transmission system combines the advantages of traditional direct current transmission(LCC-HVDC)and flexible direct current transmission(VSC-HVDC),becoming The research hotspot in the transmission field.In this paper,the LCC-MMC hybrid HVDC transmission system is adopted,that is,the rectifier side uses a grid converter(LCC),and the inverter side uses a modular multilevel converter(MMC).First,the principle of the two-terminal converter station is analyzed,and the mathematical model in different coordinate systems is established.The station-level controller of the LCC-MMC hybrid HVDC transmission system is determined.The LCC converter station adopts constant DC voltage control,the outer ring of the MMC converter station adopts fixed active power and fixed reactive power control,and the inner ring adopts direct current control to ensure the direct pressure stability of the hybrid HVDC transmission system and both ends of the converter station.Active power balance.Then,in order to improve the unbalanced voltage during the operation of capacitor voltage between MMC converter sub-modules,a sub-module capacitor voltage equalization controller is designed.This control strategy improves the frequent switching of sub-modules in the traditional sequencing method.It effectively reduces the switching frequency of the switching device.In order to suppress the double frequency circulating current of the MMC converter station,a general-purpose circulating current suppression controller is designed.The control strategy does not need to perform double frequency negative sequence transformation and phase decoupling control.And a wide range of applications.The simulation model of the system is established in the PSCAD/EMTDC simulation platform.Simulations are carried out for the designed control strategy.The simulation results show that the capacitor voltage is stable and the double frequency circulation is effectively suppressed when the system is running.Finally,in order to ensure the stability of the LCC-MMC hybrid HVDC transmission system,a three-stage startup control strategy based on the auxiliary power supply is designed for this system,which lays a foundation for the stable operation of the system.Simulation results demonstrate the feasibility of the proposed startup control strategy.