Research On LCC-HVDC And MMC-HVDC Interconnection System Based On Un-interrupted DC Autotransformer

According to the resource distribution characteristics of China and the current state of the development and construction of high voltage direct current(HVDC)projects,new modular multilevel converter-based HVDC(MMC-HVDC)can be considered to interconnect the existing line commutated converter-based HVDC(LCC-HVDC)to enable renewable energy integration and large-scale long-distance power transmission.Specifically,renewable energy generations are collected by means of MMC-HVDC and then sent out over long distances through LCC-HVDC.This solution takes the respective advantages of the two types of HVDC technology and can reduce investment costs.In order to meet the future demand of interconnecting DC lines of different voltage levels or different types,an interconnected system of LCC-HVDC and MMC-HVDC based on un-interrupted DC autotransformer(DC AUTO)is proposed in this thesis.Some key problems such as the topology structure,operation modes,and DC fault ride-through strategy of the system have also been studied.The main works are listed as follows.(1)The topology of the interconnected system of LCC-HVDC and MMC-HVDC using un-interrupted DC AUTO is proposed.The application fields and advantages of the 4-terminal interconnected system are analyzed.The basic structure and configuration method of the critical equipment,i.e.,un-interrupted DC AUTO,are also studied and discussed.(2)The steady-state control strategy of the interconnected system is given,which includes both the control strategy of each converter in the system and the power flow optimization strategy in systematic level.Simulations on PSCAD/EMTDC platform show that the local controller of the converters can maintain power balance and voltage stability of the system automatically.Meanwhile,the efficacy of the communication-based power flow optimization strategy on realizing the systematic optimal power flows is also verified.(3)The control strategy of the interconnected system under N-1 conditions(converter station withdrawal)is presented.Firstly,the constant DC voltage control strategy of uninterrupted DC AUTO is proposed to ensure continuous operation of the system under different N-1 conditions.Then,the fault response of the system under each N-1 condition is analyzed and the corresponding operation control strategy is provided.Finally,simulation results demonstrate that the interconnected system can always recover to a 3-terminal DC system after N-1 fault.(4)The DC fault ride-though strategy for the interconnected system is proposed.The control strategy of each converter during DC fault is designed at first,and the configuration scheme of the system's protection devices is then investigated.By considering the coordination between different types of converters and protection devices,the DC fault ridethough strategy for the interconnected system is developed and the effectiveness is verified through simulations.