Research On Non-blocking DC Fault Ride-through Control Strategy For LCC-FHMMC Hybrid Three-terminal DC Transmission Syste

China’s energy distribution is characterized by "rich in the west and poor in the east,abundant in the north and less in the south." Meanwhile,the load distribution shows that "developed coastal regions have more load,while underdeveloped inland regions have less." This results in a separation between the centers of power supply and consumption,with a reverse distribution pattern."West-East Electricity Transmission" is a basic infrastructure strategy in the current stage of China’s economic development,which can provide a green,reliable,efficient,and flexible cross-regional new energy consumption and allocation plan to achieve "emission peak,carbon neutrality",and can vigorously promote the development of the national economy after the COVID-19 epidemic.The hybrid direct current(DC)transmission technology,which combines the advantages of traditional Line Commuted Converter(LCC)and modular multilevel converter(MMC),is like a "highway" for efficient transmission and consumption of clean energy.It provides an effective means to solve the problem of "point-to-point" energy deployment and large-scale energy interconnection in China.As the main transmission method of China’s electric power energy,overhead transmission lines have been widely used in ultra-high-voltage(UHV)DC transmission systems for cross-regional,large-capacity,and long-distance transmission,and have dominated in DC transmission projects.However,overhead lines can hardly avoid the problem of high probability of fault occurrence,therefore,the rapid and reliable removal of DC faults and the reduction of the risk of faults to the system have become an urgent need to guarantee the long-term stable operation of hybrid DC transmission systems.Accordingly,this paper focuses on the LCC-MMC hybrid DC transmission system and carries out research work on the fault ride-through problem after the occurrence of DC faults.The specific work is as follows:Firstly,the research process and development status of conventional DC transmission,flexible DC transmission and hybrid DC transmission are summarized.On this basis,the current research status of DC-side fault handling technology is analyzed and compared,and the advantages,disadvantages and challenges of the existing methods are pointed out.The necessity and feasibility of the research on DC fault ride-through technology are proposed.Secondly,the mathematical models,operating principles and features of LCC and MMC are analyzed.The full-half bridge MMC(FHMMC)composed of a mixed connection of half bridge sub-modules(HBSM)and full bridge sub-modules(FBSM)is selected as the research object of this paper.And the control system of the hybrid DC transmission model is built,and the corresponding control strategy of the converter station is designed.The electromagnetic transient simulation model of LCC-FHMMC hybrid three-terminal DC transmission system is established on the PSCAD/EMTDC simulation platform,which provides the theoretical basis and model foundation for the subsequent research of the paper.Then,the effects of various types of DC faults,MMC grounding methods and fault occurrence locations on the fault characteristics of the pseudo-bipolar and bipolar LCC-MMC systems are investigated for different main wiring methods in the LCC-MMC hybrid DC transmission system.The equivalent circuit of the system after a DC fault is established,and the flow path of the short-circuit current in the fault circuit at different stages is analyzed.and the mathematical analytic equations of the MMC arm current and sub-module capacitance voltage after the fault are derived.The results are verified by simulation analysis,which lays the foundation for designing a suitable non-blocking fault ride-through strategy later,and also serves as a control group for comparison with the non-blocking fault ride-through strategy.Finally,the non-blocking DC fault ride-through technique for the LCC-FHMMC hybrid DC transmission system is investigated.For the FHMMC composed of mixed HBSM and FBSM,the ratio between the use of HBSM and FBSM is optimized to meet the non-blocking DC fault ride-through and reduced DC voltage operation conditions by taking advantage of the ability of FBSM to output negative voltage.Subsequently,an improved non-blocking DC fault ride-through control strategy is proposed for comparison with the basic control strategy and blocked fault ride-through strategy only.The strategy has better DC fault ride-through performance while limiting and clearing the fault current,better fault current suppression condition,no overcurrent in the bridge arm current,better capacitor voltage equalization in the sub-module,and the MMC maintains the reactive power compensation capability,which can realize non-blocking DC fault ride-through,and the effectiveness of the proposed strategy is verified by simulation analysis.