Research On Protection And Reclosing Of Large Capacity HVDC-Flexible

Modular multilevel converter(MMC)has been widely concerned by scholars in recent years due to its high quality of output voltage,low harmonic and low switching losses,and is especially suitable for constructing HVDC transmission system to connect new energy generation.However,the research on fault transient characteristics and protection is very limited,so it is difficult to meet the requirements of the reliability of the system in practical engineering.Therefore,according to the MMC-HVDC system,the fault transient characteristics and protection strategies under different faults are studied in this paper,and the specific fault handling methods are proposed according to the system structure.All the conclusions have been verified by a large number of simulations.Firstly,the structure of converter and main wiring in the flexible DC transmission system is introduced,including the MMC sub-module topology,the mathematical model of the MMC-HVDC system and the main wiring form of the MMC converter station.Taking MMC-HVDC as the research object,the transient process of the single pole-to-ground fault and bipolar pole-to-pole short circuit fault in the DC side is analyzed from both theory and simulation,and the fault equivalent circuits are established.The fault propagation characteristics between the AC side power grid and the DC side power grid are studied,and the possible influence of AC side faults on DC side is analyzed.Secondly,based on the transient characteristics of DC fault,a new fast protection scheme for HVDC transmission lines is proposed.The proposed scheme is used to detect the pole-to-ground fault by measuring the abrupt change value of DC voltage and current,and its action time is within a few milliseconds.The coordination strategy between MMC-HVDC DC side and AC side is analyzed,and the influence of AC/DC side fault on mutual protection is studied.Finally,the high speed fault isolation method for HVDC transmission system is analyzed.The DC power automatic reclosing operation process is summarized,and a set of adaptive reclosing scheme for DC power system is proposed to determine the restart condition under different fault types.A novel sub-module topology with DC fault self-clearing capability is proposed for two terminal HVDC transmission system.