Research On Fault Characteristics And Protection Scheme Of High-voltage Large-capacity HVDC Systems

Modular multilevel converter(MMC)is especially suitable for high voltage direct current(HVDC)transmission systems due to its modularity structure,high quality of output voltage,low harmonics and low switching losses.With the increase of voltage level and transmission capacity,multi-terminal HVDC technology has been popular in actual projects in recent years.Many scholars focus on the topologies and control strategies,but the research on fault characteristics and protection scheme are very limited.In this paper,the fault transient characteristics and protection under different faults for the modular multilevel converter based high voltage DC(MMC-HVDC)are studied and the fault handling methods which are applicable for MMC-HVDC are proposed.Every conclusion is verified by simulation results in PSCAD/EMTDC.Firstly,the topology structure of converter and basic working principles in the MMC-HVDC system are introduced,including the MMC sub-module topology,the mathematic models and the main wiring mode of the MMC converter.The electromagnetic transient characteristics of MMC-HVDC under pole-to-ground fault and pole-to-pole short circuit fault are analyzed from theory and simulation.Fault equivalent circuits are established.Fault current and voltage are qualitatively analyzed and quantitatively calculated.Besides,DC fault characteristics and electromagnetic transient process of different wiring mode MMC-HVDC are revealed.Secondly,based on the electromagnetic transient characteristics of DC fault,a new protection scheme for bipolar HVDC transmission lines is proposed.The scheme,with short operation time and high reliability,can detect fault,recognize fault type and locate fault position by measuring the change value of single-terminal DC voltage and single-terminal DC current.Finally,converter AC grounding fault characteristics and protection scheme are studied.AC three-phase grounding fault transient characteristics in a multi-terminal HVDC based on bipolar wiring mode are analyzed.And mathematic expressions of each component of fault current are derived.A fast protection scheme is proposed for bipolar HVDC with metallic return according to the fault transient characteristics.The scheme utilizes arm current for fault identification and is verified to have fast action speed,high reliability and the ability to resist transition resistance.