| China’s wind energy,light energy and water energy are enriched in the west and north,but the load centers are mostly concentrated in the east and south,and the power situation is generally characterized by the transmission of electricity from the west to the east.DC transmission technology has irreplaceable advantages over AC transmission in many aspects,such as large capacity,stable operation,flexible control,etc.It is absolutely the best choice to solve the problem of new energy consumption.The traditional DC transmission technology has the advantages of large capacity,mature technology and low cost,while the flexible DC transmission technology has various and flexible operation modes.The transmission of new energy in China is generally characterized by concentrated primary energy,various and scattered load points.Therefore,the hybrid multi-terminal DC transmission combining the two DC transmission technologies can effectively solve the above technical problems.The fault of DC transmission system develops rapidly in case of line fault,which requires high speed of protection.As the distribution center of electric energy,it is necessary to propose new protection methods to meet the demand of DC transmission.Firstly,this paper analyzes the topology,control logic and working mode of LCC and MMC to verify the feasibility and reliability of the station-level control strategy of the simulation model.Discharging behavior of converter station in case of line fault is analyzed,and the particularity of three-terminal hybrid DC transmission line protection is the existence of its bus.Therefore,the basis and significance of this paper are determined.According to the current protection methods of busbar,three new methods of busbar protection with advantages are proposed.Firstly,the frequency domain characteristics of the fault transient current at the bus are analyzed,and there are differences in the frequency band energy distribution and phase.A new method of busbar protection based on wavelet energy and a busbar protection based on the phase characteristics of bipolar differential current are proposed.The former is almost unaffected by the fault distance,while the latter has a strong tolerance to the transition resistance.Based on the analysis of traveling wave propagation process,it is determined that in the initial stage of fault,the measured impedance of the protection device is the equivalent impedance behind it.The fault of each section of the three-terminal hybrid DC transmission system is analyzed,and the equivalent impedance calculation formula under different conditions is obtained.The impedance amplitude-frequency characteristic diagram is drawn through Matlab,and then the fault criterion in the form of setting value is formed.This method has strong tolerance to fault distance and transition resistance. |
