Research On New Principle Of Relay Protection For AC/DC Interconnect System Based On Fault Transient Characteristics

In recent years,China's power demand has grown rapidly,and ultra-high voltage and long-distance transmission are the general direction of power grid development.In order to realize long-distance transmission,more and more high-voltage DC transmission networks have been built and put into operation,and an AC-DC interconnection network has been formed,which makes the operation mode and fault state of the power system more complicated.In addition,with the continuous development of power electronics technology,the power supply and transmission path of the power grid are no longer constant,so the traditional power frequency protection can not ensure fast and reliable protection of the line.This requires a new relay protection principle,which can identify faults and remove faults more quickly and reliably in AC/DC interconnect systems.When a high-voltage line fails,the transient component of the fault contains a large amount of fault information,and the generation of the transient quantity is not affected by the power frequency phenomenon,which makes up for the deficiency of the traditional protection.Therefore,in-depth study of the transient characteristics of the AC-DC interconnect system in the event of a fault,and propose a new protection principle based on this feature,is of great significance to ensure the safe and stable operation of the AC-DC interconnected power grid.First of all,this paper expounds the purpose and significance of carrying out the research work of this subject,and makes a comprehensive summary of the current research at home and abroad,and summarizes the main contents of this paper.Secondly,the source of transient signal in power system is discussed.The spectral characteristics of fault transient component and the high-frequency characteristics of traveling wave of each bus line are theoretically analyzed,which are used as the time-frequency domain characteristics and fourth of the transient component of the system.In the chapter,the theoretical support of the transient protection of the direction of each bus line information is considered.The influence of the bus-to-ground capacitance on the transient component is analyzed.The wavelet transform is introduced and the wavelet singular entropy is defined.The feasibility of applying wavelet singular entropy to power system transient signal analysis is discussed.Thirdly,the AC system simulation model is built in PSCAD/EMTDC.When the transmission line fails,the fault current is discretely decomposed into wavelets to analyze the time-frequency domain characteristics of the faults in the region and outside.The wavelet singular entropy is used to analyze the characteristics of fault transient signal complexity when different types of faults occur in the area and outside the area.The characteristics are analyzed qualitatively and quantitatively,and the principle of transient protection based on signal complexity is proposed.What's more,build a more complex AC system model with multiple outlets,and analyze the signal complexity of the outlet protection installations on the busbar when different types of faults occur at different locations.According to this,the line direction protection criterion is proposed.Based on the principle of transient protection of the outgoing line information of the bus,and through simulation,the impact of different fault conditions on the protection scheme is explored.Finally,the AC/DC interconnect system simulation model is built on PSCAD/EMTDC to analyze the change of electric quantity in the AC line in the AC/DC interconnect system during the faults in the area and outside,and compare with the change of electric quantity in the case of pure AC system fault.This paper explores the applicability of transient protection methods in pure AC systems in AC/DC interconnect systems,and summarizes the new principles of transient protection for AC/DC interconnect systems.