Research On Fault Location Method Of Hybrid Double Ended HVDC System

Hybrid high voltage direct current(hybrid-HVDC)transmission technology has advantages of large transmission capacity,long transmission distance,rapid control and adjustment,low line loss,low construction cost,no synchronous operation stability issue,and a variety of topologies to choose from.It provides an effective way for longdistance power transmission,cross-regional grid interconnection,and renewable energy grid integration,is increasingly used in power systems.Statistics show that the transmission line is the component with the highest frequency of fault in the HVDC system,and fault location is an important mean to find the fault point as soon as possible and then restore the power supply.Therefore,it is of great theoretical significance and practical value to study the fault location principles of hybrid HVDC transmission line for reducing the impact of faults and economic losses,improving the reliability of power supply and the safe operation of power systems.Due to the obvious difference in system topology,the fault location principles of line commutated converter-based HVDC system(LCC-HVDC)and voltage source converter-based HVDC system(VSC-HVDC)are doubtful to the applicability of hybrid HVDC system,and the fault location principle of Hybrid HVDC system transmission line is not perfect yet.In view of the above problems,this thesis has carried out a series of researches on "fault location method of hybrid double ended HVDC system".The main work is summarized as follows:(1)Based on the catadioptric principle of traveling waves,this thesis established Peterson’s equivalent circuits of at the both side DC buses and the fault point,respectively,then analyzed the catadioptric characteristics of fault traveling waves in the LCC-MMC hybrid double ended HVDC system,as well as the relationship between the reflected wave polarity at the LCC side DC bus and the amplitude-frequency characteristics of the DC filter.On this basis,the applicability of the existing traveling wave fault location principle was studied when the system adopts type-A DC filter and type-B DC filter with different amplitude-frequency characteristics,and it is revealed that in the case of type-B DC filter is adopted,and the existing single-ended traveling wave fault location principle is not applicable because the fault point reflected wave cannot be distinguished from the opposite bus reflected wave according to their polarities.(2)A traveling wave propagation characteristics-based novel fault location method for hybrid double ended HVDC system is proposed.For pole-to-ground fault,this thesis decoupled the fault network into line mode network and ground model network,according to the characteristics that the fault traveling wave has different velocities in different modal networks,it deduced the functional relationship between the time difference of modal traveling wave reaching the ends of the line and the fault distance,then gave the fault location formula.For pole-to-pole fault,it analyzed the fault traveling wave propagation path in the cases of different fault sections and different transition resistance at the fault point,derived the functional relationship between the propagation path of the first three wave heads reaching both ends of the line and the fault distance,then located the fault point by using the fault traveling wave propagation path ratio.In order to improve the fault location accuracy,a data pre-treatment method was studied in this thesis,which is combined with empirical mode decomposition to extract the fault traveling wave heads.(3)An equivalent characteristic impedance-based hybrid double ended HVDC system fault location method is proposed.According to the change of the conduction state of the power electronic devices,this thesis divided the evolution process of the DC line fault into several stages,explored the influence of the transition resistance at the fault point on the fault evolution process,and analyzed the transition conditions between different evolution stages.It proposed an adaptive criterion to judge the moment when the fault evolution process enters the AC feed-in stage.Based on the characteristic harmonics of the electrical quantities at the AC feed-in stage,this thesis defined the equivalent characteristic impedance at any point of the DC line,and deduced the functional relationship between the fault distance and the equivalent characteristic impedance at the inverter side DC bus,so as to realize fault location.It also studied an algorithm for extracting characteristic harmonics at the AC feed-in stage.(4)A fault current differential equation coefficients-based hybrid double ended HVDC system fault location method is proposed.Based on the capacitor discharge stage of the fault evolution process,this thesis established the fault equivalent circuit of the Hybrid HVDC transmission system,respectively deduced the fault current differential equations of the rectifier side and the inverter side,utilized the functional relationship between fault current differential equation coefficients and the fault distance to locate the fault point.In order to improve the fault location accuracy,it compared the LCC side fault current with the MMC side fault current and drew the conclusion that the latter one is more conducive to the fault location in the LCC-MMC Hybrid HVDC system.In addition,the influence of differential equation coefficients extraction error on the fault location accuracy was analyzed,and the Prony algorithmbased extraction method for the differential equation coefficients was studied.(5)A fault current characteristic parameters-based hybrid double ended HVDC system pole-to-pole fault location method is proposed.By constructing the fault equivalent circuit in the capacitor discharge stage,this thesis deduced the fault current expressions of the rectifier side and the inverter side,obtained the functional relationships between the two fault current characteristic parameters(the oscillation frequency of inverter side fault current and the decaying constant of rectifier side fault current)and the fault distance,respectively,then combined them to construct a comprehensive fault location equation.Besides,it studied the extraction algorithm for fault current characteristic parameters: the oscillation frequency of the inverter side fault current is extracted by combining Taylor expansion and the undetermined coefficient method,the attenuation constant of the rectifier side fault current is extracted by instead the differential with local difference.(6)Aiming at the current situation that the existing pole bus protection of LCCMMC hybrid double ended HVDC system cannot protect the DC filter branch,an extended pole bus protection principle is proposed.By constructing the equivalent circuit in the normal operation state,internal component fault,and DC line fault,the directions of DC current at each measurement point in each case are analyzed,according to these,a DC current direction-based extended pole bus protection criterion is proposed.In addition,this thesis also studied the selectivity of the proposed protection in the case of AC system fault.Finally,the simulation results verified that the proposed protection has a clear selectivity to internal fault and external fault.