| As the scale of AC-DC power system is expanding gradually,the requirement of operation stability for DC transmission system is getting higher and higher.Served as the core component of DC transmission system,the reliability of DC line is directly related to the safety of entire DC system and even the AC system that it connects to.Aiming to solve the problem that the accuracy of traveling wave protection for DC line is not sufficient and the malfunctions caused by lightning strike can not be completely avoided,this paper studies the DC line faults and the characteristic analysis of traveling wave protection based on actual engineering projects.Moreover,this paper proposes a DC line traveling wave protection optimization method,which mainly consists of the analytical setting and risk assessment of lightning strike malfunctions considering transition-resistance-resisting and anti-lighting-disturbance game theory.In this paper,a fault model and an analytical method for DC line based on Berlon method are introduced,which including system and the inverters from AC-side,model of the traveling wave propagation characteristic information and the DC control system model.By using this model in bipolar DC line,the analytical analysis of midpoint short-circuit fault of DC line,valve side fault of smoothing reactor in inverting substation and non-breakdown lightning strike of DC line are carried out.The correctness of analytical results in this paper is verified by the waveform of simulation model based on actual parameters.In order to solve the problem that the maximum of characteristic quantities during a fault process can not be obtained because of the randomness of protection sampling,this paper proposes a corresponding analytical method based on the characteristic quantity algorithm.The voltage and current waveform information is scanned with a time interval that is much smaller than the sampling frequency so as to attain a series of analytical sampling points.During the calculation,the sampling points are strictly substituted according to the actual sampling interval of devices,and finally all the possible characteristic quantities during a primary fault process are attained.Moreover,this method can also obtain the dynamic characteristic curve of characteristic quantities,which can provide preconditions for the analysis of traveling wave protection characteristics,the setting study and the malfunction risk assessment under lightning strike conditions.Based on the analytical results of DC line fault and characteristic quantities of traveling wave protection,this paper studies the analytical setting method for traveling wave protection.The concept of transition-resistance-resisting boundary and lightning strike disturbance boundary of a certain criterion are proposed for the first time,and therefore the protection boundary of traveling wave protection is quantitatively analyzed.Based on the logical constraint of each protection criterion,along with the balance requirements of selectivity and sensitivity,the optimal setting program is selected according to the protection boundary of different characteristic quantities.Finally,aiming to solve the problem that the DC line malfunctions caused by lightning strike can not be completely avoided,the fifth chapter in this paper establishes the corresponding malfunction probability analysis model and risk assessment system.The lightning strike malfunction probability model for traveling wave protection is established,which consists of the lightning strike grouding rate,shielding failure rate,wave-head parameter distribution rate and the traveling wave protection misoperation interval considering lightning strike positions,etc.A risk acessment model is also constructed,whose indicators include lightning strike malfunction rate of traveling wave protection and malfunction frequency.This model can carry out quantitative evaluation for the risk of operating lightning strike malfunctions,which can enrich the risk assessment system of high voltage DC lines and provide guidance for the safety operation of HVDC project. |
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