| Due to the increasing demand for power energy and the unbalanced distribution of energy resources and load centers in China,HVDC plays an increasingly important role in the modern power system.At present,traveling wave protection and differential under-voltage protection are the main protection and current differential protection is the backup protection in the actual operation of DC transmission project.However,there is a delay in the cooperation between the existing traveling wave protection and the current differential protection,which will cause the lack of corresponding protection in the DC control transient phase during the fault process,which greatly reduces the rapidity and reliability of the existing DC line pilot protection.Therefore,the existing DC transmission lines still lack reliable,economical and applicable pilot protection.Because the entire process of DC line faults actually contains available fault information,most related scholars currently study the entire process of DC lines and their characteristics,mainly focusing on the initial stage of traveling waves and the steady state phase of DC control,while the research on the transient stage of DC control is relatively slow.The reason is because the dynamic response of the control system introduces non-linear characteristics,so that the analysis of the transient process of DC line faults becomes an analysis of a complex non-linear system with the interaction of the control system and non-linear circuits,increasing the difficulty of fault feature analysis and extraction.Therefore,considering the DC control characteristics,studying the correlations between the control quantities,observable measurements of the DC system and the electrical quantities of the DC line faults is of great significance for improving the rapidity and reliability of the existing DC line pilot protection.Based on the above analysis,the following research work is carried out on the DC control characteristics of the LCC-HVDC transmission system and its DC control transient process:(1)According to the DC control characteristics and DC control transient process of LCCHVDC transmission system,the variation characteristics of DC control firing angle is studies,on the basis of which,the fault characteristics of the firing angle change rate are constructed.At the same time,based on the relationship among the fault current,the firing angle,DC voltage and AC bus voltage,the influence mechanism of DC control characteristics on the fault current on both sides of the converter station will be quantitative analyzed.Therefore,it will lay the foundation for further research on new principle of pilot protection for HVDC transmission lines.(2)Considering the fault characteristics of the firing angle change during the DC control transient process,a high-voltage DC line pilot protection scheme based on the change of the firing angle is proposed.This scheme studies the change characteristics of the firing angle from the qualitative and quantitative perspectives.The DC characteristics of internal and external failure of the line are accurately identified by the cosine similarity of the "firing angle change rate average" at the two converter stations and the "firing angle change" fault characteristics at fixed intervals.Simulation results verify the effectiveness of the proposed protection scheme.(3)Considering the influence of DC control characteristics on the fault currents on both sides of the converter station,a high-voltage DC line pilot protection scheme based on compensation control current is proposed.This scheme compensates the influence of DC control characteristics on the fault current by establishing the relationship between the triggering angle of the DC control quantity and the fault current and the AC bus voltage,and constructs a compensation control differential current protection criterion to identify faults inside and outside the area.Simulation verifies the feasibility of the proposed protection scheme. |
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