| Power system is always influenced by various internal and external disturbances.Power system will be in a non-fault abnormal operational state such as power oscillation and overload resulting from disturbances when massive active power imbalance occurs.Power oscillation,overload and line overheating that comes along with them contribute to cascading failures.Therefore,with the improvement of relay protection against oscillation and overload,it is necessary to ensure the electro-thermal safety of the transmission lines,which is of great significance to the safe and stable operation of power system.This thesis focuses on the electro-thermal safety of conductor under non-fault abnormal operating conditions such as power oscillation and overload,overload thermal protection and the improvement of backup protection.The main contents of the thesis are as follows:a)Conductor temperature rise is a long-neglected problem when power system oscillating,and conductor temperature rise response under oscillation current is different from conductor temperature calculation on steady state or short-circuit thermal stability calibration.To solve this problem,the numerical integration method was adopted to calculate the Joule heat power based on the analysis of oscillation current characteristic,the resisitcance was corrected by real-time skin effect coefficient and core loss efficient,and the transient thermal balance equation was used to calculate the process of conductor temperature rise during oscillation.The effectiveness of the proposed method was validated by case study.The results showed that under the condition of severe weather,conductor temperature will exceed the maximum allowable temperature in a relatively short time during power oscillation.The proposed method can accurately calculate conductor temperature rise response during oscillation,which is suitable for establishing thermal protection strategy of transmission lines and improving backup protection scheme under non-fault abnormal operation.b)Power frequency and transmission line current show a dynamic process when massive active power imbalance occurs.Attention should be paid to the electro-thermal safety of conductor,and reasonable protection strategy should be adopted during the recovery of power system.Therefore,the dynamic characteristics of power frequency and line current were analyzed in this paper.Furthermore,the influence of different UFLS schemes on the frequency recovery rate and conductor temperature rise response was studied.Thermal overload protection of transmission lines considering the influence of UFLS under active power imbalance was proposed.The effectiveness of the proposed protection strategy was verified by simulation case study.The simulation results showed that different type of disturbance,different arrangement and setting scheme of UFLS have a different influence on conductor temperature rise response and action time of overload thermal protection.The proposed overload thermal protection can excavate the ability to withstand emergency overload of transmission lines and interdict cascading fault due to conductor temperature off-limit during the period of power frequency recovery.c)The oscillating block module is usually adopted to avoid false action of distance protection,but the blocking and opening of distance relay do not consider the the electro-thermal safety of transmission lines during oscillation.To solve this problem,the distance protection module with oscillation block was built based on the analysis of the action behavior of distance protection and the principle of oscillation blocking.The real-time temperature calculation module was also established.An improved backup protection based on the combination of temperature and impedance was proposed.Finally,the effectiveness of improved protection scheme was verified by case study.The results showed that proposed backup protection can improve protection performance of electro-thermal safety of transmission line during oscillation. |
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