Study On The Transient Characteristic And Protection Principle Of Transmission Lines Containing FACTS Components

The extra-high voltage and ultra-high voltage transmission technology and the new generation FACTS devices,such as unified power flow controller(UPFC),have been put into actual operation nowadays.In the power electronic power system,when the AC system occurs fault,the inverter type power supply and the dynamic adjustment characteristics of the FACTS devices will have a serious impact on the traditional protection based on power frequency.And the filtering link of the protection based on power frequency greatly limits the increase of the action speed.Fully exploring the fault transient characteristics of transmission lines can effectively improve the performance of relay protection,which is of great significance to the safe and stable operation of the power system.This paper focuses on the study of fault transient characteristics and new protection principles of transmission lines containing FACTS components.Firstly,the working principles and transient equivalent models of different FACTS components are analyzed.Based on PSCAD/EMTDC simulation platform,the models of transmission lines with STATCOM,SSSC and UPFC devices are built,respectively.The influence of the new FACTS components on the fault transient characteristics of the line is studied in detail.The transient protection of the transmission line containing UPFC components is studied.Trough the analysis of the fault characteristics of the busbar connected lines,it is found that the transient current spectrum distribution of the fault side line and the non-fault side line is significantly different.Therefore,a new scheme for pilot protection is proposed.By using wavelet energy entropy to extract the transient characteristics of each outgoing line of the busbar,the fault direction can be identified after comparing the relative values.By comparing the fault signals of both sides of the line,it is able to further identify internal,external and busbar faults.The simulation results show that the scheme avoids the adverse effects of FACTS components on transient protection and dose not rely heavily on protection settings.The problem that transient protection is susceptible to lightning disturbance is studied.The additional components of the transient current of each phase are analyzed under non-lightning fault,lightning disturbance and lightning fault,respectively.It is concluded that the essential difference between fault and lightning disturbance is that the whole trend of fault transient waveform has obvious power frequency sinusoidal characteristics.Therefore,the main waveform of the transient current is obtained by the average sliding denoising method.A 50Hz sine function is proposed to fit the main waveform,then the lightning disturbance and fault of each phase can be identified based on the fitting coefficient.Simulation results verify the effectiveness of the proposed scheme.The problem that transient protection usually depends on high sampling frequency is studied.Through the analysis of the transient differential current of fault component,it is found that the fault phase current of internal fault changes with power frequency.Both the non-fault phase of internal fault and each phase of external fault change with high frequency,and there is a theoretical minimum frequency.Therefore,a new pilot protection principle based on the transient current waveform was proposed.The least-squares fitting method is used to extract the main frequency and the corresponding protection criterion is constructed.Through simulation verification,the proposed protection has low requirements for sampling frequency.The action speed of protection and the performance under high transient resistance scenario are improved.