Research On UPFC Protection Scheme In Back To Back VSC-HVDC Grid Connected Composite System

In recent years,there have been many blackouts in the interconnected power grid at home and abroad.The main causes of the accidents include two aspects:slow recovery of the splitting subsystem after troubleshooting,and poor power control of the tie line.In order to improve the reliability,controllability and stability of interconnected power grid,more and more flexible AC transmission devices will be put into actual power grid operation.As themost powerful new generation of flexible AC transmission device,UPFC can quickly and flexibly control the power of interconnection line.However,when UPFC detects the failure of interconnection line,in order to avoid the internal power electronic equipment In case of damage,UPFC must quickly turn on TBS to bypass the series side converter,and the electric quantity in the line will suddenly change again,which may lead to the deterioration of the operation performance of the conventional protection.Therefore,it is necessary to propose a protection configuration scheme suitable for UPFC lines.First of all,based on the back-to-back VSC-HVDC grid connected composite system that has been studied,this paper analyzes the impact of back-to-back VSC-HVDC grid connected composite system switching from grid connected mode to UPFC mode on the conventional protection of AC lines,and summarizes the reasons for the deterioration of the operation performance of conventional protection in UPFC lines.Secondly,the characteristics of transient fault current waveforms at both ends of the line in and out of the line area with UPFC are analyzed.It is found that the short-circuit current at both sides with UPFC presents two different waveforms before and after the TBS is turned on,while the current waveforms at both sides without UPFC are always power frequency sine waves with exponential attenuation,and the similarity of current waveforms at both sides is weak When a fault occurs outside the C line area,the two ends of the line flow through the same size and opposite direction of the short-circuit current,and the reverse similarity is very strong.Finally,according to the difference of the transient current characteristics between the two ends of the UPFC line when there are internal and external faults in the line area,a scheme of UPFC line pilot protection based on the similarity of fault time-domain current waveform is proposed,and the protection action criterion is constructed by Pearson correlation coefficient.The proposed protection scheme is not affected by TBS conduction and rejection,and can identify faults quickly and accurately under different power flow levels,fault types and fault positions.It has a certain resistance to transition resistance and noise,and the data window is only 10ms.Therefore,it is used as the main line protection in the UPFC mode of VSC-HVDC grid connected composite system.Because UPFC has little influence on the operation performance of conventional distance protection,conventional distance protection is still used as the line backup protection in UPFC mode.The feasibility of the proposed scheme is verified by PSCAD/EMTDC simulation.