Fault Component Direction Protection For Power Grid With DFIG

Doubly Fed Induction Generator(DFIG)is the mainstream model of wind power generation currently.Affected by the grid-connected topology and control strategies,DFIG shows the fault characteristics which differs from conventional synchronous generator.Therefore,with large-scale integration of DFIG to power grid,the fault characteristics of the power grid will be bound to change significantly,which causes traditional relay protection principle based on the characteristics of synchronous generator facing severe challenges.Considering that the performance of traditional relay protection deteriorates greatly and there is no relay protection principle for DFIG currently in power grid with DFIG,therefore,it is of great significance to improve and develop the relay protection application of DFIG.Based on the research content mentioned above,the dissertation implements an in-depth study on the theoretical and technical problems of fault component protection in power grid with DFIG.As a typical fault component protection,negative sequence direction protection is widely used in the pilot protection of transmission lines.The theoretical basis of traditional negative sequence directional protection is that keeps the negative sequence equivalent impedance at back system of protective installation stable.But with DFIG connected to power grid,the negative sequence equivalent impedance is no longer fixed,which causes degradation in the performance of traditional negative sequence directional protection.Thus,an improved scheme of negative sequence direction protection is proposed in this paper.Firstly,the DFIG mathematical model for negative sequence equivalent impedance calculation is established;then the basic characteristics of negative sequence equivalent impedance of DFIG are derived,especially the characteristics of phase angle change rules;on this basis,the improved scheme of negative sequence direction protection is proposed and its effectiveness is verified by numerical simulation.Theoretical analysis and simulation results show that the proposed negative sequence direction protection has higher sensitivity and reliability,as well as can be well adapted to various operating states of DFIG,accordingly,the proposed negative sequence direction protection has significantly better performance beyond the traditional.With DFIG integrated into power grid,its fault current is featured with deviation from power frequency and high harmonic content,which makes degradation in the performance of relay protection principle based on characteristic in frequency domain.In this respect,a new idea of protection based on time domain features is proposed.Combined with the mind of fault component,a new pilot protection principle based on time domain is constructed by using the waveforms similarity of current before and after fault.Firstly,the characteristics of transient current waveforms are analyzed;then,the obvious difference in the similarity between fault transient current waveform and pre-failure current waveform provided by DFIG and synchronous generator is verified;further,Hausdorff distance algorithm is used to characterize the above differences,and a new protection principle based on time domain is constructed,and the vertical action criterion is designed as well.In order to ensure the defect that the time domain protection algorithm is affected by individual abnormal data seriously,a new abnormal data recognition method is proposed,which can be more improve the anti-interference ability.Through theoretical analysis and simulation verification of the new principle and the new criterion,it is illustrated that the proposed new principle of pilot protection in time domain can identify all kinds of internal and external faults accurately and reliably.In the end,the summary about the research is given and the further work is prospected.