Study On Novel Pilot Protection Of Transmission Line With Large-scale Renewable Energy Power Plants

Over the past two decades,renewable power generation,such as wind power generation and photovoltaic power generation,have increased explosively.By the end of 2021,the installed capacity of renewable power plants is 635 GW.At a short time section,the largest proportion of renewable energy generation is close to 50%in Xinjiang and Inner Mongolia.The equivalent potential of renewable power source is not constant,and the waveform of fault current is seriously distorted.The performance of protection based on fault current characteristics of synchronous generators has reduced drastically.Incorrect operation of protection will lead to disconnection of large-scale renewable energy power plants,which threaten the security of the power system.Therefore,correctly understanding the variation characteristics of fault current of renewable energy power sources and then studying the reliable novel protection is the key issue.This dissertation proposes the novel protection for the transmission line with renewable power sources based on the cognition of fault characteristics of renewable energy power supply.A protection prototype is developed and verified by the artificial short circuit test.The specific research work and innovative achievements are as follows.(1)Based on the switching boundary of fault-ride-through control strategy of converter,the difference of fault current waveform characteristics of different converters with different time scales is deduced.It is proved that there are essential differences in the waveform characteristics of fault current between a converter and a conventional synchronous generator or between different converters.The mechanism of performance degradation of conventional pilot protection is revealed.It solves the problems that it is difficult to correctly recognize the fault characteristics of renewable power sources and the performance analysis of protection is lack of theoretical basis,and lays a foundation for the study of novel protection.(2)The current waveform similarity based protection is proposed,and it is suitable for the scenario with the renewable power plants connected to the strong power grid.A protection design based on fault current waveform feature extraction is proposed.It breaks through the limitations that the conventional protection uses the comparison of power frequency amplitude and phase angle to identify the faults.A method for extracting the difference of fault current waveform based on cosine similarity is proposed.A practical criterion considering the stability factor is constructed,which improves the accuracy of fault identification in the scenario of weak output or zero output of renewable power plants.The problem of the sensitivity degradation of the conventional pilot protection is solved.(3)The current waverform singularity based protection is proposed,and it is suitable for the scenario with the renewable power plants connected to the weak power grid.Based on the difference of fault current in eigen response stage caused by different control strategies of line double ended converter,a waveform singularity feature extraction method based on singular value decomposition is proposed.The start-up criterion based on singularity detection and the main criterion of singular value comparison are given.The problem of difficult application of conventional pilot protection for the renewable power plants connected to the weak power grid is solved.(4)A pickup setting method based on specific signal error analysis of waveform characteristic protection is proposed.It provides the setting basis for the industrial application of the new protection principle.Based on communication mode of sampling value transmission of current waveform,the protection prototypes of transmission line with large-scale renewable energy power plants are developed.Furthermore,a comprehensive protection test scheme based on model of renewable power sources and playback of recording data is proposed,which comprehensively verifies the dynamic performance of the novel protection.Finally,the effectiveness of the proposed novel protection is verified by artificial short circuit test.It solves the problem that the data requirement of novel protection is difficult to match with the performance of existing protection platform.