Influence Of Fault Of UHVDC Hierarchical Access System On AC Protection

China’s energy resources and load centers have the characteristics of reverse spatial distribution,which determines that UHVDC transmission technology with large capacity and long-distance transmission capacity has become an important means to alleviate contradictions.However,with the increase of DC transmission capacity and landing density,the existing single-layer DC access mode will cause difficulty in power flow evacuation of receiving system,and bring a series of problems to voltage stability.UHVDC hierarchical access AC system is an innovative access mode,which greatly improves the ability of safe and stable operation of power grid,and also puts forward new requirements for relay protection of receiving AC system.In this thesis,the transient characteristics of UHVDC hierarchical access system and its impact on relay protection of receiving terminal AC power network and countermeasures are studied in depth.The main research contents and results are as follows:Based on dynamic phasor theory,the equivalent power frequency current model of DC system injected into AC network under commutation failure is established.Based on the mechanism of commutation process,the expression of current switching function of commutation bridge in typical commutation failure scenarios and the transient characteristics of corresponding fault equivalent power frequency current are analyzed.According to the actual engineering parameters of Ximeng-Taizhou(±800 kV)UHVDC hierarchical access system,the electromagnetic simulation model of the DC hierarchical access system is established on PSCAD platform.The simulation results verify the correctness of the theory and obtain the transient characteristics of the fault equivalent power frequency current of the DC hierarchical access system.On the basis of analyzing the transient characteristics of fault equivalent power frequency current in DC hierarchical access system,the adaptability of differential current protection and directional protection of longitudinal sudden change in end-to-end AC network under DC hierarchical access mode is analyzed,and the influence of DC hierarchical access on differential current protection and directional protection of longitudinal sudden change in AC network is revealed.Li.Based on the electromagnetic simulation model of DC layered access system,the correctness of the theoretical analysis is verified.Further,in the interconnected power grid of the UHV DC tiered access AC system,the fault circuit current of the DC system is affected by the control strategy and the distortion waveform containing a large number of aperiodic components may be caused by the commutation failure of the DC system,which may result in the traditional power frequency protection of the AC grid.There was a mistake.Aiming at the above problems,based on the analysis of the characteristics of fault current waveforms of synchronous generators and DC systems,a new principle of longitudinal protection based on waveform similarity is proposed.The protection criterion is constructed by using Euclidean distance and Pearson correlation metrics.And through the simulation calculation,the accuracy of the protection criterion is verified.