Study On The Relay Protection Of UHVDC Transmission Line

Ultra high voltage direct current(UHVDC)transmission technology has the advantages in low line loss,high efficiency of transmission corridor and flexible operation mode which is the important development direction and inevitable trend of UHVDC transmission.However,the UHVDC transmission projects are usually long distance transmission,so it has much probability suffering from lightning strikes and grounding failure.Statistics show that the dc transmission lines have the highest probability of failure rate in the UHVDC transmission projects,but the correct action rate of dc line protection is only 50%.At the same time,a large part of the existing protective devices are affected by the lightning interference and so on.Therefore,trying to improve the protection performance of UHVDC transmission lines and to effectively identify the lightning transient signals is important to ensure the safe and stable operation of the power system.Based on the PSCAD/EMTDC simulation platform,the simulation model of UHVDC transmission system is built.And the building process of transmission line model,lightning current model and insulator flashover model is introduced which plays a foundation for the coming fault simulation of transmission line and the simulation validation of the proposed protection scheme in this paper.The high-frequency component caused by lightning strokes on the UHVDC transmission line is the main factor of the mis-operation of travelling wave protection and transient protection.Due to the existence of fault current pathway to the earth,when lightning strokes failures and the common earth faults occur on the transmission lines,the voltage amplitude of the signal detected at the installation place of the protection attenuates quickly,thus with a low signal complexity;while lightning strokes occur without forming faults,there is no fault current pathway to the earth,the voltage amplitude of the signal detected at the relay-installed place attenuates slower,thus with a high signal complexity.Based on the multi-scale wavelet transform analysis of the attenuation of voltage signal,this paper depicts the complexity of the signal by wavelet singular entropy and shows the ratio of data-windows before and after the mutation moment to describe the change of signal complexity.With the combination of energy spectral distribution characteristics of the voltage signal,the identification of lightning transient signal is achieved.For the identification of faults on the UHVDC transmission line,a directional pilot protection method of fault component based on Hilbert-Huang transform is proposed.On the basis of the analysis of Fault superimposed circuit of faults in or outside the UHVDC transmission line,a conclusion can be reached that for faults in the transmission line,the polarities of the fault components for voltage and current which are detected by the protection devices located on the rectifier and inverter side are opposite.When the faults occur outside the transmission line,the polarities of the fault components near the fault place are the same while the polarities away from are opposite.Thus,the criterion for identifying the internal and external faults of the transmission line is formed,and the fault pole selection of the line can be further carried out based on the direction features of the the fault components for voltage and current.Based on the simulation model of the UHVDC transmission system,the simulation verification for the methods of identification of the lightning transient signal and identification of faults in or outside the UHVDC transmission line are carried out separately.And the simulation results show that the proposed methods can effectively identify the various faults with high speed and reliability and is less affected by the transition resistance.