Effects Of Rail Transit Stray Current On Transferring Characteristics Of Current Transformer And Relay Protection

With the expansion of urban railways,their close proximity to the power grid has led to an increasing intrusion of stray currents.The networked,high-density rail conditions and the large topology of the power grid pose new challenges for analysis,risk assessment and suppression of stray currents.The following three aspects are mainly manifested:the time and frequency domain characteristics of stray currents and their flow direction within the system are unknown;the mechanism of the impact of stray currents on current transformers for protection is unknown,which in turn makes it impossible to assess relay protection’s risk;the above-mentioned situation leads to the internal factors as a reference for stray current suppression measures that is easily overlooked,which may lead to the setting of inappropriate values.Based on this,this thesis focuses on the distribution characteristics of stray currents,the mechanism of influence on the steady-state and transient transfer characteristics of current transformers for protection,the risk assessment of relay protection and the suppression measures for stray currents.The research work of the thesis mainly includes.(1)Based on the measured data of stray current,fourier analysis and comparison of waveforms in different time domains to obtain the waveform characteristic law of stray current;establishing a regional local grid simulation model,realizing the dynamic input of stray current by controlling the ground potential,and studying the propagation of stray current in the area grid.(2)Using the P-level protection current transformer equivalent model and its core hysteresis return model to establish a model of the steady-state and transient transmission characteristics under stray currents and remnant magnetism,providing a model basis for further risk analysis of relay protection.The significant difference between the hysteresis return and the core magnetization curve during steady-state operation of the protection current transformer,coupled with the small value of the stray current and the alternating phenomenon,results in a long-term change in the steady-state transmission characteristics under tray currents.In addition,the effect of stray currents and remanence on the transient transformer characteristics is measured in terms of the onset of saturation time based on a model of the steady-state transformer characteristics,and the waveform characteristics after transient saturation has occurred are analyzed.(3)Assessing the risk of relay protection under stray currents.On the basis of the grid topology model,models for transformer differential protection,transmission line distance protection,longitudinal differential protection and zero sequence overcurrent protection are established.Theoretical analysis of the mechanism of the impact of stray currents on various relay protections is carried out,and the correctness of the theory is verified through simulation to provide a model basis for suppressing the impact of stray currents.(4)Optimize the suppression measures for stray currents.Firstly,measures are proposed to optimize the saturation characteristics of current transformers under stray currents.Secondly,the risks to the power system of installing capacitive isolators at the neutral point of transformers are analyzed.Finally,the switching strategies of capacitive isolators are optimized by considering the riskiest operating conditions for each relay protection under stray currents.