Study On Fault Analysis And Protection Adaptability For AC-DC Transmission Lines Shared The Same Tower

HVDC transmission has developed rapidly in China,due to its advantages of long transmission distance and large transmission power.The construction of DC projects will occupy a large amount of land,which will lead to the lack of land resources in China.Building AC-DC transmission lines on the same tower is a feasible method to deal with the transmission corridor constraints and reduce construction cost.For AC-DC transmission lines sharing the same tower,two problems must be considered due to the close distance between the AC and DC lines.One is the fundamental frequency induction in dc line under neighboring ac system fault conditions.Another concern is a touch line fault between ac and dc lines.To solve the problems above,this paper has done the research as follow.Asymmetric faults in AC lines will generate 50 Hz induction in DC lines through electromagnetic coupling.This paper firstly presents a theoretical analysis for the coupling effect and calculate the induced components in dc lines.Moreover,factors such as tower configuration,the location and length of the parallel part of the hybrid system and conditions including DC power transfer capacity,system strength,fault type and fault location are discussed.The impacts of these factors are considered and the solution for seeking the boundary value is presented.Simulation results demonstrated the correctness of the method.At last some constructive proposals on protection scheme from the aspect of safe operation of ac and dc system were recommended.AC/DC touch line faults are a new fault type,and related research is still in a blank stage.In this paper,the equivalent circuit of AC/DC touch line fault is used to analyze the distribution characteristics of AC and DC currents.It is found that DC line has the characteristics of conventional DC line fault,and exhibits cyclic fluctuation characteristics due to the superposition of AC power.The component is propagated to the converter,and the converter transformer on the valve side causes obvious second harmonic component.Then,based on the PSCAD/EMTDC AC/DC grid simulation model,the DC power,AC voltage level,fault type and transition resistance are deeply studied.The influence on the power frequency component of the DC line and the characteristic quantity of the AC line.The potential challenges of AC and DC relay protection system under the fault situation is of great concern in the paper.Based on the AC-DC hybrid line model on EMTDC simulation platform,this paper analyses the adaptability of DC line protection to touch line fault such as traveling wave protection,AC/DC line-touching protection and the adaptability of AC protection such as differential protection,distance protection and zero-sequence protection under touch line fault.Simulation results found that the fundamental frequency component introduced by touch line fault will reduce the operation rate of traveling wave protection.The sensitivity of AC distance protection to line-impact fault will decrease,and the sensitivity of differential protection to touch line fault will increase.The research results in this paper provide a reference for fault analysis methods of AC/DC transmission lines on the same tower,as well as theoretical and data support for design and selection of AC/DC co-tower erection,optimization and improvement of relay protection and safe operation of power system.