Protection And Fault Location Principles For Hvdc Ground Electrod Lines

High voltage direct current?HVDC?transmission project has remarkable advantages for long-distance and large-capacity power transmission,which has been widely used in China.As the important part of HVDC system,ground electrode system mainly plays the role of providing the path for unbalanced current and holding neutral-point voltage.Ground electrode line suffers a high failure rate due to its large length and complex environment.With the fast development of HVDC transmission technique in China,the potential threat of electrode line fault to electric power system could not be underestimated.Obviously,ground-electrode line protections and fault loction play significant roles in the security and stability of an AC/DC hybrid system.Therefore,this paper researched on the key issues of ground-electrode line protections and fault location.One is that the traditional protection methods have low sensitivity under the high resistance and remote ground faults,and the other is that the positioning accuracy of existing fault location approaches is low.The main work in this paper are as follows:Firstly,this paper established the bipolar HVDC system model with ground-electrode system according to a practical project.Based on the simulation model,characteristic harmonics of HVDC ground-electrode system are analyzed in detail,which is able to lay solid foundation for subsequent research.The research results show that,there are abundant and stable characteristic harmonic signals in the electrical signals of ground-electrode system,and the content of 12^th harmonic is dominant in each characteristic harmonic.Secondly,the feature of electrical signal under ground-electrode line faults are analyzed in this paper.Digging in the electrical characteristics under faults is the key to study the new ground-electrode line protection algorithm and fault location method.According to the feature of ground-electrode line in HVDC system,the fault signal characteristics of ground-electrode line under different operation conditions are analyzed in detail from the theory and simulation.The analyzing results show that,the characteristic harmonic signals in the voltage and current of ground electrode contain abundant fault information.With the increase of fault distance and transition resistance,the characteristics of differential current between two lead wires used in the unbalanced protection evidently weaken.While the characteristics of characteristic harmonic components are still marked and present certain regularity.Therefore,it is available to research ground-electrode protection methed by adopting harmonic features.Thirdly,a protection principle for ground-electrode line is proposed based on characteristic harmonic measured impedance?CHMI?in this paper.In order to solve the problem of low sensitivity of protection when there are high resistance and remote ground faults of ground electrode lines in HVDC transmission system,a new principle of HVDC ground electrode line protection based on the CHMI is proposed.First,based on the equivalent circuit of the ground electrode lines,the CHMI of ground electrode line is defined,which is the ratio of characteristic harmonic voltage to the characteristic harmonic current of one line from the measuring end.And then the characteristics of the CHMI pre and post line fault are analyzed and simulated.Furthermore,a fault protection criterion based on the ratio of CHMIs'amplitudes is constructed.Finally,in order to verify the performance of the proposed protection principle,the simulation results based on PSCAD/EMTDC show that the protection principle for electrode line proposed in this paper can reliably identify the faults of almost the whole range of ground electrode lines with high transient resistance and noise interference.In addition,there is no need to install additional hardware.Hence,it has good applicability and practical value in power system.Lastly,this paper presents a fault location method for ground-electrode line based on CHMI.Locating faults accurately corresponding to long distant and high impedance values in the ground electrode lines poses the great challenge for the HVDC transmission system.In this paper,an accurate fault location algorithm is proposed.On the basis of the analysis of the CHMI characteristics mathematically,the fault branch is identified by the amplitude ratio of CHMI,and the fault location function derived from the equations of CHMI is applied to locate faults.The proposed algorithm is validated through numerous simulations covering various fault scenarios.The results demonstrate that proposed algorithm can locate the fault accurately in different fault scenarios.Above all,this paper analyzes the fault features of ground-electrod line in HVDC system,and discusses how to realize line protection and fault location utilizing the CHMI characteristics.The problems of the low sensitivity of tradition protections and the low positioning accuracy of existing fault location approaches under the high resistance and remote ground faults are inproved,which can provide supports for the performance of ground electrode line protection and fault location in HVDC system.