Research On Fault Location For VSC-HVDC Transmission Lines

With the continuous innovation and development of power electronics,the voltage level and transmission capacity of Voltage Source Converter based High Voltage Direct Current(VSC-HVDC)transmission system are continuously improving.Once the fault occurs on HVDC transmission lines,accurate and reliable fault location is extremely important for repairing the fault timely and guaranteeing the normal and stable operation of VSC-HVDC system.Therefore,it is very important to research on accurate and reliable VSC-HVDC fault location algorithms.At present,the currently used fault location device for VSC-HVDC projects are mainly based on traveling wave algorithms.But the reliability of traveling wave algorithms relies on detecting and identifying the traveling wave heads accurately.But the most commonly used identification methods based on wavelet transform have low applicability in long distance fault and high transition resistance fault.The existing traveling wave algorithms for VSC-HVDC transmission lines have failed to consider the wave velocity variation with frequency adequately,and there is less research on fault location algorithms for DC cable lines presently.With the in-depth research on basic theory of S-transform,it is used to analyze the fault traveling wave signals for VSC-HVDC system in this paper;and the fault location algorithms based on S-transform for DC overhead lines and DC cable lines are proposed in this paper,respectively.The main research contents and conclusions are as shown as follows:1.The in-depth research on feasibility of fault location for VSC-HVDC transmission lines based on S-transform is researched.With researching the basic principle of S-transform,the advantages of its basic properties in processing fault traveling signals are analyzed.This paper analyzes the characteristics of S-transform results deeply to research the basic principle of detecting traveling wave heads with S-transform.To verify the accuracy and reliability of detecting traveling wave heads with S-transform,the simulation is implemented in this paper;and compared with wave transform,the advantages of detecting traveling wave heads with S-transform are analyzed.2.A single-terminal traveling wave fault location algorithm for VSC-HVDC overhead lines based on S-transform is proposed in this paper.With analyzing the propagation law of fault traveling wave in VSC-HVDC system,this paper proposes a method to identify the traveling wave heads with their polarities.And S-transform is applied to extract different amplitude-time curves of the fault traveling wave signal on different frequency components,to detect different traveling wave heads.And the results are more intuitive,accurate and easier to be identified.With using the amplitude-time curves of different frequency components to detect different wave heads,the applicability of the whole line and the accuracy under high transition resistance are ensured.Then the fault location algorithm is implemented with the traveling wave heads,which can avoid the influence of wave velocity.The PSCAD simulation results show that the proposed algorithm has great applicability for different fault distance,without being affected by transition resistance.3.With analyzing the structure and parameter characteristics of DC cable,a phase-mode transformation methods for DC cable is proposed.And based on the analysis of the time-frequency characteristics of fault traveling wave in DC cable,a DC cable fault location algorithm based on time-frequency characteristics of traveling wave and S-transform is proposed in this paper.(1)This section researches the model of DC cable with armor deeply,and analyzes the frequency-dependent variation characteristics of its parameters.Then the characteristics of its parameter matrixes are researched.Based on the above analysis,a phase-mode transformation method for DC cable is proposed.Then the characteristics of the modulus after phase-mode transformation are analyzed,which provides a theoretical basis for researching the DC cable fault location algorithm.(2)This section analyzes the time-frequency characteristics of fault traveling wave in DC cable to summary the influence of attenuation and dispersion for traveling wave heads.Based on this,the proposed fault location algorithm applies S-transform to process fault traveling wave signals,for detecting the arrival time of each frequency component,which has realized the frequency-dependent detection of wave heads and improved the accuracy of detection with overcoming the influence of transition resistance.And the wave velocity is confirmed optimally with the application of least squares fitting.The polarities of wave heads are identified with the modulus maximums of Stationary Wavelet Transform(SWT),then the fault distance is calculated accurately with the fault type confirmed.The PSCAD simulation results show that the proposed algorithm has higher accuracy,better applicability and capability to tolerate transition resistance,than the fault location algorithm using fixed wave velocity value.