The Research On The New Principle Of HVDC Transmission Line Protection Based On Boundary Characteristics

With the development of electronic technology,the promotion of new energy and DC loads,and the expansion of the power grid,The advantages of DC transmission systems are becoming more apparent.China’s HVDC transmission started late,but the pace of development has been accelerating.At present,many core technologies have become autonomous in China.However,for high-voltage DC line protection,ABB and Siemens’ traveling wave protection devices are still widely used in China.The actual operating experience shows that the conventional traveling wave protection developed by ABB and Siemens are weak with large transition resistance and the malfunction rate is as high as 50%.Based on the analysis of the line boundary characteristics,this paper studies new principles of DC line protection which is less influenced by transition resistance and line parameters.Firstly,the paper analyzes the topological structure of LCC-HVDC and VSC-HVDC system,functions of main components,the control characteristics and the changes of voltage and current after the fault,and compares the differences in the control strategy,control performance and transient characteristics of the two systems,and specifically discussed the frequency characteristics of the line boundary.Studies have shown that after a fault in the DC system,the control system directly acts on the transient component of the fault,making the system as steady as possible,and the development of the fault changes the control strategy through feedback regulation.Therefore,it is difficult to quantitatively analyze the law of the transient component of the fault.In order to effectively control the fault development process,the extraction time of fault information should be limited to the initial stage of the fault.The study of the characteristics of the boundary frequency of the line provides a new idea for proposing a reasonable protection scheme.Secondly,the paper further studies the transient characteristics of the boundary-related faults of LCC-HVDC lines,analyzes the attenuation of high-frequency signals on long lines and the current-suppressing effect of smoothing reactors,and proposes a protection scheme of traditional DC line based on the characteristic sub-harmonic current variation.The presence of the smoothing reactor makes change rate of the characteristic sub-harmonic current significantly different with the fault inside and outside the line.According to this,the fault information of 600 Hz current is used to construct the start-up criterion,protection criterion and selection criterion.simulation waveforms in PSCAD and computing results under different fault position,transition resistance,and line length validate the reliability of the proposed protection scheme.Thirdly,for the VSC-HVDC line boundary,this paper studies the ratio of transient voltage and its influencing factors and forms of expression with the fault inside and outside line,and proposes a protection scheme of flexible DC line based on the ratio of transient voltage cumulants.The boundary structure of the flexible DC line is relatively simple,which makes the ratio of the high-frequency transient voltage is easy to calculate,and the ratios of the faults at different locations are relatively different.According to this,a double-end protection criteria is constructed with the fault information of 1000 Hz voltage.simulation waveforms in PSCAD and computing results under different fault position,transition resistance,and line length validate the reliability of the proposed protection scheme.Finally,in order to verify the applicability of the proposed protection scheme,the paper builds a test platform for the new principle of high-voltage DC line protection based on the boundary characteristics,and presents the implementation flow of the hardware design and software programming of the platform.The main control protection module realizes functions such as opening and exit,fault identification,event recording,and communication.Harmonic acquisition module applies the energy measurement chip ADE7880,can quickly and reliably sample and passed to the main control chip.WiFi modules were added to the platform to provide a more human-friendly interface.Using the relay tester to simulate fault conditions,experiments show that the test platform can reliably implement protection functions.