Research On HVDC Line Protection, DC Line Fault Recovery And Fault Location

According to the advantage of transmission stability, controllability and economy efficiency, High Voltage Direct Current transmission(HVDC) is gradually becoming the important means of Chinese long-distance & high-capacity power transmission and regional grid interconnection. The reliability of HVDC system is not only related to the safety of itself and internal electrical equipment, and will even affect the stable operation of the whole grid. Related statistics shows that, HVDC system fault is mainly composed of DC line fault. Therefore, sensitive and reliable DC line protection, accurate and quick DC line fault location and DC line fault recovery sequence(DFRS) are of great significance to HVDC system security.In the aspect of DC line protection, traveling wave protection is usually used as main protection, while low voltage protection and longitudinal differential protection taken as backup protection. The line protection can operate correctly in the vast majority of line fault conditions, however, it may refuse to operate in high resistance grounding fault. In the aspect of DC line fault location, currently, the fault location methods used on HVDC transmission lines are mostly based on traveling wave theory, but the fault locator may fail to get a result under harsh environment such as high resistance grounding fault. In the aspect of DFRS, it operates blindly and practices employing a fixed time. In the case of permanent fault, it couldn’t reclose the line, however, causes a second shock to the system.In order to solve these problems mentioned above, the main works are described as follows:(1) Research on open line voltage of HVDC open line testBy comparing the Open Line Test(OLT) principle and open line voltage establishment with/without DC line, an improved open line voltage formula, applied to OLT with DC line, is proposed. The influence of DC filter and DC line parameters is discussed. The improved formula can be used to fix the threshold value of OLT voltage protection, to avoid the possible misoperation due to the error between the measured and calculated results. The results are also used in research of HVDC line voltage character during line fault recovery sequence.(2) Research on Improved HVDC Line Fault Recovery Sequence based on DC Line VoltageBy discussing the relationship of the fault point status and DC line voltage, an improved scheme of DFRS base on the DC line voltage characteristic is proposed. Before system restart, the improved scheme judges the fault status first, the line will be reclosed only when the line fault has eliminated.(3) Research on HVDC line fault location method based on intelligent algorithmThe fault location methods based on genetic algorithm and neural network algorithm are proposed separately, discussing the advantage and disadvantage of intelligent algorithm based fault location method. By using intelligent algorithm, the fault location method is free from the influence of fault position and grounding resistance. What is more, these methods use the transient fault record data, at any time, directly to finish the fault locating.(4) Research on high resistance grounding fault backup protectionBy analyzing the reason of the mal operation under the high resistance grounding fault, proposes an improved solution is proposed to avoid the miss trip problem during high resistance grounding fault.The conclusions in the paper are all aimed at the existing problems of HVDC system, has important practical significance in improving the line protections and pole control, enhancing the safety and stability of HVDC system.