Research On Protection And Fault Location Principles For HVDC Transmission Lines

As important parts of the protection system in HVDC (High Voltage Direct Current) projects, the protection and fault location for transmission lines have to detect and clear faults rapidly, and their operating performance will directly affect the safe operation of HVDC transmission systems and the grid. Statistical studies show that the HVDC transmission line had become a component with the highest failure rate in HVDC transmission systems. However, the correct action ratio of protections for HVDC transmission lines was only50%. Development of HVDC projects in China is at the forefront of the world, but we have few core technologies of existing protection and fault location devices for HVDC transmission lines, and there is lack of systematic theories. Protection systems in existing HVDC projects mostly adopted technologies of ABB and SIEMENS. Improvement of the theoretical level of protection and fault location for HVDC transmission lines and to speed up the independent R&D of HVDC protection system in China are imperative.The main protections for HVDC transmission lines being put into operation are vulnerable to disturbance of transient process, such as lightning and commutation failure, and have limited anti-grounding-impedance ability and complex setting calculation of criterion threshold value. Scholars are committed to improve the main protections for HVDC lines being put into operation or to propose novel main protection principles. Some of the proposed novel protection principles have shortcomings of incomplete theory, signal used in protection criterion having small energy, limited anti-grounding-impedance ability, high requirements of sampling frequency, lack of basis for setting calculation or the setting calculation depending on simulation results, etc. The currently methods for locating faults on HVDC transmission lines include traveling-wave-based methods and fault-analysis-based methods. Fault location devices for HVDC lines being put into operation adopted the traveling wave principle, and the reliability of traveling-wave-based methods will be limited for detection failure of the wavehead and high grounding impedance faults. Fault-analysis-based methods have stable performance. However, accuracy of fault-analysis-based methods is generally lower than the traveling-wave-based methods, some of which have limited anti-grounding-impedance ability.Depth fault transient analysis of HVDC transmission systems can lay the foundation for research on protection and fault location principles for HVDC transmission lines. In this paper, considering influence factors for fault transient characteristic analysis of HVDC transmission systems are studied, corresponding influence mechanisms are deeply analyzed; simulation cases and their implementation schemes are given, and transient characteristic analysis of electrical quantities on HVDC lines is carried out. Main protection principles for HVDC transmission lines are proposed, which have high anti-grounding-impedance ability, and their setting calculations of criterion threshold values do not depend on simulation results; a novel two-terminal traveling-wave-based fault location algorithm for HVDC transmission lines is presented, which has good reliability, high positioning accuracy and high anti-grounding-impedance ability. The main research contents and conclusions are as follows:(1) Research on fault transient characteristics of HVDC transmission systems.The existing related contents of fault transient analysis for HVDC transmission systems are mostly considered in theoretical analysis and simulation test of research on protection and fault location for lines, which are limited by the literature space and test contents. From the protection and fault location for HVDC transmission lines point of view, more comprehensive fault transient analysis of HVDC transmission systems is carried out in this paper. The basic control principles of HVDC systems and hierarchical structure of the control system are studied, response time of the control system to faults and steady values of electrical quantities after faults are analyzed; operating conditions and control strategies of various possible operation modes of both ends bipolar HVDC transmission systems are studied, and transient characteristics of fault voltages and currents on DC lines under different modes are compared; analysis of impedance characteristic of the DC filter link and its influence on the fault transient process are put forward, and it is pointed out that the DC filters and smoothing reactors can be taken as fixed impedances in characteristic analysis of low frequency of fault transient voltage and current signals on the DC lines; influence of fault types, fault positions and the values of grounding impedance on the transient characteristics of faults on DC lines are analyzed; calculation model of lightning discharge of DC transmission lines are studied, simulation modeling methods for lightning current source, tower, flashover model of insulators and transmission lines in lightning transient simulation are given, transient characteristics of voltages and currents on DC lines under lightning induced fault and lightning disturbance are analyzed; The formation mechanism and determination standard of commutation failure in HVDC systems and the simulation methods of monopolar and bipolar commutation failure are studied, analysis of transient characteristic of voltages and currents on DC lines for commutation failure is presented. This work can provide direct and effective reference for subsequent studies on fault transient analysis of HVDC transmission systems, and lays a foundation for the research on protection and fault location principles for HVDC transmission lines.(2) Research on the protection principle for HVDC transmission lines considering faulted pole selection.Transient amplitude characteristics of voltage fault components on DC lines are analyzed, and it is found that amplitudes of voltage fault components on faulted pole lines are larger than those on the healthy pole, as the integral values of voltage fault components can better reflect the characteristic. Criteria for faulted pole selection are proposed, using the ratio of amplitudes of voltage fault components on bipolar lines, and the setting method for threshold values of the criteria is also given. Transient polarity characteristics of current fault components on DC lines are analyzed, and it is found that polarities of current fault components at the two ends of faulted lines are opposite for internal faults, which are same for external faults. Polarities of current fault components at two terminals of the faulted line are used to propose the criteria for internal faults. The stationary wavelet transform is used to extract polarities of current fault components. The selectivity and rapidity of the proposed main protection for HVDC transmission lines are satisfied, and the setting method for threshold values is simple and does not depend on simulation results. The protection can be realized with a sampling frequency of10kHz and above, only needs the polarity information of the current fault components at the other end, and has low requirement for reliability of communication channel, which is especially applicable in need of faulted pole selection.Based on parameters of the±660kV Yindong HVDC transmission project in China, a PSCAD simulation model for the±660kV HVDC transmission system is built, and is used to test the performance of the protection principle for HVDC transmission lines considering faulted pole selection. Simulation results demonstrate that the protection can select the faulted line rapidly and distinguish internal faults under different operation modes of a two terminals bipolar HVDC transmission system. The protection has high anti-grounding-impedance ability and can realize high speed protection on the whole DC transmission line.(3) Research on the protection principle for HVDC transmission lines based on directions of voltage and current fault components.The protection for HVDC transmission lines considering faulted pole selection will not act correctly for line-to-line fault, which happens rarely. Therefore, this paper presents a protection principle that discriminates internal faults for HVDC transmission lines. Transient characteristic analysis of voltage and current fault components on DC lines shows that, the directions of voltage and current fault components measured at the two terminals of the DC lines are different for internal faults and external faults. Direction criteria for voltage and current fault components are proposed by comparing the integral values of voltage and current fault components in a period of time with setting threshold values; a protection principle for HVDC transmission lines based on directions of voltage and current fault components is proposed by comparing directions of voltage and current fault components measured at the two terminals of the DC lines. Setting calculations of threshold values in this protection do not depend on simulation results. Only the direction information of voltage and current fault components at both ends of the line are used to identify internal faults on the HVDC transmission lines, so this protection principle has low requirements on the communication channel. This protection principle does not need synchronization of sampling data. Criteria for directions of voltage and current fault components only calculate the integral values with a data window of5ms, and have low requirements on the data calculation speed and sampling frequency. A sampling frequency of10kHz and above can satisfy the calculation of criteria in the protection.A PSCAD simulation model of the±660kV HVDC transmission system is used to test the performance of the protection under different fault conditions. The proposed protection acts accurately for lightning induced faults, high grounding impedance faults and line-to-line faults, and does not act for lightning disturbance and external faults. It can realize accurate and fast protection for the whole DC transmission line, being applicable for a variety of operation modes of bipolar HVDC transmission systems, with high practicability.(4) Research on a novel two-terminal traveling-wave-based fault location algorithm for HVDC transmission lines.Propagation characteristics of fault traveling wave on DC transmission lines are analyzed. It is found that a propagation characteristic curve of traveling wave head can be used to express the propagation process of fault traveling wave along the transmission line, and the fault location is calculated with propagation characteristic curves of traveling wave heads at the local and remote terminals of a DC line. To obtain propagation characteristic curves of traveling wave heads, initial surge arrival times of distribution currents along the DC line are detected. A novel two-terminal traveling-wave-based fault location algorithm for HVDC transmission lines is presented, which does not need additional measurements, and has low requirements of the reliability of channel. Since the data window of this algorithm is short, the algorithm will not be influenced by adjustment of the control system. Simulation results based on PSCAD show that the algorithm can realize accurate fault location on the whole DC line under various possible operation modes of two terminals bipolar HVDC transmission systems, and the location accuracy increases when the fault point is close to the middle of line when, as the location accuracy is not affected by the transition resistance value and close faults the on HVDC lines, with a maximum fault location error of0.1129%of the whole length of the DC line. Compared to the two-terminal traveling-wave-based fault location used in existing HVDC systems, calculation in the proposed fault location algorithm is more complex, but it is of better reliability, anti-interference and anti-grounding-impedance ability. It retains the advantage of high positioning accuracy of the existing two-terminal traveling-wave-based fault location method. Therefore, the fault location algorithm in this paper can be used as an effective supplement to the existing two-terminal traveling-wave-based fault location for HVDC transmission lines.Research contents and results in this paper can provide theoretical support for improvement of theory level of protection and fault location for HVDC transmission lines, construction of rapid and reliable protection system for HVDC transmission projects and independent R&D of protection devices. The work done by this thesis has important academic value and practice meanings.