| HVDC is more economical than high-voltage AC transmission in long-distance,and high-capacity power transmission.HVDC transmission has many advantages such as long distance,flexible control,lower line loss,fast response and large transmission capability.It is widely used in modern power system and plays a important role in asynchronous network.At present,the number of HVDC transmission projects in China is increasing,and has become the world's leading position.However,in general,due to the long distance of HVDC transmission lines,the transmission lines have to cross different kinds of terrains and climates,so the failure probability of long-distance HVDC transmission lines is relatively high,accounting for 50%of the fault rate of HVDC transmission system.Therefore,it is of great significance to strengthen the safe operation and stability of HVDC transmission line protection and quickly and reliably detect and isolate faults.At present,the principle of HVDC line protection mainly includes traveling-wave protection,differential under-voltage protection,low voltage protection and longitudinal differential protection.Among them,traveling-wave protection and differential under-voltage protection have fast operation speed and are often used as the main protection of the line.However,traveling-wave protection is vulnerable to high resistance,fault distance and interference.The traditional traveling-wave protection method uses the voltage change rate as the protection starting criterion and the voltage and current mutation amount after fault as the protection action criterion.However,with the increase of the transition resistance,the voltage and current change rate and mutation amount will decrease significantly,which makes traveling-wave difficult to be detected.Aiming at the defects of traveling wave protection principle,some scholars have done a lot of research.As a time-frequency analysis tool of transient signal,wavelet transform can effectively extract the traveling wave head,which greatly improves the traveling wave protection principle.However,for different types of signals,different master waves and different types of waveform extraction are needed,which also increases the difficulty in the analysis of traveling wave protection.Therefore,it is of great application value and practical significance to study a kind of high voltage direct current transmission line protection with high reliability,quick response and high sensitivity.The main work and achievements of this paper are as followsThe research background and practical significance of this topic are introduced,including the development of HVDC transmission at home and abroad,the analysis of the main technical characteristics and advantages and disadvantages of HVDC transmission,the components and main functions of HVDC transmission system,and the main significance of this topic.In addition,the research status of HVDC transmission in China in recent years is described,and the advantages and disadvantages of the existing protection of HVDC transmission lines in China are pointed out.Taking S transform as the basic tool,this paper studies the basic principle,derivation and properties of S transform and the derivation process of the discrete S transform,compares it with wavelet transform and short-time Fourier transform,and analyzes and compares their advantages and disadvantages in detail.Through the analysis of them,it is concluded that S transform can be used to extract the time-amplitude characteristics of the frequency component of the voltage in the transmission line,so as to detect the fault and determine the fault time according to the abrupt change point of the frequency component.Based on a lot of analysis and simulation,the propagation characteristics of different frequency components of voltage in dc transmission lines are analyzed,and a new single-terminal protection method for HVDC transmission lines is proposed based on S transformation.Firstly,the frequency component of the line-mode voltage is extracted by S transformation.Then,according to the boundary characteristics of HVDC lines,the time-amplitude characteristics of the extracted high-frequency components are used to detect faults,and faults in and out of the region are distinguished at the same time.Then the fault and interference are identified according to the amplitude of the extracted zero frequency component.Finally,the fault pole is judged according to the ratio of the integral values of the voltage mutants at the positive and negative poles after the fault.Finally,the simulation model of China Yinchuan to Jiaodong ±660 kV bipolar 12 pulse HVDC transmission project was built on the platform of PSCAD,and the proposed protection method was verified.A large number of simulation results show that the relay protection method proposed in this paper can quickly and accurately fault the region within and outside the region,eliminate lightning strikes and noise interference,correctly determine the fault pole,and when the remote high resistance grounding can also be accurate and effective action,indicating that the proposed protection method has good reliability and quick action. |
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