| Transmission lines and distribution networks play an important role in the power system.With the rapid development of economy,the level of urbanization is getting higher and higher.In order to satisfy the stable operation of the power system,beautify the image of the city,and based on the reality of sustainable development,the transmission mode of electric energy has gradually developed from a single overhead line to hybrid cable-overhead line.It is very important to determine the fault location quickly and accurately after the circuit fault occurs for the fast recovery of power supply and safe and stable operation of power system.The existing fault location methods have various shortcomings,especially for hybrid cable-overhead lines.Based on this,the hybrid cable-overhead lines are studied in depth.When the hybrid line fault occurs,the traveling wave of the fault attenuates or distorts in the transmission process,resulting in certain errors in the detection of the traveling wave head.Based on this,this thesis presents a hybrid line fault location method based on traveling wave natural frequency.It defines the concept of natural frequency of traveling wave and its forming principle.It also introduces the method of measuring fault distance by using the idea of natural frequency of traveling wave.At the same time,it deduces the quantitative relationship between boundary conditions of system end,fault end and fault distance,and natural frequency of traveling wave.Finally,the method and process of fault location based on natural frequency of traveling wave are expounded.Compared with the traditional time-domain analysis method of traveling wave signal,the advantage of using frequency-domain angle is that it does not need to accurately acquire the arrival time of traveling wave,nor need to identify the fault traveling wave head.Secondly,traveling wave velocity is a frequency-domain correlation,so obtaining the natural frequency of traveling wave can accurately calculate the traveling wave velocity.Based on VMD decomposition algorithm and considering the natural frequency of harmonic second traveling wave,a fault location ranging method for overhead line-cable hybrid transmission lines with more complex characteristics is proposed.The VMD parameter optimization based on EMDH is used to effectively screen the decomposition mode number k and accurately decompose the traveling wave signal.Finally,the fault distance is calculated by using the ranging formula taking into account the natural frequency of harmonics.EMTDC simulation results show that the ranging method has high accuracy and adaptability,and has a certain reference significance.A fault location method based on traveling wave natural frequency is proposed to solve the problem of complex structure in hybrid distribution network.Firstly,the propagation of traveling wave in mixed distribution network with different complexity is introduced andanalyzed,and the corresponding relationship between natural frequencies of different waves and traveling wave paths is sorted out.Then,the relationship between the length of traveling wave paths calculated by signals from multiple measuring points and the length of known lines is compared,and the fault occurrence interval is determined,and the fault location is determined.In order to increase the reliability of multi-ends fault location,the method of fault location ranging based on single-end observation signal is also introduced.The simulation results show that this method can achieve better fault location in hybrid distribution network,and have certain positioning accuracy and high applicability. |
