Research On The Vibration Mechanism And Broken Strands Recognition Of Overhead Transmission Lines

Strands Broken often occur on overhead transmission lines because of the extreme weather,such as lightning,wind blowing and gas corrosion.Some time it even developed into broken lines.Almost all transmission lines will experience aeolian vibration.In order to reduce the strand breakage caused by aeolian vibration,many studies have been carried out on the mechanism of aeolian vibration and the identification of strand breakage,but the phenomenon of strand breakage has not been eliminated.Modal analysis is a method of extracting modal parameters and evaluating the state of a structure by analyzing its dynamic response.This method is a common technical means in the field of structural engineering,but it has not been fully studied in the identification of transmission line strand breakage.Therefore,considering the particularity of overhead line structure,vibration characteristics and operation environment,it is of great significance to study the vibration characteristics of transmission lines,to find out the modal parameters which can effectively characterize the broken strands,to explore the modal parameters identification method in the field environment,and to solve the monitoring technology in the actual environment.It can effectively identify the broken strands of transmission lines and improve the operation safety of transmission lines.At the same time,it also promotes the further research and development of modal analysis technology in the field of transmission lines.In this dissertation,the vibration characteristics of transmission lines are studied,the model of cable vibration considering stiffness is studied,and the calculation methods of natural frequencies and modes are given.In order to find the characteristic modal parameters to characterize the strand breakage of transmission lines,the natural frequency variation law after strand breakage of transmission line was analyzed.At the same time,the influence of temperature on the elastic modulus,tension and winding angle of transmission line is studied.The linear relationship between temperature and natural frequency is illustrated through the analysis of three types of conductors.An experimental platform for conductor vibration is set up,and the regularity of frequency decrease of LGJ-95/15 conductor after strand breakage is studied under the condition of installing anti-vibration hammer and not installing anti-vibration hammer.Experiments show that the natural frequency decrease value above 30 Hz after strand breakage of conductor is larger,which can be used as a characteristic modal parameter to judge strand breakage of conductor.The current flowing through the conductor is changed by upflow method to change the temperature of the conductor.The change rule of natural frequency of the conductor after the temperature change is further studied through experiments,which can be used as the temperature compensation basis for the identification of strand breakage.The identification method of transmission line modal parameters under environmental excitation is studied.The principles of short-time Fourier analysis(STFT),Wigner-Ville distribution,empirical mode decomposition(EMD)and random subspace analysis(SSI)are explained respectively.For the problem of false modes in stochastic subspace analysis results,the principle of eliminating false modes is given,and an automatic modal frequency identification method based on dynamic clustering is given.The modal analysis of 110 kV transmission lines is carried out,and the variation of the vibration response intensity of the conductors with wind speed is studied.The natural frequencies of the transmission lines are extracted by the above methods.The experimental results show that the stochastic subspace analysis has the best effect in the modal analysis of transmission lines,and the maximum absolute error of multiple measurements is 0.57 Hz.The key technologies of the system are introduced in detail,the hardware design scheme and the work flow are given,and the key technologies of the sensor are mainly studied.A method for determining the installation position of sensors based on finite element analysis is proposed,and it is pointed out that better measurement results can be achieved by arranging sensors to avoid modal nodes.Aiming at the non-linear problem of sensors,a sensor non-linear calibration algorithm based on linear regression and BP neural network is proposed.Aiming at the problem of trend term of measurement signals,an EMD combined with correlation coefficient is proposed.Discriminating trend term filtering method;aiming at the problem of signal loss in wireless communication between sensor and condition monitoring device,the method of data compression and data reconstruction is given to reduce the loss rate;for the power supply problem of wireless sensor,transformer and charge-discharge controller are designed according to the actual load law of transmission line to realize the self-power supply of sensor.The identification method of wire strand breakage under the action of aeolian vibration is studied,which provides a new technical means for the identification of transmission line strand breakage,and provides a new basis for real-time acquisition of transmission line operation status.