Research On On-line Monitoring Method Of Overhead Transmission Line Ice Coating Based On Φ-OTDR

Electricity occupies a very important position in our national economy and is an important foundation for social and economic development.Transmission lines are an important part of the power grid.With the rapid development of our country’s social economy,the demand for electricity in various fields has increased sharply,which puts forward higher requirements for the safety,stability and reliability of power transmission.The long-distance,large-span overhead transmission line route is complicated in terrain,and the environment is harsh.It is prone to be destroyed by disasters.Among them,freezing rain,sleet,rime,snow,etc.condense around the cable and form icing and the galloping in which the wind-induced vibration amplitude of the cable increases due to the uneven icing of the cable in the circumferential direction is the most common and the most harmful.It is extremely urgent to monitor the icing of transmission lines,detect and deal with hidden faults in time,and ensure the safe and stable operation of the power grid.Traditional electronic ice-coating monitoring technologies such as tension sensor-based weighing method and image monitoring method have disadvantages such as difficulty in power supply and networking,poor anti-electromagnetic interference ability,weak environmental adaptability,high cost and difficulty in high-density deployment.The optical fiber sensor based on Fiber Bragg Grating(FBG)relies on the grating in the optical fiber and cannot use the existing Optical Fiber Composite Overhead Ground Wire(OPGW)and all-dielectric self-supporting type in the power grid.Optical cables such as All-dielectric Self-supporting Optical Fiber Cable(ADSS)and Optical Phase Conductor(OPPC).Moreover,FBG technology can still only realize scattered point measurement,and the number of arrays is limited,which cannot be used to monitor long-distance transmission lines.The distributed optical fiber sensor uses the entire optical fiber as the sensing element and signal transmission medium for external parameters.It has the advantages of high sensitivity,strong anti-electromagnetic interference ability,continuous measurement without blind areas,and many measurable parameters.And it can be well integrated with the structural characteristics of the existing transmission line,and overcome the shortcomings of traditional ice coating monitoring technology.At present,among many distributed optical fiber sensing technologies,Brillouin optical time domain reflection technology(BOTDR)for monitoring temperature and static strain and phase sensitive optical time domain reflection technology(Φ-OTDR)for monitoring vibration,Polarization-sensitive optical time domain reflectometry(POTDR)has achieved certain application results in the monitoring of icing galloping on transmission lines.However,there are still shortcomings.First,the temperature distribution data of the transmission line obtained by monitoring alone cannot directly determine whether the transmission line is iced and the degree of ice coating.The comprehensive analysis of the accumulated temperature time,the temperature difference between the days and the meteorological data can only qualitatively evaluate the the probability of icing.The judgment result is slightly far-fetched,and the false alarm rate is high.Second,Φ-OTDR and POTDR can only detect galloping,and cannot judge whether the transmission line is iced and the degree of ice coating.In view of the above-mentioned shortcomings,this dissertation attempts to cut from the perspective of the influence of icing on the vibration characteristics of transmission lines,and propose a new method of icing monitoring.Firstly,the finite element model of the contour ice-coated overhead conductor was established in ANSYS finite element analysis software and the modal analysis was carried out.The analysis results showed that the thicker the ice,the greater the horizontal tension of the overhead conductor and the greater the sag.The natural frequency of each mode of overhead conductor decreases monotonously as a function of ice thickness.Secondly,according to the monotonous decreasing mapping relationship between the natural frequency of overhead transmission lines and the thickness of icing,a method for measuring the natural frequency of transmission lines based on Φ-OTDR is proposed,and the change of the icing thickness is inverted from the change of its natural frequency.Finally,a transmission line icing simulation device with a high degree of simulation effect was built,and the suspension fiber was coated with an attachment to simulate the transmission line icing,and the commercial Φ-OTDR was used to detect and analyze it.The test results show that the vibration frequency of the overhead suspended optical fiber is only related to the system properties,not the excitation;the natural frequency of the suspended optical fiber decreases continuously with the increase of the number of icing layers.The first-order natural frequency f of the in-plane vibration is fitted with the reciprocal n-1/2 of the icing index,and the correlation coefficient reaches 0.9614.The experimental results prove that the natural frequency measurement method of transmission line based on Φ-OTDR proposed in this dissertation,the online monitoring method of inverting the change of ice thickness from the change of natural frequency has high reliability and strong feasibility.This is of great significance to promote the better application of Φ-OTDR to the engineering application of transmission line icing monitoring.