Optical Fiber Sensing System for Online Monitoring Wind-induced Vibration on Power Transmission Tower Survey

High-voltage transmission towers in coastal areas can be damaged by destructive hurricanes and strong seasonal winds. With real-time condition monitoring systems, severe and critical damage can be detected in the early stage, and appropriate solution taken. In this paper, we investigate possibilities for real-time monitoring of wind-induced vibration of transmission towers, using an all-optical fiber sensing system. The optical sensing probe is made by splicing coaxially a thin-core fiber, with tilt fiber gratings inscribed into its core, to a lead-in single mode fiber. The sensing probe is well packaged and directly mounted on the transmission tower. The sensor simultaneously provides two wavelength-separated spectral signatures, one originating from the core-guided mode and the other from the cladding modes. The optical power of the cladding modes shows the real-time vibration information, both amplitude and frequency, of the transmission tower. The power of core mode provides in-situ temperature measurement and an inherent self-calibration to remove the fluctuations of light source intensity and optical transmission loss. In collaboration with a power grid company, we demonstrated the feasibility of the proposed all optical fiber sensing system during field trials. A stable and reproducible correlation between the real-time wind speed and vibrational acceleration of the transmission tower was demonstrated. The measurements also showed that the mechanical resonant frequency of the transmission tower remained constant to within 0.01 Hz, independent of the wind speed. Any change in the resonant frequency would thus signal damage to the tower's structure, thereby giving early warning of a dangerous situation.