Research On Thermal Performance And Temperature Monitoring Of OPGW

With the development of smart grid, the integration of electric power transmission and communication network becomes the main trend. The optical fiber composite overhead ground wire(OPGW) is not only used for ground wire to protect transmission line, but also acts as optic cable to transmit information. It is now widely used in high voltage transmission lines. Under the short circuit conditions, the temperature of OPGW cable will rise rapidly, which will affect the safety and stability of the electric power network directly. Materials and structures are two important factors that influence the thermal properties of optical cables, and choosing the proper constituent material and structure can effectively improve the thermal performance. To further improve the safe and reliable operation of the transmission line, real-time monitoring of the running state is another effective way. Due to the advantages of long sensing distance, high positioning accuracy and fully distributed measurement, distributed optical fiber sensing technology based on Brilliouin optical time domain reflectometry(BOTDR) has broad application prospects in the temperature monitoring of the high voltage transmission lines. This thesis focuses on the improvement of the thermal performance of OPGW cable and using BOTDR technology to monitor temperature information along the optical cable.On the basis of analyzing the temperature rise mechanism of optical cable under short circuit condition, the simulation model is established by using COMSOL Multiphysics software. The impact of armored layer materials and casing materials on the thermal properties of the cable are analyzed. The improved structures are also presented. By comparing the thermal performance of the modified structures and the traditional ones from the short circuit current and the temperature limitation, the thesis gets the following conclusion that the thermal performance of modified trapezoidal structure has been greatly improved.The experimental scheme of temperature sensing based on Brilliouin optical time domain reflectometry is investigated and the sensing principle is studied. It gives emphasis on some key technologies in the system and parameters selection of important devices in the experiment. A Brillouin laser is developed as the local beams of coherent beat frequency, whose linewidth is narrow, and the output power is relatively stable. With the optical coherent detection technology, the Brillouin frequency shift can be shifted from 11 GHz high frequency to 1.28 GHz. In the system, the high speed data acquisition card has been used to collect multiple sets of data. The signal processing and extraction are implemented by the method of overlapping average, short-time Fourier transform and Lorenz fitting.Experimental platform is built for temperature sensing. The influence of different pulse amplitude, pulse width and sensing length on the Brillouin scattering signal is analyzed. It is indicated that at the threshold of nonlinear effect, the sensing distance is the longest. As for the 10 km sensing fiber the spatial resolution 15 m can be achieved and ±4? temperature accuracy can be reached. The experimental results verify the linear relationship between Brillouin frequency shift and temperature and the result shows that the system can measure the temperature changes along the optical fiber more accurately.