Research Of Direct Lightning Measurement With Optical Fiber

Measurement of lightning plays an important role in lightning protection technologies, such as obtaining the precise amplitude and waveform of lightning current. The current can be measured directly by observing natural lightning and obtaining the related data through established tower. However, indirect measurement, based on popular lightning orientation system and analyzing the relationship between the current and emitted electromagnetic radiations, is also an important approach. In this paper, a direct measurement system of lightning current by using Faraday Effect is demonstrated, and novel fiber-optic sensing technologies are utilized here instead of traditional CT.Firstly, the characteristics of lightning current are analyzed in detail. In most case, the lightning header is about 1~5μs (2.6μs in average), and the half peak value time is about 50μs in average with an amplitude of several KA. The frequency spectrums of amplitude and energy, calculated by the use of continuous Fourier transform, are analyzed emphatically in order to determine the system requirements, such as measuring range, detection bandwidth and response times.Secondly, the detailed principle of optical current sensor is introduced. Then, a real time system of measuring high lightning current, which can satisfy the requirements very well, is proposed and designed. Although there are several kinds of sensing architectures, interference fiber sensor is most competitive in this system. Faraday phase shifts are converted into interference signal that can be detected, while the reciprocal reflection structure can make the interference beam travels in a single fiber. Since the light waves traveling in opposite directions have the same optical path, reciprocal noises, such as strain and temperature changes, can be eliminated to a large extent and the accuracy of measuring Faraday phase shifts can be improved by four times. Considering all the factors which would affect the measurement accuracy, a system theory model is established based on Jones matrices. Effects of each factor are simulated and analyzed in detail so that the curves and effecting rules of scale factor error are acquired in conclusion.Finally, optimum working conditions of the system are discussed. Included are also selections of devices for the measuring system, which is expected to optimize the experimental platform.