Research On Signal Processing System Of All Fiber Optical Current Transducer

According to the requirement of electric power field, the importance of current transducer (CT) was analyzed, the shortcoming of traditional CT and the purpose of researching on all fiber optical current transducer (AFOCT) was shown.Base on the principle of Faraday effect and Sagnac interference, the configuration of AFOCT was put forward and the theoretic model was built up. In allusion to the kinds of the factors which affect the system measuring precision, theoretic analysis and simulation research were carried through and the influence laws were found; By analyzing the characteristic of photo-detector's output, a measurement and signal processing scheme based on sine wave modulation and demodulation was put forward; The signal processing circuit and software were also designed and the experiments were carried though.Firstly, the configuration model of AFOCT and the complete theoretic system model were built, the effects of 1/4 phase delay, linear polarization, circular birefringence were analyzed. According to the calculation results, the error caused by 1/4 phase delay is small and can be controlled through manufacture technics; the effects of all the factors on system error are related to the linear birefringence of sensing fiber, and the larger the linear birefringence is, the larger the effects of the factors are; the circular birefringence could not cause error independently if without linear birefringence, it also can counteract parts of effect caused by linear birefringence. The effects of sensing fiber, transmitting fiber, light source and phase modulator were also discussed, low birefringence fiber is selected as sensing fiber, high birefringence fiber is selected as transmitting fiber, and Piezoe-lectric Transducer used sine signal driving is selected as phase modulator.Secondly, based on the analysis of traditional correlative demodulation, a improved algorithm was put forward to eliminate the influence of light intensity change and modulation degree change. By analyzing the characteristic of photo-detector's output signal, a measurement and signal processing scheme based on sine wave modulation and demodulation was put forward either for alternating coupling or direct coupling mode. According to the simulation and experiment results, both alternating current and direct current signal can be processed correctly, light intensity change and modulation-degree change can not affect the measurement result, the effect on measuring precision by SNR of photo-detector's output signal can be decreased by low pass filter but accompanying with time delay, the fact that impact signal can be properly demodulated shows the high efficiency of the scheme.Lastly, grounded on the algorithms above, signal processing electronic circuit and software were designed, and experiments for validation and calibration were carried out. The experimental data show that the mean ratio error meets the precision requirement of 0.2 grade for measurement and 5P for protection, and the correlation coefficient of linearity is up to 0.9999847, and the response speed is less than 2ms when amplitude demodulation used. The frequency characteristic of measurement results fits with the measured current. The fluctuation caused by temperature drift is in the range of±0.6%, and has nothing to do with current value and position of HB fiber.