Practical Research On Faraday DC Optical Current Measurement System

With the rapid development of HVDC project in State Strong Smart Grid, the power system puts forward a higher requirement for the dc current transformer which is the basic measuring equipment. The Faraday magneto-optical effect optical current transformer has been paid more and more attention to, for it has many advantages, such as high precision measurement, optical fiber transmission and digital output, etc.As there are still some problems in the original Faraday Effect DC optical current transformer, such as signal to noise bands overlap and temperature drift, which could reduce the measurement accuracy, the DC optical current transformer has not been widely applied. Therefore, this paper presents a spectrum shifting measuring method based on modulation and demodulation and designs and realizes a new Faraday DC optical current measurement system.In the opening of the paper, with the analysis of the noise of the Faraday DC optical current transformer, the signal to noise bands overlap and low frequency noise is determined. To solve this problem, on the base of modulation and demodulation, the spectrum shifting measuring method is presented. According to the method, the theoretical model of the Faraday DC optical current measurement system is designed. Then, the mathematical model of the optical modulation section is deduced and its circuit is realized and testified. With the computer program, this paper presents and realizes three demodulation methods. The section on signal demodulation is realized with analysis and selection on the methods. Finally, the error analysis contrast tests on the measurement system are also completed. The results show that the system has high accuracy (±0.3%) which is higher than the DC measuring devices based on hall element. Meanwhile, it is proved that the system is able to overcome the signal to noise bands overlap and has practicability.