Research On Signal Processing Technology In Optical DC Measurement System Based On TGG Crystal

With the gradual increase in the proportion of renewable energy power generation,HVDC transmission technology emerged.The DC current transformer is an important device in the HVDC transmission system which provides input signals for line control,metering and protection of the system.The optical DC current transformer has the advantages of large dynamic measurement range,wide frequency response range,easy communication with digital devices and better meets the development needs of intelligent power grids.Therefore,studying a high-precision optical DC current transformer is of great significance for the stable operation of HVDC transmission systems.However,when the optical current transformer is directly used for DC measurement,there are still some problems,such as low signalto-noise ratio of the output signal and overlap of the signal-to-noise band.These problems directly affect the measurement accuracy of the optical DC current transformer.In order to solve the above problems,this paper starts from two aspects of improving the sensing unit material and signal processing algorithm and mainly does the following work:First of all,this paper analyzes the input and output of each component by establishing a mathematical model of the sensing system.It is concluded that the noise of the sensing system mainly comes from the photoelectric conversion.The characteristics of noise are analyzed to provide a theoretical basis for subsequent signal processing algorithms.It is proposed to use TGG crystal as the sensing material and use its high Verdet constant to improve the signal-to-noise ratio and measurement sensitivity of the output signal.The Faraday optical experiment platform is built to compare the TGG crystal with the commonly used ZF-7 magneto-optical glass to verify the superiority of the TGG crystal.The Verdet constant theory of TGG crystal is studied.The change of the Verdet constant with temperature is studied based on experiment and a temperature compensation scheme is proposed according to the variation law.Secondly,in order to extract useful signals from noise,this paper proposes a double correlation detection algorithm based on multiple auto-correlation algorithm and lock-in amplifier from the perspective of weak signal detection technology.Through simulation,the measurement errors of different signal processing algorithms are compared and the influence of signal-to-noise ratio on measurement error is explored.For the power transient signal,this paper proposes a windowed double correlation detection algorithm.Signaling the short-circuit current to verify the validity of the transient algorithm.The effect of the length of the window function on signal processing is studied and the length of the window is determined.Finally,an optical DC measurement system model is built and a complete set of experimental systems including hardware systems and software systems based on Labview is built.Steady-state experiment and transient experiment are performed separately.The measurement error of different sensing materials and signal processing algorithms is compared by steady state experiments to verify the superiority of the measurement system.Steady-state experiments show that the linearity of the measurement system is good.Compared with the traditional signal processing method,the measurement error can be effectively reduced and the measurement error is controlled at about 0.3%.The feasibility of the system for transient signal measurement is verified by transient experiments.The amplitude error is 0.5% in the transient experiment.It can reflect the waveform shape and amplitude of the current to be measured and meet the transient measurement requirements.