| With the increasing voltage level and capacity of power system as well as the requirements that future power grid asks for more intelligent,automatic,digital equipment,tradtional current transformer cannot meet the requirements of power system development due to its own defects.Optical fiber current transformer(FOCT)is becoming the mainstream of the development of power system for its advanced sensing mechanism,reliable insulation,impossibility of magnetic saturation phenomenon,large dynamic range,wide frequency response,small size,etc.However,the transformer would lose its reliability in power measuring,monitoring,protecting and so on because its internal optical components could be easily affected by temperature,vibration,electromagnetic interference and other factors,which leads to an unsteady Verdet constant,besides,generates complex noises with linear birefringence,and therefore limits its popularization and application to a larger scale.The internal noise of FOCT and the complex ambient noise directly reduce the signal to noise ratio of the output signal of FOCT,which affects the maximum measurement limit of the FOCT.The frequency of internal noise generated by the FOCT and the frequency of measured current signal are overlapped,and the internal noise is time-varying,coupling,with inaccurate statistical characteristics,while external environmental noise could be much more complicated on account of its changes result from FOCT’s operating environmental variation,which makes the traditional method of signal noise separation unavailable.Thus,the study starts on noise characteristics of FOCT,considered the characteristics of the output signal of FOCT and its work environment,based on the analysis the advantages and disadvantages of traditional adaptive algorithm and several typical variable step adaptive algorithm,combined with multi-scale analysis in wavelet transform theory,came up with an improved multi-scale domain variable step adaptive algorithm,and proved that the improved algorithm has faster convergence speed,higher steady-state precision theotetically.In the simulation,firstly the MATLAB is used to analyze the improved algorithm and the other two typical algorithms,the steady-state error of the learning curve of the paper improved algorithm can be kept within 0.1dB under two SNR conditions of 10 dB and 0dB respectively.Then,the article design a variable step size adaptive algorithm testingsystem based on ActiveX technic,proposed by Microsoft,which uses component object models.Using of the system can adjust the parameters of algorithm quickly,achieve the best filtering performance,and compare the output signal of the algorithm and the original signal.The MATLAB simulation and the test system based on Activex technology both verify that the improved algorithm has higher accuracy and has a stronger anti-interference performances.Finally,the article used the embedded system DSP to develop a processing circuit board of FOCT.Then designed three kinds of noise generator,namely noise generator based on DSP,triode tube noise generator and OCT simulator.The algorithm hardware testing system is set up through the signal processing board and noise generator.In the OCT simulator experiment,when the input signal is 1.5A,the AC signal of the output of the photodetector is 30 m A,the signal to noise ratio of the output signal is improved by12.9387dB;in 2.5A,the signal to noise ratio is improved by 13.255dB;in 5A,the signal to noise ratio increases 21.809 dB.The noise generator based on DSP and the OCT simulator are more complex,and the measurement error is relatively high,but it can be guaranteed to be within 1.5% and can meet the requirement of the accuracy of FOCT measurement(2%).The hardware test results are consistent with the theoretical analysis ans simulation analysis,proved that the improved algorithm has a unique advantage in FOCT applications. |
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