Research On The Key Technologies Of Signal Detection And Processing Of Optical Direct Current Transformer

The rapid development of high voltage direct current (HVDC) technologymakes the direct current parametric measurement increasingly important. Thetraditional direct current (DC) measurement methods are generally based onelectromagnetic induction principle, leading to the disadvantages of huge volume,complex structure, high insulation requirements and etc. Optical current transformer(OCT) has the merits of small volume, light weight, simple insulation structure, nomagnetic saturation issues. It is important to carry out researches on DC OCT andpromot its producibilibty and practicality for the development of HVDC technology.As to signal detection and processing, there are two main problems for DCOCT. One problem is that useful signal is very difficult to extract and recover frominterference noise because the signal-to-noise ratio is low and the noise overlaps thesignal band, the other one is that measuring accuracy is low when general signaldetection methods are applied to DC OCT because the current to be measured andthe inherent constant in data processing are both DC signal and cannot be separated.Aiming at these two issues, this paper made researches on the key technologies ofsignal detection and processing of DC OCT.For the problem that signal-to-noise ratio is low and the noise overlaps thesignal band, this paper analyzed the signal to noise characteristics of DC OCT, thenproposed to use modulation and demodulation method to realize the separation ofsignal and noise and the extraction of signal. Besides, this paper discussed theprinciple of modulation and demodulation method. Theoretical derivation andsimulation analysis were made on the anti-noise performance of envelope detectionmethod and synchronous demodulation method. The results showed that whenmeasuring DC component only, the two demodulation methods had the sameaccuracy, but when measuring harmonic current as well, the anti-noise performanceof synchronous demodulation method was superior to envelope detection method. Inthis paper, synchronous demodulation method was selected because of its betteroverall performance.For the problem that the current to be measured and the inherent constantcannot be separated, this paper analyzed the signal detection methods of DC OCT,pointing out that high measuring accuracy cannot be achieved when general signaldetection methods, such as single optical path method and double optical pathmethod, were applied to DC OCT. Based on that, signal detection methods of DCOCT were optimized and single optical path compensation method was proposed. The theoretical analysis and simulation results showed that this method made up forthe defects of single optical path method and double optical path method and had ahigh measurement accuracy.Besides, this paper designed DC OCT experiment system, completed thewelding and debugging of circuits, and carried out experimental studies. Thestability of light intensity of light source was analyzed theoretically andexperimentally, pointing out that temperature was the main influencing factor, andproposed the experimental means to reduce its impacts. Based on theimplementation of signal extraction by using modulation and demodulation method,the characteristic curves of the experiment system were determinated whichdemonstrated the superiority of the proposed signal detection and processingmethods of DC OCT. At last, DC measurement error experiment was conducted.The experiment results showed that the DC OCT system designed had an accuracyof±0.6%.