Investigation Of High-Sensitive Gas Laser Raman Spectroscopy And Its Application

Laser Raman spectroscopy is widely applied to analyze solid and liquid materials as a power analysis tool of matter structure, capacity. The sensitivity is not enough for the trace-gas detection. So it is importance to enhance the laser Raman spectroscopy of gases. Here, we will analyse the dissovied gases in transformer oil using of laser Raman spectroscopy enhanced technique. Dissolved Gas Analysis(DGA) is considered the best method to monitor and diagnose the potential faults, prevent tragic accidents in power transformer. There are various transformer diagnosis technologies including IEC three-ratio method, Expert System, Artificial Neural Network, Pattern recognition systems etc. Generally they synthetically analyze the datum of DGA, physical chemistry, electrical performance etc. and then give the final judgement. Whereas, diagnose of gases dissolved in transformer oil is the best important step in all transformer fault analysis technologies. It has an important significance to seek after a new method of Dissolved Gas Analysis in electrical power transformer.There are often more than ten gas species in transformer oil. Multi-trace gas detection still is a hot research field in recent years. Different gas-detection techniques have achieved rapid development and have been widely applied in many areas. Today, the gas chromatography has been widely used to diagnose the gases separated from the power transformers. However, it is well known that this technique requires separating the gas mixture by a gas chromatography column. And different gas species detection need different special gas detector. Aside,gas chromatography technology has complex process and performs poor quantitative repetition. So it is important to seek after a simple direct method of simultaneous multi-gas species detection. Raman spectroscopy technique has above advantages. It needn't separate the gas mixture and can detect multi-gas species using a single frequency laser. Aside, Raman spectroscopy technology has advantages of accurately qualitative ability, finely quantitative repeat. However, it is hard to achieve high sensitivity detection due to the small Raman scattering cross section of molecules and strong Rayleigh scattering disturbance. In electric power system, the investigation of gases diagnosis using of the Raman spectroscopy technique almost is blank for a long time. With the development of laser and detector technique, we try to analyze the gases dissolved in transformer oil by Raman spectroscopy technology. For improving the detection sensitivity of gas molecules, we tried the methods of Surface-Enhanced Raman Scattering (SERS) and laser Raman spectroscopy respectively. Good results are obtained.Firstly, some typical gases are detected out by using of SERS. Compared to Raman spectroscopy of free gases, the enhancement of 2～3 order-of-magnitudes has been observed in the surface-enhanced Raman spectroscopy (SERS) of gases adsorbed on nitric acid-roughened metal foil. This improvement of detection sensitivity is important for many gases analyze in other fields. However, some Raman lines of gases adsorbed on these active substrates showed larger frequency shifts and linewidth broadening, comparing with the Raman spectroscopy of free gases. These greatly limit application of this technique in the field of gas analysis. In our work, we mainly investigate the problem of relative Raman frequency shifts by using two-oscillator electromagnetic model on the base of the mechanism of electromagnetic enhancement. According to our calculation, we find that the surface-plasmon dispersion, retardation effect, radiation damping, and the radius of particles play crucial roles on the relative large Raman shifts and linewidth broadening. Especially the surface-plasmon dispersion induced by the large particle influence Raman shift severely. Up to now, the exiting theory mostly takes small particle approximation for simplify formula in the numeral calculation. We further calculate the Raman frequency shifts of gas molecules adsorbed on large metallic particles. Our investigation has the direction significance for metallic active substrate preparation theoretically.Finally, we have built a complete Raman detection system for analyzing the dissolved gases in electric power system. A near-confocal cavity is designed in the gas sample cell to improve the detection sensitivity of gases. A power of >9W is achieved on the two focuses of the near-confocal cavity, which enormously enhance the excitation light intensity of the gases molecules in the cell. At the same time, the detection sensitivity of gases is greatly enhanced. We also designed the compound lens and a wide band reflector in the sample cell to enhance the Raman signals collection. Using this Raman detection system, we successfully detected the gas mixture separated from standard transformer oil provided by East China Electric Power. In the experiments, the mixture of eight gases with the same volume ratio is used to simulate the typical gas species in electric power system. High sensitivity detection of gases dissolved in transformer oil is achieved. Today this experimental Raman system is transferred into commercial industrialization. The success of this Raman gas detection system will has revolutionary meaning for defaults diagnosis in the electric power system.