Research On Multi-Parameters On-Line Detecting Of Oil-Immerse Power Transformer Condition Based On Optical Fiber Sensing Technology

In recent years, the level and capacity of power transformer improve continuously, with the increase in domestic demand for electricity and the grid size. However, at the same time transformer failure rate and repair time are also increasing. Long-term researches show that concentration and composition of oil-gas and internal temperature are related to the degree of faults and life of transformer. For this reason, the study of novel on-line inspection technology of internal temperature and gas in transformer has great significance both in theory and practical. However, the internal environment of transformer has features of high voltage, powerful electromagnetic influence, small space and strong Corrosion. It has become a key technology problem to be solved that detecting transformer condition and forecasting transformer faults at high accuracy and timely. The development of technologies of fiber and semiconductor sensing and semiconductor laser provide of feasible approach to solve this problem. Unfortunately, for now, the detecting technology of transformer internal condition still has disadvantages of single parameter, low anti-interference capacity, sensibility and accuracy, slow response and multi-gas interference, which can not well satisfied with the needs of real-time monitoring transformer and forecasting transformer at high accuracy currently.In response to above problems, combining fiber semiconductor sensing technology with gas spectrum measurement technology and transformer fault analysis technology, optic-fiber semiconductor temperature and optic-fiber gas sensing models are established based on researches of theoies of semiconductor temperature sensing, gas absorption spectrum and photoacoustic spectroscopy. Meanwhile, a novel GaAs probe with high coupling coefficient and a novel gas cells with high sensitivity are designed. Further temperature and gas inspection systems in transformer are also presented, which can be used in the measurement of multi-parameter at high accuracy, good sensibility and quick response in real time. As a result, the research effectively improves the level of transformer condition detection and fault prediction. The main research contents are as follows:(1) Based on the structure of oil-Immerse transformer, the characteristics of temperature increase and principle of fever and cooling are discussed. The reason of transformer overheating fault and its characteristics are also analyzed. Meanwhile, the limit of transformer temperature rise is provided. Furthermore, a detailed explanation of the principle of transformer gas at normal or non-normal work state and the dissolve and diffusion process about gas in transformer oil are introduced. Then the relationship between the types of transformer internal failures and concentration and composition of oil-gas is disclosed. Finally this part research lays possibility and a theory foundation for study of on-line inspection technology for oil-immersed transformer condition.(2) For the problem that the technology of on-line detection of transformer temperature has some disadvantages of slow response and poor anti-interference capability and matching, the relationship between temperature and the absorption wavelength of GaAs is revealed because of in-depth analysis of GaAs optical properties and research of temperature sensing principle of semiconductor. On the basis of above this relation, the GaAs temperature sensing model is established and the temperature sensing system is constructed in which a novel reflective sensing probe and wavelength demodulating system based on CCD and diffraction technology are also designed. The novel GaAs probe has advantages of small volume, high coupling coefficient, perfect insulation, strong resistance to corrosion and good matching and the novel wavelength demodulating system can eliminate the influence of intensity fluctuation of the light source and the disturbances of optical path. Reflectance spectra at different temperatures have verified the sensing characteristics of GaAs semiconductor in further. Finally the experimental results indicate that the system has accuracy of±0.5℃, resolution of0.1℃and response time of less than6s with the range of0-235℃, the maximum temperature fluctuations is±0.3℃and the accuracy of temperature measurement is also±0.5℃at strong magnetic field environment. In short, the system has advantages of wide measure range, high accuracy, strong anti-interference ability, good stability and quick response, which is very suitable for on-line monitoring of transformer internal temperature and pre-warning for transformer in time.(3) In view of the problems of single parameter and gas component, low anti-interference capacity and mutual interference of multi-component gas in transformer oil-gas inspection, the detection models of open-difference, multi-component gas with single laser and multi-parameters are established based on studies on the theories of gas absorption spectrum and Beer-Lambert Law. Simultaneously, the new gas detection systems according to the three models are developed and also verified by experiments.Firstly, CO2absorption spectra around1572.66nm at different temperatures are measured and the multi-point inspection of CO2at high accuracy are also achieved by means of combining the ECDL characteristic with SDM and TDM technologies, difference detection model and a low noise and long optical-path gas cell. The structure of the difference detection system is simple so this system can be accomplished easily. In addition, the system can eliminate the interference of other gas and dust and is so suitable for transformer oil-gas detection in open environment. Especially experimental results of the difference detection system is so similar to wavelength modulation system's. Secondly, a series gas cell with small size is designed and the high precision measurement of multi-component gas with single laser is carried out as well based on characteristic of Ultra-Narrow-Linewidth laser, high precision adjustment of laser wavelength, taking the sawtooth signal to modulate the spectrum of laser and harmonic detection technology. Furthermore the experiments indicate that the measurement relative error is less than2%, the system dynamic response time is less than10s and the relative concentration fluctuation is less than1%.Therfore, the system can eliminate the mutual interference of multi-componet gas in transformer oil-gas detection. Lastly, by measuring and analyzing the infrared absorption spectra of CO around6354.179cm-1and6383.09cm-1at different temperatures, a kind of detection system is designed based on the characteristics of Ultra-Narrow-Linewidth laser, the spectrum scanning technology and difference detection model which is simple and easy for handing. The results of experiments reveal that the temperature measurement relative error is less than4%, the relative temperature fluctuation is less than3.5%, the concentration measurement relative error is less than5%and the minimum detectable carbon monoxide is0.05%. Consequently, CH4, CO and CO2are the major components of fault gas of power transformer which are measured simultaneously at multi-parameter and high accuracy. It can improve the level of on-line monitoring of oil-gas and forecasting faults of power transformer.(4) For the problem that detection technology for transformer trace oil-gas has disadvantages of low accuracy and mutual interference of multi-component gas, the high sensitivity measurement models of multi-component gas with photoacoustic spectroscopy and ring-down time are built based on depth analysis of fundamentals of photoacoustic effect and theory of cavity enhanced absorption. Simultaneously, the new trace gas detection systems according to the two models are developed and also verified by experiments.According to operation-modes, structural characteristics of resonant photoacoustic cell and the source of photoacoustic system noise, an optimal design theory of resonant photoacoustic cell is built. Therewith, a novel first-longitudinal resonant photoacoustic cell with multi-optical path is designed which can effectively reduce the noise disturbance and improve sensitivity. Meanwhile, according to the theory of laser coupling and the characteristics of laser transmission and laser frequency overlaps with one of cavity modes in cavity, a low noise tunable optical cavity which is consisted of two mirrors with high reflectivity also is used as the absorption cell which can simultaneously tune operation between laser frequency and cavity length. As a result, by taking the sine signal to modulate the wavelength of laser and combining second harmonic signal detection technique with laser wavelength scanning technique and characteristics of Ultra-Narrow-Linewidth laser, the detection system of multi-component trace gas with single laser is achieved which is able to effectively eliminate the background noise and the influence of intensity fluctuation and the relationship between modulation depth and measured peak amplitude also is obtained. Multi-component gas detection experiments indicate that the measurement relative error is less than2%, maximum relative fluctuation is less than1.5%and the sensibility can reach the order of magnitude of10-6. So the system can eliminate the mutual interference of multi-component gas and improve sensitivity in trace oil-gas detection. Meanwhile, the input laser is switched off by using laser detuning technique then detection system of trace acetylene wih high sensibility and accuracy is constructed based on ring-down time measured at different concentrations. Therefore, the absorption spectra of CH4around6518.824cm-1and the relationship between concentration and ring-down time are measured and also analysed. Cavity enhanced absorption gas experiments indicate that the concentration measurement relative error is less than2.5%, the dynamic response time is less than10s, and the lowest detection limit is2×10-6. So the system can improve accuracy in trace oil-gas detection. In conclusion, the multi-component detection system with single laser and photoacoustic spectroscopy technology and cavity enhanced gas detection system can measure single or multi gas at magnitude of10-6. It is suitable for on-line detecting for trace oil-gas and forecasting the early internal faults of transformer in time.