| In power systems, oils are widely used primarily for lubrication, insulation and heat dissipation. With the improvement of our voltage level and the increasing of the oil-filled equipment, oil deterioration problems will lead to a series of equipment failures which affects safety and stability of the power system. So it is very important to monitor the oil deterioration and equipment failures timely.The existing online monitoring method is not accurate enough, or not economic or complicated. In this paper, we introduced a low-field nuclear magnetic resonance?NMR? technology to monitor the deterioation status of the oils in electric power on-line. For this purpose, we carried out the preliminary research on the deterioation status of the oils in electric based on low-field NMR principle. The main work in this paper is as follows:?1? The design of NMR detection sensor. The design of the NMR sensor include two aspects: the magnet structure and coil structure. To design the magnet structure, the Ansoft Maxwell simulation software was used to optimize the given region of interest?ROI?. Passive shimming method was adopted to improve the uniformity of ROI. Finally a V shaped magnet was desgined, which is composed of two identical cuboid permanent magnet blocks and two iron shimming sheets of a special shape, generates a relatively homogeneous static magnetic B0 field?? 364 ppm? on a 10 ×10 mm2 ROI and the magnetic field at the center of ROI is 390 mT. In the vertical direction, it produces a constant gradient of 15.4 T/m from 13 mm to 18 mm above the magnet plane which allows for self-diffusion measurements. According to the application, we chose a solenoid coil as the RF coil, because its use is convenient for inserting the pipe and accessing the oil sample and it also has the better SNR.Through the simulation of the RF magnetic field calculation, the optimal structure for solenoid coil.A optimal structure for solenoid coil was obtained by the simulation of RF magnetic field.?2? NMR experiments of different deterioration state of oils from electric power system. The NMR sensor was used to assessing the deterioration status of phosphate ester fire-resistant oil and transformer oil. Firstly, five phosphate ester oil samples with different service time was measured and the equivalent transverse relaxation time is obtained by one dimensional inverse Laplasse transform. Results demonstrate that an increase in the service time of the phosphate ester oils clearly decreases equivalent average transverse relaxation time T2mean. In order to confirm the reliability of the results, the acid value of phosphate ester oil samples were measured which was found consistent with the deterioration rule of phosphate ester oil. Then, two transformer oil samples with different service time were measured and the results obtained by the inverse Laplasse transform show that an increase in the service time of the turbine oils decreases equivalent average transverse relaxation time T2mean. Because of the less samples of transformer oil, a high accuracy of the spectrometer was used to detect the chemical shift information for the further analysis of its deterioration condition. Results show that with the deteriorating of the oil, the chain length of the oil becomes shorter. At last, the self diffusion of liquid is studied by using the portable sensor. The self diffusion coefficients of two oil samples with different viscosity were measured.Results show that this NMR sensor can distinguish the different viscosity of the liquid, but the detection accuracy is limited and can not distinguish the minor viscosity changes of the liquid.In this paper, a series of work of the low field nuclear magnetic resonance method to detect oil deterioration state of electric power and the sensor design were carried out,which provided an economical,small bulk,simple and quantitative evaluation new method for the on-line monitoring of oil deterioration state of electric power. |
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