Study On Magnetic Resonance Method For In Situ Measurement Of Silicon Rubber Insulator Aging

Silicone rubber insulators?SRIs?are widely used in power transmission lines,the aging of SRIs threatens the safety and stability of power grids.Therefore,the power grids needs an in situ measurement method which can quantitatively evaluates the aging status of SRIs.In this paper,we study a portable single-side nuclear magnetic resonance?NMR?.The traditional way to optimize magnet structure is tedious and time consuming,so an uniform design method was introduced to optimize magnet structure.To eliminate the inaccuracy of measurement introduced by different chemical structures of SRIs produced by different companies,the aging status of the SRIs was represented by the changes in transverse relaxation time?T₂?between the surface and internal materials of the SRI.The main works of this paper are consisted of five parts as follows:?1?Principles of low field NMR and its signal measurement method were summarized.The sequence for measuring transverse relaxation time was introduced.?2?The single-side NMR sensor with curved shape of magnets structures was optimized.And a U shape of magnets structures was optimized by uniform design method.Using the finite element simulation software?Ansys Maxwell?as the calculation tool to optimize the coils structures used by different sensors.The sensor structure with high temperature stability is simulated in the finite element software.?3?In this paper,we summarized several NMR signal inversion methods,and the Nelder-Mead method is applied.?4?The performance of the two sensors was measured,such as the magnetic field temperature stability,RF field uniformity and measured thickness.The magnetic field in the ROI of both sensors decreases linearly as the temperature increases.The measured RF field matches the simulation results.The measurable thickness of two sensors are489?mand 309?m,respectively.?5?The aging status of surface and interior of two SRI with different service time were measured by the two abovementioned sensors.The measured signal was inversed to obtain T₂ distribution to compare the aging status.Aging status of SRIs were characterized by the relative change of T₂ difference between surface and interior of insulator.We compared the CPMG echoes of the same SRI measured by the two abovementioned sensors,the amplitude of CPMG echoes which measured by U shape sensor was higher than that of curved shape sensor.The aging status of the same SRI with different temperature were measured to reveal the influence of temperature.The results show that:for the SRI with increasing service time,the decay rate of CPMG fitting curve increases,and the dominated component of T₂ distribution move toward left,the T₂ value decrease,and the aging status difference between surface and interior increases.The aging status of SRIs can be measured by single-side NMR sensor.The T₂ value measured at different temperatures increases linearly with the temperature.