| The composite insulators have been more widely used in high voltage transmission lines, but the aging problem of composite insulators has been more serious with the increasing of service time. In order to avoid transmission line accident caused by overage of the composite insulators, we should discover the defect of composite insulator in time. The study on non-destructive assessing the deterioration of composite insulators, is greatly significant to the safety and reliability of the power supply.On account of the shortcomings of existing composite insulator detecting methods, a portable unilateral nuclear magnetic resonance（NMR） sensor has been designed to detect the aging status of composite insulators in this paper, target-field method and stream function method were used for frequency-ratio（RF） coil design. The main work in this paper can be expounded as follows:①According to the structure characteristics of composite insulators, two unilateral NMR structures were designed here. The first one is composed of a cylindrical magnet made of Nd Fe B35 and yoke iron（steel1008）, the size of diameter and height is 30 mm and 10 mm, respectively. Magnetic flux density B0 along vertical direction is 377.52±0.04 m T in region of interest（5mm×5mm） 3mm above from the surface of magnet. The other one is composed of three rectangle bar magnets made of Sm Co, and the sizes of magnets are all same as 10mm×10mm×30mm. The magnetic field distributes along horizontal in region of interest（5mm×5mm） 3mm above from the surface of magnet, and the strength of magnetic field is 120±0.4m T.②Theory of RF coil design based on target-field and stream function was studied, and optimize different coils based on maximizing relative signal to noise（SNR）1 acB i R. We obtained a planner RF coil, its relative SNR can reach 12001400, and magnetic field induced by the coil distributes along horizontal in target area.③The portable NMR sensor was used for measuring the composite insulators with different service times in the same transmission line, analyzed the NMR signals, we have obtained the equivalent transverse relaxation time which can represent the aging degree of the sheds of composite insulators. And the equivalent transverse relaxation time of insulators was decreasing while the aging level was increasing.According to the experimental results, the equivalent transverse relaxation time measured by unilateral NMR sensor designed in this paper can represent the aging degree of insulators more sensitive than static contact angle method. The portable NMR sensor can evaluate the service life of composite insulator effectively and be used in power system. The new coil design method can be used for designing RF coils which can match any main magnet structure, it also has practical value in the development and application of nuclear magnetic resonance. |
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