Novel Unilateral NMR Sensor for Assessing the Aging Status of Silicone Rubber Insulator

The aging of a silicone rubber insulator threatens the safety of the power grid. An accurate estimation of the insulator aging status is necessary for taking precautions against insulation failure. In this paper, we propose a novel unilateral nuclear magnetic resonance (NMR) method to quantify insulator degradation attributed to aging. First, a unilateral NMR sensor was designed specifically for transverse relaxation time ( T_{2}) measurement. The sensor static field possesses a constant gradient along the vertical direction to facilitate depth-selected measurements. Second, a step motor system was employed to control the penetration depth of the measurement area precisely. At each depth, the transverse relaxation decay curves were obtained with the sensor by using the Carr-Purcell-Meiboom-Gill sequence. Third, a data-processing method based on fast Fourier transform and band-pass filter was utilized. With this method, the transverse relaxation information of the layer with the desired thickness was extracted. Finally, 1-D T_{2} distributions of each layer were obtained by applying inverse Laplace transformation on the filtered transverse relaxation decay curves. The 1-D depth profiles taken from three kinds of 220-kV silicone rubber insulators with different service times were compared. The results demonstrate that the insulator with a longer service time is characterized by a shorter relaxation time. Moreover, an increase in the penetration depth into the insulators clearly results in an increase in T_{\text {2long-mean}} . The potential applications of the unilateral NMR method in electrical engineering include the nondestructive examination for defect detection and the aging evaluation of the insulating material.