Assessments of Low-Temperature Aging Test Method for the Dielectric Materials Immersed in Liquid Nitrogen

Among the various factors influencing the service life of the electric equipment, the performance of dielectric insulation materials has an important role to determine their whole service life. Degradation of insulating materials is fatal to insulation breakdown and finally lead to stop their operations. So the evaluation methods for the degree of degradation and aging for solid, oil and gas insulation have been developed and utilized very effectively to determine their performances. In order to determine the degradation of insulating materials immersed in extremely low temperature media such as liquid nitrogen, the abrupt temperature change from cryogenic to normal room temperature should be considered. But the assessments of low-temperature aging test method for the dielectric materials immersed in liquid nitrogen considering these conditions were not fully reported. Therefore, for the fundamental step to establish the suitable degradation test methods for cryogenic dielectric materials, we focused on the evaluation of aging test methods for dielectric materials exposed to low temperature environments considering thermal shock by cool-down and warm up test, thermal stress test and electric field stress test. In order to verify the effect of various degradation methods on cryogenic dielectric materials, Kraft, Kapton(polyimide) and Nomax type 410 sheets have been prepared and degradation experiments have been conducted. Then dielectric breakdown tests were performed. According to the test results, it was found that thermal shock degradation could be effective factor determining the degradation of cryogenic dielectric materials.