Determination of Threshold Electric Field for PPLP Specimen in Liquid Nitrogen Based on the Measurement of Electrical Conductivity

The externally applied electric field causing sudden increase in electrical conductivity is called the threshold electric field. And the information of this threshold electric field is very important for reliable insulation design of dc power apparatus because dc electric field distribution is governed by electrical conductivity of the insulating material. There are several reports on the threshold electric field of XLPE and polymeric materials used for dc power cables, operated at room temperature. However, there are no previous works on threshold field of cryogenic insulating materials, which is necessary for the reliable insulation design of a superconducting dc cable. Therefore, in this study, we tried to determine the characteristics of threshold electric field of a polypropylene laminated paper (PPLP) specimen in liquid nitrogen by varying the number of PPLP layers. From the experiments, we could observe that there exists the threshold electric field of PPLP in a cryogenic environment. Furthermore, it was found that there was no difference in the measured threshold electric field in spite of different number of PPLP layers. In addition, it was found that if electrical stress exceedes the threshold field on PPLP specimen, the conductivity can be varied according to the number of PPLP layers.