A study on the measurement of electrical conductivity of PPLP in LN2 for a stop joint box of DC HTS power cable

DC HTS power cable has drawn attention as an ideal solution for transmitting massive electric power for long distances. As one of the accessories of the DC HTS power cable, the stop joint box (SJB) is an essential component for long distance transmission cable connections and separation of the cooling section. The main insulation of the SJB is a polypropylene laminated paper (PPLP) which is the same as that of the DC HTS power cable. In order to separate the cooling section, an epoxy spacer is located at the center of the SJB. For the insulation design of the SJB, DC electric field analysis should be done to determine the shape of insulation structure and their physical dimensions. As DC field distribution was mainly determined by the ratio of electrical conductivity of the insulating materials, information about the electrical conductivity of dielectric material should be obtained. But the reliable data of electrical conductivity of insulating material, especially in cryogenic environment could not be easily obtained. In this paper, we focused on the measurement of electrical conductivity of PPLP in LN₂ according to the existence of copper deposition. Due to the infiltration of LN₂ between the edge of the electrode and PPLP, the accuracy of measured electrical conductivity might be lowered. Thus, in order to prevent the infiltration of LN₂ into PPLP specimen, PPLP specimen deposited by copper was adopted. On the basis of the measurement of PPLP in LN₂, the initial SJB model was designed based on the joint box for a conventional AC oil-filled (OF) power cable. Then, two kinds of improved SJB models with different epoxy spacer configurations were considered and simulated using the finite element method (FEM). Consequently, it was possible to design the insulation structure of the SJB of a DC HTS power cable based on the DC electric field analysis.