High voltage testing of insulators for a medium voltage liquid nitrogen insulated line

The dielectric strength of two insulator designs for a liquid nitrogen insulated line at distribution network level was investigated with 1-phase AC, 3-phase AC, and standard lighting impulse voltage. In the described test series, a rigid ground tube was used in combination with discrete insulators resulting in a short liquid insulated line configuration. Firstly, the radii and conductor position were calculated which cause the lowest maximum field strength on the three inner high voltage conductors. Then a device was built up which allows high voltage testing on a straight length of 0.7 m. All tests were performed under ambient pressure. Copper tubes were employed instead of superconductors. One type of insulator was a simple PE disk and the other type was 3D printed with PLA material. One phase AC test was performed up to 72 kV RMS. Three high voltage transformers were used for the 3-phase AC test in order to obtain a voltage phase shift of 120 degrees, each. Voltages up to 86.6 kV phase to phase (RMS) were reached. Standard lightning impulse tests up to 145 kV were successfully performed.