High voltage testing of insulators for a medium voltage superconducting cable

The dielectric strength of simple electrical insulator designs for a liquid nitrogen insulated cable at distribution network level was investigated with 1-phase AC, 3-phase AC, and standard lightning impulse voltage. An outer ground tube was used in combination with insulators forming a short cable sample with a corrugated length of 0.67 m. All tests were performed with an open cryostat with the insulators embedded in liquid nitrogen. Several continuous types of insulators were investigated which provide mechanical support all along the three inner copper tubes. Some insulator types withstood the 10 kV grid level tests. However, none of the short length samples with continuous insulator withstood the complete test series for the 20 kV grid level. A version with discrete insulators withstood the 20 kV grid level tests. The discrete insulators provided mechanical support to the three inner conductor tubes at certain locations only, followed by the interspace to the next solid insulator where the inner tubes were completely surrounded by liquid nitrogen. The investigations allow a preliminary selection of simple solid electrical insulators before using expensive superconductor or more complex test setups for the development of superconducting 3-phase power transmission cables for the distribution network.