Dielectric strength of liquid nitrogen for the design of Resistive Superconductive Fault Current Limiters

Abstract

Resistive Superconductive Fault Current Limiters (RSFCL) are protection devices that can be used on AC and DC grids to reduce the amplitude of fault currents. As an example, they can advantageously avoid the replacement or complete upgrade of a congested substation when prospective short circuit currents exceed the ratings of the installed switchgears and busbars. The RSFCL is made of windings of superconductive tapes that can be parallel and / or series connected and immersed into a liquid inside a grounded tank. The operating temperature being in the range of -200°C, liquid nitrogen (LN2) is used to maintain the superconductive properties of the high temperature superconducting (HTS) tapes. LN2 also ensures the dielectric insulation to the tank and between windings, making the understanding of its insulating properties key for the design of the RSFCL.

Previous works have shown the difference of behaviour of nitrogen in its liquid phase and in diphasic condition. The liquid phase is found during the grid normal operation. The diphasic condition results from the boiling of LN2 in the event of a short circuit current limitation when the tape switches from its superconductive state to its resistive state. We will focus on the characteristics of nitrogen in its liquid phase, which drives the sizing of the RSFCL cryostat in respect to the required insulation level. Although experiments have been carried out for a significant number of electrodes configuration (size, shape, gap), voltage levels and wave shapes, we have identified the need of performing dielectric characterizations of LN2 for the specific combination of parameters found on RSFCL for AC as well as DC applications.

Therefore, we have conducted dielectric tests on a real size mock-up under atmospheric pressure to characterize the withstand and breakdown conditions of LN2 in an environment representative of a RSFCL (grounded tank and a typical set of internal corona rings at potential, whose function is to shield the windings). In the experiment, the tank has a diameter of 1.31 m 10299 D1 -Materials and emerging test techniques PS2: Materials for electrotechnical purposes and modelling

Diego a P Brasiliano, Christophe Creusot, Nicolas Deveaux, Alain Girodet, Laurent Mathray

Presented at CIGRE 2024.

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