Phd Raphael CHASSAGNOUX
“Dielectric study of liquid and boiling nitrogen –
Application to a superconducting fault current limiter”

The increasing number of interconnections in electrical networks and the massive integration of renewable energies nowadays comes with an increase of short circuit currents, and more constraints on high voltage circuit breaker during the current clearance. To solve this problem, a solution consists in inserting a fault current limiting device on electrical lines. Among the available technologies, the superconducting fault current limiter is ideal from the perspective of transmission system operator. However the design of this device is non-trivial, especially the electrical insulation, which is very specific to this apparatus: electrical insulation in a cryogenic environment (liquid nitrogen at – 196°C), superconducting tapes inducing electric field reinforcement, and strong transient heating generating numerous vapor bubbles.

In this context, the aim of this work is to gather data and knowledge supporting the design of future superconducting fault current limiters. First, the case study of a fault current limiter model inserted on an electrical line allowed to deduce the main stresses on this device during its working sequence. These constraints are then reproduced within a high voltage cryostat. Experimental results gathered include breakdown voltages measured in various conditions of voltage type (DC or impulse voltage), pressure (from 1 to 4 bars), temperature (from 65 to 77 K), inter electrode gap (from 5 to 10 mm), with and without transient heating of the tape. The observation of boiling conditions and pre breakdown phenomena evidenced several new phenomena, and allowed to perform a qualitative interpretation of the breakdown voltages. An important result is the observation of the complex and sometimes adverse effect of subcooling (temperature decrease or pressure increase) on breakdown voltages, due to the variation of nitrogen bubbles size.

Olivier LESAINT