Effect of Magnetic Fields on DC Interruption by Liquid Nitrogen in the HTS Electrical System

Abstract

The high on-state loss of cryogenic solid-state circuit breakers makes it a heavy burden in realizing the High-Temperature Superconducting (HTS) electrical system. Due to the ability to work in cryogenic circumstances and extremely low on-state loss, the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switch shows excellent application potential in the HTS electrical system. However, the effect of the magnetic fields on the arcing process in the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is still unclear. This paper aims to obtain the effect of the magnetic fields on the arcing process in the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Neodymium (NdFeB) magnets are used to provide the changeable magnetic fields. DC interruptions ranging from 200 A to 1800 A are conducted under 0 mT, 20 mT, and 50 mT respectively. A high-speed camera is also employed to capture the arcing process. Arcing in the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> under magnetic fields is characterized as the arc dwell period and arc voltage rapidly increasing period, which is quite similar to that in the air, but more frequent arc restrikes are observed. The magnetic fields strengthen the convective arc cooling by mixing the arc with LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and the vaporized N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , contributing to the sharp increase in arc voltage. Thus, the magnetic fields significantly increase the arc voltage and its raising rate. 630 V/1038 A is successfully interrupted under the magnetic field of 50 mT in the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The successfully interrupted current increases by 8 times when magnetic fields are applied.