Experimental investigation on the velocity of cathode spots in a vacuum arc with high d<i>i</i>/d<i>t</i>
Liming Liu, Zhao Yuan, Lixue Chen, Shan Liu
Abstract
Abstract With the development of DC transmission and aerospace technology, the demands on the breaking capacity of vacuum circuit breakers (VCBs) under high <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:mrow> <mml:mrow> <mml:mtext>d</mml:mtext> </mml:mrow> <mml:mi>i</mml:mi> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mtext>d</mml:mtext> </mml:mrow> <mml:mi>t</mml:mi> </mml:mrow> </mml:mfrac> </mml:math> condition are getting higher and higher. The breaking capacity of VCBs is determined by cathode spots, which provide electrons and metal vapor to maintain the arc. In this paper, the experiments of cathode spot observation were carried out based on the detachable vacuum interrupter and the cup-shaped axial magnetic field contacts. The images of cathode spot distribution were obtained by an ultra high-speed camera. In the initial expansion stage of arcing, it was discovered that the distribution of cathode spots is ring-shaped and symmetrical. The initial expansion velocity of cathode spot ring is approximately proportional to the value of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msqrt> <mml:mfrac> <mml:mrow> <mml:mrow> <mml:mtext>d</mml:mtext> </mml:mrow> <mml:mi>i</mml:mi> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mtext>d</mml:mtext> </mml:mrow> <mml:mi>t</mml:mi> </mml:mrow> </mml:mfrac> </mml:msqrt> </mml:math> and is independent of frequency. During the whole arcing process, the expansion velocity of cathode spot ring rises first and then drops with arcing time. The effects of the magnetic field and metal vapor density on the expansion process of cathode spot were analyzed. Based on the experimental conditions and results, the values of magnetic field and metal vapor density were calculated which can reasonably explain the variation of expansion velocity of cathode spot.