the origin of current chopping in vacuum arcs

IEEE TRANSACTIONS ON PLASMA SCIENCE. VOL. 17. NO. 2. APRIL 1989

303

The Origin of Current Chopping in Vacuum Arcs RENE PETER PAUL SMEETS and behavior of localized emitting sites on the cathode (“cathode spots”) is available nowadays, new attempts can be made to understand the phenomenon from a fundamental point of view. The purpose of this paper is to connect the rapid microscopic processes at the cathode of a vacuum interrupter with the current-chopping phenomenon in ac circuits. This connection will be established through intermediate steps. In each of these, the time scale on which the relevant phenomena take place becomes shorter: From typical 50-Hz effects via submillisecond dc arc lifetimes to“instabilities” appearing during some ps. Finally, the relevant transients in the arc signals will be related to physical cathode processes on a sub-ps time scale. BETWEEN AC THE 11. STEP 1: THE RELATION CHOPPING CURRENTN D THE DC ARC LIFETIME A It is a well-known fact that a low-current dc vacuum arc has a finite lifetime ( 7 ) which is a strong function of the arc current ( I ). A lifetime is defined as the time between arc initiation and spontaneous extinction. From experiments under well-defined conditions, an empirical relation is obtained[3]-[5]:

Absrrucl-The semi-empirical model, described in this work, relates the phenomenon of“current chopping” of vacuum arcs to rapid microscopic processes on the cathode surface. First, for a resistive circuit the approximate chopping current as a function of power-current amplitude was calculated using two dc arc lifetime parameters which were obtained from experiment. Secondly, it is shown that spontaneous arc extinction is the result of an accumulation of characteristic arc instabilities, each of which can be described as a randomly occurring transient arc resistance. Qualitatively, the“severity” of these instabilities is demonstrated to depend on arc current, arc length, and circuit parameters. Thirdly, the presence of instabilities seems to be a manifestation of an ion deficiency in the near-anode region. This deficiency may be due to a discontinuity in the ionized mass flow into the plasma, occurring at a failure to initiate a new cathodic emission site“in time.” Following the three steps, outlined here, the nature of the currentchopping phenomenon, hitherto explained by a minimum cathode spot operation current, can thus be coupled to the dynamics of the spot.

I. INTRODUCTION DEALLY, in a power interrupter called upon to break an ac current, a stable discharge should persist between its separating contacts until the current reaches its natural zero. In practice, however, the current flow is interrupted prior to this moment at actual current values between 210 A, depending upon the contact material. Failure to carry the current gradually to zero is called“current chopping. This phenomenon may result in substantial overvoltages roughly proportional to the value of the choppe

d current. These arise due to the magnetic energy, still stored in the inductances, and may be especially troublesome when switching off inductive loads such as locked motors and transformers in a no-load operation during inrush. In modern vacuum interrupters, however, overvoltages due to current chopping are minimized by a careful selection of the contact compound, and can be handled by standard surge-protection techniques. It has been shown that precocious arc extinction in gaseous interrupting media can be attributed to arc columnar effects. In a vacuum interrupter, however, the absence of columnar effects at low currents suggests that the sudden arc cessation is dominated by cathode processes. This was already recognized 25 years ago by the work of Lee et al.[ l],[2], who postulated that current chopping is due to a breakdown of necessary conditions to maintain emission of electrons at the cathode. However, since much more (experimental) evidence on the nature

I

In? ( I )

=

a

+ 0 In ( I )

(1)

with? the average of a number of lifetime measurements at the same current. The parameters a and 0 can be determined experimentally: a is a function of the surge impedance of the feeding circuit, 0 depends on the vacuum-breaker contact distance and motion, and both a and 0strongly depend on the contact material and surface microstructure[6],[7]. In an ac circuit, the near-zero current variation with time is i(t)=[ut

(2)

where l i s the current amplitude, and w is the power frequency. The remaining time before current zero at a momentary current value i can thus be given as In ( t )= a‘

+ In ( I ),

with a’

=

-In ([ U ) .

(3)

Manuscript received April 4, 1988: revised August 26. 1988. The author is with the Faculty of Electrical Engineering, Eindhoven University of Technology, P.O. Box 513. 5600 M B Eindhoven, The Netherlands. IEEE Log Number 8927087.

Comparing (1) and (3) makes clear that if 0> 1, the expected lifetime of a dc arc decreases at a faster rate of current than the remaining time before current zero. This implies a chopping of current from a level Ich to zero at a time somewhat before current zero.

0093-3813/89/0400-0303$01.00@ 1989 IEEE

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