IEEE Transac-t on, T I-- t i l EI-0 ,. 4, A°tg- t*st t9

THE EFFECT OF THE ELECTRIC CIRCUIT ON THE CURRENTTRANSIENTS OF THE VACUUM NIEAR THE CURRENT ZERO

F. R. Froncek

Technical University of WroclawInstitute of Electric Power Engineering

Wroclaw, Poland

ABSTRACT

The results of the detailed quantitative and qualita-tive analysis of the current transients of the vacuumarc near ac zero are presented. Based on the currentoscillograms obtained experimentally during unstableburning of the vacuum arc, we found some characteristictransient forms have been distinguished as well astheir close relation to the switched-off circuit param-eters. The set of theoretical dependences which enableevaluation of the influence of several switched-offcircuit parameters on the current transients of thearc near the current zero, have been elaborated, Theresults of theoretical analysis are in accordance withthe results obtained experimentally.

INTRODUCTION mentioned times, however, are much greater than timesof transient processes and inertia in the arc cathode

In ac circuits when the arc current ia decreases zone, which are estimated at several ns [5,6]. In thefrom certain value Imin the arc becomes unstable. It author's opinion the direct reason for the loss of arcis manifested by the occurrence of some high frequency stability in the interval Atj is the instantaneousintermediate transients which superimpose on the decay of the arc spot which due to the rapid effectsteady current and voltage of the arc. After some of the circuit can reappear after short time, of thetime the arc extinguishes, finally causing rapid cur- order of several ns, generating a sheath of spacerent decay in the switch while the instantaneous value charge. This sheath moves in the direction of theof the current is not zero in the switched-off circuit. anode at the speed v., close to the speed of positiveThis value of current is called chopping current Ich ions in the diffusive discharge plasma. The hypothe-(Fig. 2). First of all, the effect of the electric sis of creation of the double sheath when currentcircuit on the current transients of the arc for changes are very fast and positive, was presented byia<Imin is of great interest. Experimental investiga- Proskurovskij and Puckariev [6]. For the values of thetions [2,3,4,7] showed this effect to be very strong. intercontact distances of the vacuum switches andThe changes of the mean chopping current Ich obtained speed VS%104 m/s the travel time of the sheath to theby changing of circuit parameters can reach 500 to 600%. anode- can amount to 1'Ps.The effect of the electric circuit on the current trans-ients during unstable burning of the arc has not beenelucidated fully, so far. The reasons for an unstable RESULTS OF EXPERIMENTAL STUDYarc when the current is smaller than Intin are the in-stability of the arc cathode zone, mainly due to un- The experimental study was performed by means of thesatisfactory conditions of applied and output power at test system shown in Fig. 1. The principle of opera-the arc spot [1,5,7]. The cathode material is con- tion of this system had been discussed earlier [3,4].sidered the basic factor determining I . Some ob- The frequency values f, voltage supply amplitude Um,servations of the current transients of the arc near inductances of switched-off circuit Lo and intercon-the current zero make it possible to distinguish [3] tact distances d amount to Um=900 V, f=50 Hz, Lo=52 mH,very short time intervals of the arc current decay and d=6 mm. Some characteristic current ransientsAtt, followed by longer intervals Att1 containing during the unstable burning of arc for the differentattenuated oscillations of the arc current tmhf which circuit parameters are shown in Fig. 3. For the loware superimposed upon current component of power-line inductance values L2, of the order of 1 paH, and simul-frequency (Fig. 4). The times At1 are very different. taneously for high value C2, of the order of 1 pF,The experimental study indicated that the greatest transients shown in Fig. 3a and 3b can be observed.value At1 can reach is in the order of 1 us [3]. The The chopping current values under these conditions are

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IEEE Tranmectionrs on Electrical Insulation Vol. EI-20No.4. August 1985

very large and very close to the value Imin. For small 2. When inductance L2 increases in the range 5 to 500inductances L2 and progressively decreased capacitances pH, a decrease of chopping current Ieh as well as AIsmxC2 the packets of current attenuated oscillations ihf may be observed, and the differences of current valuesappear. The number of these packets N increases, 1ch and Isr= increase with L2. The values 'ch- TsmxFig. 3c and 3d. A packet of attenuated oscillations indicate the significance of the current ihf in thecontains 3 to 6 cycles. The greatest current ampli- current chopping mechanism, Fig. 3e.tude 'hf in each packet has a tendency to increaseslightly when the power frequency current decreases.Increasing of inductance L2 results in decreasing N. UFor the inductance L2>L2Cr transients of growing cur-rent amplitude, (Fig. 3e), appear successively. InFig. 3f the current oscillograms of arc for theR2>2/L2/C2 are given.

S3

SiiLo Is

Rh I R2~Lh __

Fig. 1: The test system. VS - vacuum switch; RM .measuring resistor; COLO - capacitance, inductanceof current circuit 50 Hz; VD - voltage divider, Fig. 3: Oscillograms of the arc current near theVDL - voltage diode limiter; HVIG - high voltage current zero. Copper electrodes.impulse generator; OSI,0SI1 - oscilloscopes,TCD,ICD - controZ devices.

THEORETICAL ANALYSIS

By switching VS when the arc current ia<Imin, thecurrent consists of at least two components: ipf -component at power frequency f=so Hz, and ihf' - compon-

_ * -b ent at circuit vibration frequency given by%2-C2-R2-VS. If the amplitude of cut-off current is 1m> (2 to3)1min then

al b Vpf min fIm (1)

Fig. 2: Osci2o2gram of the current (a) and arcvoltage (b) near ac zero. The current transient At11 during interval k is deter-

mined by the approximation

ihfu -i t Ihf exp[-a-c ]sin(wrkr O<k<I (2)The analysis of the negative current rush values

Ais in intervals At1, Fig. 4, was carried out by the 2 0.5author. The greatest values Aiso ... Aism for one r.12 [Auc(O)tAuck]transient Alsmx were read and its mean value AIS= and 1hfk [=2(L) + 11skl] (2a)maximal value AI5smx were computed based on several L2/C2- (aL2)dozens of oscillograms. The relations of the currentvalues ATsm= and AIsm' on the capacitance C2 and induc- (i2(0)+|AiAsktance L2 are given in Figs. 5 and 6. The mean values y arcsin Ihf (2b)of the chopping current Ich are marked in the Figure.Very similar relations of the chopping current Ich wereobtained and_presented in [3]. From the comparison X 1 -_ (2c)of Ich and AI,= the following conclusions result: [L2C2

1. For small inductances L2%1 pH and wide range of capa- Au (0) = u (0) - u (2d)citances C2=4 to 2000 nF, current chopping occurs mainly c c aas a result of fast current decay during At1 and inthis case the role of the current component ihf may be The variables in Eq. (2) are ua - voltage drop at theneglected, Fig. 3a. vacuum arc in the steady state, a - oscillation atten-

uation coefficient, uc(O) and i2(0) - respectivelyvoltage at the capacitance C2 and current flowing

through the inductance L2 at the beginning of the in- and shape of transient in Fig. 3e are made clear. Interval At,, Auck and Aisk - increments of voltage uc order to elucidate the dependence shown in Fig. 5, aand current i2 during k-interval At,, AucC(O)- value of more detailed analysis of phenomena during the intervalvoltage at the capacitance C2 reduced by constant Atj is required. In this analysis the author assumesvalue of arc voltage u. at the beginning of the inter- that after a short time, of the order of 1 ns, fromval At,; 0'k<n,n - number of intervals AtI during un- arc spot decay to its regeneration, the sheath of spacestable burning of arc, Fig. 4. charge appears and moves in the anode direction. Assum-

ing, for a simplification, that charge concentration inthe discharge plasma decreases with the square of thedistance from cathode, nq%1/x22 the voltage drop uat onthe moving sheath may be estimated by means of

h(n-i) Ihf1 hat Aln[1 (x0+b+v.-At )](3)

|hfk where A and xo are constants depending on parameters and___ - - / shape of plasma, b - thickness of sheath, v5 - speed of

HN ~~~~~~~~~~~~~sheathdislocation. Taking into account Eq. (2d) and

'di A di2 =u +ui(+)Ai sk

_c Lg it 2dt2

c at a (4)MI (+A' s(n-1)

is 2 t =at+u0

AISt< <l <A <& Atl At

'\sr<Ais(n-1) sk so sn = Ismx t __

OAIsmx Oa L =4-5,uH | Ismxucmn 20

()A&Ur(n-1)A

Fig. 4: Styled transients of arc current (i5) and ofvoltage (uc) at the apacitace C2 during unstable Fig.5burning of vacuum arc near' the current zero. value ofDpnd eothechopinacuretaeCch thmeanvleA5Uharacteristic values. v1u ftecopn urn e,ma au Im

and maximal current rush AI5m; durPing udnstabl-eThe following cases may be distinguished vacuum arc at the copper contacts.

(1) Intervals At1 of unbalance arc stability occur whenthe values of current i2(°J and voltage Auc?(OJ arewefnpositive. In this case the current amplitude 1hf< will di di u - au - au (0)grow. 2 s at clk c(2) Intervals At1 occur when current i2(0) and voltage dt dt (5)uc<(O) values are negative. In this case the currentamplitude will decrease. The voltage increment Au0k at the capacitance C2 and(3) Intervals At1 occur for i2(0)>O and Auc(0)<0 or current increment Aig =IAiskf are given by the follow-ig(0)<0 and Auc(0)>O. In this case current amplitude ing expressions1hfk: will increase or decrease depending on participa-

C ~~~2Fig. 4f Stylese trapnsents of arccurrent (is)andMofttvoltagenty however, thecaacitanci2(during] unstabre Fii : Deedec ontecpctne. ftemainfurnceing of vacumiacnaearthe currentcy r.vau of tecopncurrentAuk=-1d 6

ampliCtude Ihfks* 2 I

The changes of conditions in the arc spot during itsmotion over the micro nonuniformities of the cathode, arc at.th cpA c-A t_and the related phenomenon of arc stability unbalance i2n2d -Atk - cLduring intervalsAto , are random. The smaller the arccurrent, the greater e theprobabi lityf ose phenomena. At AtThe higher probability of intervals Atl is when simul- 1 r k2 r ktaneous the current ihf is negative and current i2 = L J uAst - J LAuc + Auc(t)]dtpositive. In this way, the relations given in Fig. 6 0 0

CONCLUSIONS

lC| lg2= 022.SF R2t OQ 71 1. For arc current ia<Imin the strong effect of elec-C2 0,22. F R2 0 -Q tric circuit on the form of current transients of the20 arc and on the value of chopping current lch has been

'a>< ismx determined.

15 2. The role of high frequency current component ihincreases in the current chopping mechanism with the

10 /ch /> _growth of the inductance L2 of the limb L2-C2-R2 con-ach/ hIsrl t/ ot>_,nected in parallel to the contacts of the vacuum

5 ______ ^switch. For small inductances L2 of the order of 1

i H, the influence of current ihf on the value of thel cchopping current 'ch is insignificantly small.

2 5 10- 2 5 10-4 2 H 5 3. The accepted hypothesis, which assumes that duringL2 --- fast arc spot decay, of the order of 1 ns, creation

occurs of a sheath of space charge moving in the direc-

Fig. 6: Dependence on the inductance L2 of the mean tion of anode in the time of the order of 1 ps, cor-value of the clhopping cwrrent Ih, mean value Alsmx rectly explains the results of experimental investi-and maximaZ current rush AIsmx during unstable gations.vacuum are at the copper contacts.

REFERENCESThe graphical dependences (3)-(7) are presented inFig. 7. After AtI, the value di2/dt changes to nega- [1] G. Ecker, "Das Chopping - Phanomen," Beitrag a.d.tive, i.e. the current i2 begins to decrease and cur- Plasma-physik, No. 3, p. 67-78, 1974.rent is increases. The relations given in thisFigure enable the estimation of the effect of induct- [2] G. A. Farrall, "Arc Extinction Phenomena inance L2 and capacitance C2 on the current transients Vacuum,," J. of Appl. Phys. Vol. 42, p. 3084-3088,near the current zero. Decrease of capacitance C2 1971.results in faster growth of voltage (uc in Eq. (6))and increasing of A. and at the same time decreasing [3] F. R. Froncek, Overvoltage Generation at Inter-AtI and the current Ai5k. The initial value Auc(O) rupting small Inductive Currents by a Vacuummay have an enormous effect on the value A0 and Aisk. Switch. Diss., Technical University of Wroclaw,Increase of inductance L2 results in decrease of Aisk 1978 (in Polish).(Eq. 7) and indirectly in the chopping current 'o.hThe obtained results of the theoretical analysis are [4] F. R. Froncek, H. Markiewicz, Z. Zalucki, "Strom -in excellent agreement with the experimental results und Spannungsverl-ufe des Vakuumlichtbogens in derpresented in Figsr. 3, 5, and 6. Nahe des Stromnull-durchganges," ETZ-a, Bd 99, H9,

p. 549-553, 1978.

u Aa-ArAc I[5] E.Hantzsche, "The State of the Theory of VaccumArc Cathodes," Beitrag a.d. Plasma-physik, Vol.

Um C It >C" ' 23, No. 1, pp. 77-94, 1983.

j ;3Uat Uc- [6] P. I. Proskurovskij, V. F. Puchkariev, "Reactionof a Vacuum Arc Discharge to a Discontinuity inCurrent," - Zn. Techn. Fiz., Vol. 51, No. 11,p. 2277-2282, 1981.

>7 X ~~~~~~~[7] M. Vano, A.0., "Experimental Study of the CurrentChopping Phenomenon in Vacuum Circuit Breakers,"Electr. Eng. in Japan, Vol. 90, No. 5, p. 73-83,

AUC,(O) N ~~~~~~~~~~~~1970.

u This paper was presented at the International SymposiumO L-r- _ on Discharges and EZectricaZ InsuZation in Vacuum,

-+_- I(+) ~tttlAtf1tt( A t Berlin, Germany DDR, 24-28 September 1984.

.(0) Manuscript was received 2 May 1985.

Fig. 7: Transients of arc voltage ua and uat andvolZtage u0 at the capacritancee 02 during currentdecay interval Ate-.