J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 471473

Study of Post-Arc Plasma Behavior
in an AG Circuit Breaker
BORGHEI Majid, MAHDAVI Javadr, HOJABRI Alireza2 and GHORANNEVISS Mahmood
Plasma Phlsics Research Center, Science & Research Campus, Islamic Azad University, Tehran 14665-678, IranIElectrical Engineering Department, Sharif University, Tehran, Iran"Physics Department, Islamic Azad University, Karaj, Iran(Received: 11 December 2001 | Accepted: 18 June 2002)

Abstract
In the search to obtain a better understanding of switching arc phenomena in the current intemrption,our attention has been focused on the study of arc plasma behavior in the alternative current circuit
breaker. In this paper a comparison between intemrption capability of sulfur hexafluoride (SF6) andnitrogen (Nz) ballast arcs are made and we present physical aspects and characteristics of arc plasma
specially plateau variation of the post-arc current near the intemrption limit that have been investigated inexperimental measurements with a gas mixture of sulfur hexafluoride (SF6) and nitrogen (N2).

Keywords:
circuit breaker, post-arc current, sulfur hexafluoride and nitrogen, transient recovery voltage, rise ofrecovery voltage

1. Introduction

reheating of the gas to a well-conducting arc column,
which is termed "thermal reignition" [3]. Measuring thecurrent form close to zeto is a very helpful diagnosis

When interrupting an alternating current, anelectrical arc is initiated between the contacts of aand in this paper we present results of arc behavior
according to use of a particular device that has beendeveloped for this reason before [4].

breaker. The different basic limits exist which determine
the breaking capacity of a circuit-breaker that they have

many industrial applications [].

The study of arc in a circuit- breaker enables us
better understanding of the main parameters which
successful breaking depends on. If arc cooling to be
sufficiently enough, the electrical conductivity in inter-

2. Device Principal and Experimentation
As a requirement, a device that its mechanicalprinciple is based on Eddy current effect and insert ahigh value shunt in series with the circuit-breaker has

electrode space will become so slight that the current
will tend towards zero, even when the applied voltage isconsiderable [2].

been developed and was used for measuring current
according a new method. The shunt does not disturb theImmediately after current zero, the transient
recovery voltage (TRV) drives a post-arc current

circuit, because its insertion is just in the vicinity of zerocurrent for a short period at the time chosen formeasurement (when the current approaches zero). This

through the renrains of the decaying arc plasma resulting
in a heating effect. The energy balance between thisheating and cooling effects either results in a: decay ofthe plasma and an intemrption of the current, or in aduration is adjustable, can varies and be controlled by avery accurate electronic unit during the breaking period.

Corresponding author's e-mail: majid_borghei@yahoo.com
471@2o02by The Japan Society of Plasma
Science and Nuclear Fusion Research

Borghei M. et al., Study of Post-Arc Plasma Behavior in an AC Circuit Breaker

In Fig. 2 recorded current and voltage forThis unit operation is linked with zero cunent.

Immediately after zero is reached, the shunt isshorted again and will be out of the circuit and in the

successful current intemrption in SF6 is shown. We cansee that the both voltage and current reach zerosimultaneously, and the circuit breaker performs thecurrent interruption without any post-arc current. The
test condition was to keep the opening phase angle thesame in two cases. The reason for different intemrption
cases that were observed in Fig. 1 and Fig. 2 is due todifferent characteristics of these two gases. Moleculardissociation temperature is about - 7500 K for N2 and 2500 K for SF6 and as long as an electric current goes
through the plasma, the radial temperature profile
peresents a plateau value around the dissociation
temperature, dividing arc into a hot core and peripheral
region having low thermal and electrical conductivities.
When the current reaches zero, the temperature
profile become transformed as follows: rapid cooling of
the central zone as long as the temperature is greaterthan dissociation temperature, followed by much slower
cooling [7]. Relatively low dissociation temperature inSF6 permits gas to conserve its time constant for a longtime until complete disappearing. We can evaluate arc

event of a failure to break, the full current wave will not

flow through it [5]. The device also has been tested on arotating arc gas circuit-breaker.
In order to study the behavior of a gas mixture of
SF6 and N2 as an intemrpting gas a series of experiment
was carried out using a rotating arc circuit breaked, and thepeak voltage and current were 4 kV/l 1 kA, respectively.
Measuring of current by high value shunt in series withthe power circuit of the above characteristics was done
and typical recorded results are shown in Figs. 1-3 [6].
For comparison results ofpure SF6 and N2is added.

3. Physical Aspects and Discussion
SF6 and N2 are the two most commonly usedworking gas ballast arcs in high voltage circuit breaker.
Since experiments in this field are difficult and costly
matter, usually mixture of SF6 and N2 is used. Successful current intemrption in N2 is illustrated in Fig. 1.
The post-arc current with its amplitude (> 4 Amps.) isclearly apparent and finally reaches to zero (after - 15time constant by the below equation:psec).

f=_1.dcrGdt

that t is arc time constant and G is electrical conductance. In Fig. 1 post-arc current with relatively large
amplitude (more than 4 Amps) is due to large arc time
constant in N2 and because of good thermal conductivityof gas (for high temperature equivalently some Amps)
reaches zero. But successful intemrption in SF6 dose notpermit measuring of a post-arc current, because of very
low amplitude (low temperatures). Threrfore in amoment gas pass through from conductivity to nonconductivity. In case of SF6 existence of a very low arc
time constant in the vicinity of current zero can justify
instant zero reaching of current.
Figure 3 shows a random behavior of gas mixtureof SF6 and N2l7o75 and Vo25l that it may be the same aspure SF6 Gig. 3(a)) or causes successful interruption
with an important post-arc current (Fig. 3(b)) or failureof intemrption (Fig. 3(c)). The current curve in Fig. 3(b)shows a plateau variation that this behavior is notpredicted by semi-empirical conductance rhethods. It
must be noted that the post-arc current is due torecovery voltage applied to a medium having a residual

0510152025303s40
Time (ps)Fig. 1 Interruption in Nr. P= 4 bars

\.__>

(1)

Current(A)conductance (the decaying plasma). The plateau in the510152025303s40

post-arc current is related to closing rate of rise ofrecovery voltage (RRRV) to its maximum value (limitTime (Us)

Fig. 2 lnterruption in SF.. P=2bars.
472

Borghei M. et al., Study of Post-Arc Plasma Behavior in an AC Circuit Breakercooling mechanism" in order to understand the observed

interruption behavior of SF6 and N2 gas in the ourexperimental works.

4. Conclusion
In this study recorded post-arc cunent measurement
t5 20

results have been illustrated and described. The
amplitude and the duration of the post-zero current isgreater for N2 and gas mixture of SF6 and N, than for

25

Time (ps)pure SF6. This is due to differences in the cooling ratesof this two gases mainly due to turbulence. Also aplateau region in the case of gas mixture of SF6 and N20was recorded. More longer plateau is equal to moreprobability ofreignition or in otler words such post-arc
current inject an input energy that exceeds the energy
loss by cooling mechanism. There is a competitionI
2t5 20 25
Time (|ls)

betweenjoule heating due to conduction, convection andturbulence.
Crmnt(A)

5. Acknowledgment
The first author is indebted to Dr. Ali Zolfaghari0(Schneider Electric Company-USA) and wish to expressI

his gratitude for fruitful discussions, comments and0510152025303540

other great helps from the beginning to final step of thispaper. Corresponding author also would like to express
sincere thanks to his wife (Mrs. Arezoo Bazrafshan) forTim€ (Us)

Fig. 3 lnterruption in a mixture of SF. and N, gas (%75l"/"25l.. lal Interruption without post-arc current, (b)Interruption with post-arc current and plateau

her patience and assistance in preparing the final

variation. (c) Reignition. P = 2 bars N2 and P = 1
bars SFu , Circuit: 4 kV/1 1 kA.

manuscript.

References[1] P. Chevrier et al., J. Phys. D 30, 1346 (1997).
[2] P. Chevrier et al., J. Phys. D 26,949 (1993).
t3l E. Shade et al., IEEE Trans. on PES 10, l4l

RRRV). From Fig. 3(b) it also can be seen that the postarc cunent lasts for several tens of microseconds (- 20psec) with a peak intensity of greater than 2 Amps.
Where as in pure SF6 this duration has been observed of
the order of few microseconds [8] and in pure N2 also of(1982\.

[4] J. Mahdavi et al., Rev. Gen. Elect. 4, 238 (1984)[5] J. Mahdavi et al.,IEE Proceedings 132,285 (1985)[6] J. Mahdavi et al., Rev. Gen. Elect. 5, 2l (1986)[7] A. Gleizes et al.,IEEE Trans. On PS 1, 16 (1991)

the order several tens of microseconds (- 15 psec Fig.

l).
These disagreements in behavior is due toand references therein.

differences in the cooling rates of the two gas, mainly
due to turbulence. Turbulent mixing is a "necessarv[8] W. Hermann et al.,IEEE Trans. On PAS 5 , 1546
(1997\.

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