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Vacuum 80 (2006) 599–603

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Effects of nozzle shape on the interruption performance of thermalpuffer-type gas circuit breakers

Jong-Chul Lee, Youn J. Kim�

School of Mechanical Engineering, Sungkyunkwan University, 300 Chonchon-Dong, Suwon 440-746, Korea

Received 28 June 2005; received in revised form 30 September 2005

Abstract

During the last decade the advanced interruption techniques, which use the arc energy itself to increase the pressure inside a chamber

by PTFE nozzle ablation, have displaced the puffer circuit breakers due to reduced driving forces and better maintainability. In this

paper, we have investigated thermal flow characteristics inside a thermal puffer-type gas circuit breaker (GCB) by solving the

Navier–Stokes equations coupled with Maxwell’s equations for considering all instability effects such as turbulence and Lorentz forces

by transient arc plasmas. These relative inexpensive computer simulations might help the engineer researching and designing new

advanced interrupters in order to downscale and uprate high-voltage gas-insulated switchgear (GIS) integral.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Ablation; Arc modelling; High voltage; PTFE; Thermal puffer; Turbulence

1. Introduction

The advanced interruption techniques for decreasing themechanism in order to create enough pressure and flow forinterruption have displaced the mechanical compressionprinciple, so called puffer type, due to decreasing mechan-ical stresses such as downsizing, longer lifetime, higherreliability, and so on. The types developed already aredifferent from the manufacturers because of the protectionfor their technologies. However, the basic principle ofinterruption with arc energy itself in order to increase thepressure and to create the efficient gas flow has nodifference between them. All of them use the ablation ofPTFE nozzle and the partial mechanical compression. Theimportant for development nowadays is how efficiently andhow economically they find the interruption limit of thisnew advanced interrupter [1–4].

The key to these solutions might be the usage ofnumerical techniques early in the design cycle due toincreasing of computer power and developing of numericalschemes. So many researches for numerical and computa-

ee front matter r 2005 Elsevier Ltd. All rights reserved.

cuum.2005.10.004

ing author. Tel.: +8231 290 7448; fax: +82 31 290 5849.

ess: yjkim@skku.edu (Y.J. Kim).

tional testing have been proceeding to develop the programand to find the effective scheme for arc modelling since1990s.Computational fluid dynamics (CFD) has been extended

to the various research fields as well as to the traditionalheat and fluid mechanics due to increase in computerpower and development of numerical schemes. The thermalflow driven by arc plasma in a number of arc devices, suchas circuit breakers, arc heaters, plasma torches, arc weldingapparatus, and so on, can be calculated by coupling theresults of CFD with those of electromagnetic field in onecode simultaneously. Especially it is very important todevelop the method of simulations numerically due toexpensive and dangerous set-ups for real testings of circuitbreakers [5,6]. Moreover, it might be unique to improve theperformance and to develop the new designs of innovativeproducts. Numerical tools are increasingly used in thephysical design of gas circuit breakers (GCBs) to qualifyand improve the design and to reduce time to market [7,8].During the last decade the advanced interruption

techniques, which use the arc energy itself to increase thepressure inside a chamber and to blow-off arc plasmasbetween the contacts by PTFE nozzle ablation, havedisplaced the puffer circuit breakers, most widely used

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Nomenclature

AA acute angleB magnetic flux density (A/m2)C1 turbulent parameter ( ¼ 0.2)E electric filed (V/m)EV expansion volumeF empirical factor ( ¼ 0.9)ha total energy required to break a PTFE chain (J/

kg)J current density (A/m2)OA obtuse anglep pressure (Pa)RA right angleS source termSct turbulent Schmidt number ( ¼ 1.0)V velocity (m/s)

Greek Letter

a reabsorption factorG diffusion coefficientd thermal radius (m)k thermal conductivity (W/mK)r density (kg/m3)m viscosity (N s/m2)s electrical conductivity (S)f dependent variablesj electrical potential (V)

Subscripts

l laminar partm mixturet turbulent part

J.-C. Lee, Y.J. Kim / Vacuum 80 (2006) 599–603600

interrupter, due to reduced driving forces and bettermaintainability. However, the pressurizing mechanism ofthis new principle so called thermal puffer is not verifiedcompletely yet and also turbulent effects are moreimportant for this new type circuit breaker but there arefew related works on the phenomena.

In this study, we investigated the behaviour of thermalgas flow driven by arc plasma with three different shapenozzles in a thermal puffer-type circuit breaker. Becausethe GCBs become more sophisticated and design marginstightened, defining the arc plasma management strategyearly in the design cycle is vital to ensure a cost-effectivedesign for the level of thermal recovery. In order to achievethis objective, the characteristics of thermal flow driven byarc plasma during high current region (HCP) on a thermalpuffer-type circuit breaker are analyzed and compared withthree different nozzle shapes.

2. Numerical method

2.1. Governing equations

Since the pressure of the arc plasma between the contactsin high-voltage GCBs is most of all above about 5 bar, thisthermal flow is assumed to be under LTE and LCE and thevelocity of PTFE vapour ablated can be assumed to travelwith the same velocity. The governing equations forcalculation of this problem consist of two parts; one is onthermal flow field and the other, electromagnetic field.Firstly, Navier–Stokes equation for flow field can bewritten as

qqtðrfÞ þ rðr~Vf� Gf rfÞ ¼ Sf. (1)

In order to calculate the electromagnetic field simulta-neously, the electro-potential can be written as

rðsrjÞ ¼ 0. (2)

The related information for the governing equations islisted in Table 1 and the detailed description might be seenat Refs. [6–9].

2.2. Turbulent model

Turbulent model plays an important role in determiningthe quality of calculation on arc plasma in SF6 circuitbreakers. Since direct simulation of turbulence by solvingNavier–Stokes equations is very difficult with availablecomputing power, the statistical behaviour of a turbulentflow is conventionally described by the time-averagedvalues of the flow and thermodynamic properties. There-fore it is necessary to introduce additional relations torelate Reynolds stress in the momentum equation and theenergy equation, which is the task of all turbulence models.In the previous works of turbulent modelling in arc

plasma by Professor Fang’s group in Liverpool University[9–11], the popular two-equation k2e model (KE) has noadvantage over Prandtl mixing length model (PMLM) inthat one of the five constants in the former needs to beadjusted in order to achieve agreement between thepredicted and measured radial temperature profiles in asupersonic nozzle. Namely, the Prandtl mixing lengthturbulence model to arcs in supersonic flow has hadconsiderable success in predicting the thermal interruptioncapability of a gas-blast circuit breaker, although theturbulence parameters need to be adjusted for a givengeometry of the nozzle.However, we should adjust the turbulent model on the

thermal puffer-type CBs due to having other pressurizingmechanism by arc heat energy and PTFE mass transferduring high current region as well as at low currents. In thisstudy two turbulent models are used to calculate thevelocity scale and the length scale; one is PMLM as zero-equation model and the other KE model as two-equationmodel.

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Table 1

Definitions of f, Gf and Sf for governing equations

Equations f Gf Sf

Mass 1 0 0

Axial momentum w mþ mt qp=qzþ ðJ � BÞz þ viscous terms

Radial momentum v mþ mt qp=qrþ ðJ � BÞr þ viscous terms

Enthalpy h K þ kt=cp dp=dtþ sE2 � qþ viscous dissipation

Turbulent kinetic k ðmþ mtÞ=sk rðPk � eÞTurbulent dissipation e ðmþ mtÞ=se re=kðC1ePk � C2eeÞElectrostatic potential j s 0

PTFE concentration cm rðDl þDtÞ 0

J.-C. Lee, Y.J. Kim / Vacuum 80 (2006) 599–603 601

In case of PMLM, the turbulent viscosity is calculated bylength and velocity scales directly related to the mean flow.We assume that the length scale is proportional to the localthermal radius which characterizes the boundary of thehigh-velocity core and the velocity scale is constructedfrom the local velocity gradient and the length scale asfollows

d ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiZ R5k

0

1�rr5k

� �2rdr

s, (3)

mt ¼ rðc1dÞ2 qw

qrþ

qv

qz

��������. (4)

More sophisticated turbulence models are based onadditional equations which calculate the velocity andlength scales for the turbulence. In case of KE, time-averaged turbulent kinetic energy and its dissipation ratedetermined are shown in Table 1 and the kinematicviscosity is related to k and e by

nt ¼ Cmk2=e. (5)

Turbulent Prandtl number for thermal conduction isassumed unity and the constants in governing equationsare usually set at

Cm ¼ 0:09; C1e ¼ 1:44; C2e ¼ 1:92,

sk ¼ 1:0; se ¼ 1:3,

which appear to have a wide range of applicability whenapplied to shear flow.

2.3. Radiation model

Energy transport by radiation in arc plasma systems isan extremely complex and important phenomena. And theradiation flux at a particular position is not only a functionof the local gas properties but also dependent on thetemperature and pressure field in the whole domain ofinterest. Liebermann and Lowke [12] calculated the netemission coefficient of SF6 at the centre of an isothermaland cylindrical arc column under uniform pressure. Andmost of researchers use this data for computational costexcept in some cases specialized in circuit breakers [13].

In the source term of the enthalpy equation in Table 1, q

represents the net radiation loss per unit volume and time(radiation flux vector). It has been successfully applied to SF6

nozzle arcs [10] and to the modelling of a thermal expansionCB. This model is one-dimensional and assumes anaxisymmetric arc in which radiation transport occurs onlyin the radial direction. Because we focus on the turbulentmodel in this study, the semi-empirical radiation model(NEC) is used for calculation of the radiation flux vector.

2.4. Ablation model

PTFE is almost exclusively used as the nozzle material inmodern high-voltage circuit breaker for its high mechanicalstrength and good insulation property. Moreover, theimportance of this PTFE property increases more andmore because it endures to high-temperature arcs betweenthe contacts in a thermal puffer-type circuit breaker. Sincethe physical process of the ablation of PTFE by radiation ispoorly understood, we assume a constant vapour tempera-ture of 3400K [6]. The total energy which is required tobreak the chain of the PTFE molecules and to raise thetemperature of the PTFE vapour from room temperatureto 3400K is 1.19� 107 J/kg. And the rate of ablation isdetermined by

_m ¼ ðFQÞ=ha. (6)

The composition of the SF6 and PTFE mixture isobtained by minimizing Gibbs free energy under localthermodynamic equilibrium and local chemical equilibriumconditions. The transport properties are calculated basedon the Chapman–Enskog approximation. Diffusion due toconcentration gradient is dominant in this work and theturbulent diffusivity is related to the turbulent viscosity by

Dt ¼ mt=rSct. (7)

From Chervy et al. [14] the net emission coefficient ofSF6–CF4 mixture, SF6–C2F6 mixture, pure SF6, pure C2F6,and pure CF4 are very close to each other. The net emissioncoefficient of the SF6–PTFE vapour is approximately thatof pure SF6 in the present work, since the temperature ofthe mixture in the arc core is higher than 18,000K duringmost of arcing period.

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Fig. 2. Distributions of the mass concentration of PTFE vapour near

peak current, 74 kA.C1 means the mass concentration of PTFE vapour

and the value can have 0 to 1.

Fig. 3. The evolution of the overpressure in the expansion volume of the

breaker according to the nozzle shapes.

J.-C. Lee, Y.J. Kim / Vacuum 80 (2006) 599–603602

3. Results and discussion

A schematic cross-section with the main components ofa thermal puffer CB, which is the basic design fordevelopment of new interrupter, is shown in Fig. 1. Dueto the long-range nature of electric field, the computationaldomain for electromagnetic field is larger than that for theflow field. In order to investigate the effects of nozzleshapes, the conditions about input current, arcing time andmoving velocity are fixed as the general values for this typeof interrupters. We take that input current is 50 kA(rms),arcing time 13.1ms and moving velocity 8m/s. The point 1shown in Fig. 1 should be used to compare the over-pressure in the expansion volume (EV) according to threedifferent shape nozzles, which are different from the angle,y, such as right angle (RA), acute angle (AA) and obtuseangle (OA).

Distributions of the mass concentration of PTFE vapournear peak current, 74 kA, are shown in Fig. 2. Even smallchanges for the geometry such as the shape of a nozzleshould cause a big difference for the characteristics ofthermal flow and interruption performance. The investiga-tion for these patterns should be indispensable for thedesign and the development of circuit breakers early in thedesign cycle with relatively inexpensive computer simula-tions due to increasing the reliability and decrease in thecost and time.

The evolution of the overpressure, which is one of themost important factors for design of interrupters since itgenerates the blowing-off force of the arc plasma, in theexpansion volume on three different nozzle shapes duringhigh-current region is shown in Fig. 3. Regarding the lossof pressure in the nozzle, the RA nozzle leads to other lessloss of pressure. Therefore the blast will be so stronger andso the breaking process will be improved.

In order to extinguish the arc plasma between thecontacts at the current zero effectively the temperature of

Fig. 1. Schematic diagram of the thermal puffer-type circuit breaker

under investigation.

blowing-off gas toward the contacts should be low as muchas possible. The remaining hot gas distribution in theexpansion volume should be a good information on theperformance of interruption, because providing gas ofrelatively low temperature is more effective for the actualarc interruption. It is well known that the cold stream canachieve only when the hot gas entrained during high-current period mixes well with the gas in the expansionvolume. It is one of the most important factors for designof the thermal puffer-type circuit breakers. The distribu-tions of the hot gas at the end of calculation, near currentzero, when the instantaneous current is 2 kA, are shown inFig. 4. The gas temperature toward the contacts is aboutless than 500K in case of RA (Fig. 4(a)), but in cases of AAand OA the temperatures are about more than 1000K andthose have distinctive ineffective regions which are notmixed with hot gas. It is clear that the relative hot gastoward the contacts has less probability to extinguish thearc plasma at current zero. Through this work, weconfirmed the effects of the geometry for interruptionprobability with our own CFD schemes and continue to

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Fig. 4. Distributions of the hot gas at the end of calculation, near current

zero, when the instantaneous current is 2 kA. Arrows indicate the velocity

vectors. Only a limited region of the circuit breaker is shown.

J.-C. Lee, Y.J. Kim / Vacuum 80 (2006) 599–603 603

further study development of turbulent model, radiationmodel, ablation model, and so on.

4. Conclusion

In order to investigate the thermal flow characteristicsand the interruption performance using numerical methodsin a thermal puffer-type circuit breaker, we have calculatedthe behaviour of thermal flow driven by arc plasma withthree different nozzle shapes in detail.

Regarding the loss of pressure in the nozzle, the RAnozzle leads to other lower loss of pressure. Therefore, theblast will be so stronger and so the breaking process will beimproved. And the gas temperature toward the contacts isless about 500K in case of RA, but in cases of AA and OAthe temperatures are more about 1000K and those havedistinctive ineffective regions which are not mixed with hotgas. It is clear that the relative hot gas toward the contacts

has less probability to extinguish the arc plasma at thecurrent zero.Through this work, we confirmed the effects of the

geometry for interruption probability with our own CFDschemes and continue to further study for developmentof turbulent model, radiation model, ablation model, andso on.

Acknowledgement

This work is supported by BK21 (Brain Korea 21) andthe authors thank Professor MTC Fang, Drs. JD Yan andCM Dixon of University of Liverpool, England, forproviding the computer simulation tool and the technicalsupports developed during the co-work.

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