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2003 Annual ReportConference on Electrical Insulation and Dielectric PhenomenaHeat Sink Effects in Thermal Stabil ity Tests of ZnO Arresters

Z. Zhengl (zheng@power.ele.utoronto.ca) and S. A. Boggsl (steven.boggs@ieee.org)'Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, CanadaElectrical Insulation Research Center, Institute of Materials Science, University of Connecticut, Storrs,CT

Time0-2ms

2 ms- 60 s60 s - 60 002 s

60.002 s - 60.102s60.102s - 70.102 s70.102s - 1870 s

Abstraet: The effect of heat sinks placed between theZnO elements of a surge arrester is evaluated in thecontext of the IEC test for thermal stability.

Action2 ms first power surge

60 s relaxation2 ms second power surge

0.1 s relaxationLO s overvoltage

30 min normal voltage

Introduction

Test Time not Scaled)Figure 1. IEC lype test for stability of ZnO arresters (not toscale). A short impulse of defined volumetric powerdissipation(250 Jlcm ) s applied, then the system s allowedto relax for 1 minute after which a second identical impulse isapplied. The system then relaxes for 100 ms after which anAC overvoltage is applied for 10 s followed by maximumrated operating voltage for 30minutes.

ZnO conductivity as a function of field and temper-ature is shown in Figure 2. At high electric field(current), the conductivity is essentially independentof temperature to about 500 C. At low field, conduc-tion is ohmic, but the conductivity is a very strongfunction of temperature. During the IEC AC overvoltage phase, the peaks of the AC voltage extendslightly into nonlinear conduction, while at maximumrated operating voltage, the entire AC waveform isgenerally in the low field, ohmic region. In both ACphases of the type test, the conductivity and powerdissipation increase rapidly with temperature. As canhe seen from Figure 2, the low field conductivity andpower dissipation increase by roughly an order ofmagnitude for every 100 C increase in temperature.Thus applying an AC overvoltage or maximum ratedoperating voltage to elements at elevated temperaturehas the potential to cause thermal runaway. as thepower dissipation in the elements may exceed theheat loss from the surface of the elements.-

Figure 2. Typical electrical conductivity of ZnO as a functionof temperature and electric field. Under low field conditions,the conductivity is a very strong function of temperature,while under high field conditions, the conductivity is almostindependent of temperature.One method for improving thermal stability is toplace heat sinks between the elements. This has threebeneficial effects, viz., (i) in the short term, such heatsinks absorb heat from the ZnO elements to lowertheir temperature and reduce power dissipation, (ii) inthe longer term, the elements conduct heat from thelarge ZnO heat sink interfaces to the heat sink- gas

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interface, thereby increasing heat transfer from theelements to the surrounding gas, and (iii) since thegreatest reduction in ZnO temperature occurs near theheat sink surface, this decreases the ZnO conductivityin that region compared to the middle of the ZnOelement, which results in the maximum power dis-sipation occurring near the ZnO - heat sink interfacewhere beat can he removed from the ZnO elementmost efficiently. These thermal phenomena occurbecause ZnO has a thermal conductivity of about 20Wlm-K while common heat sink materials such asAI, have thermal conductivities in the range of 250W/m-K. ZnO and AI have similar volumetric heatcapacities, in the range of 2.5E6 J/m3-K. In ourdiscussion below, we will consider ZnO elementswhich are 22 mm thick and about 85 mm in diameter.The heat sinks are of variable thickness and of thesame diameter as the ZnO elements. The heat sinksare often placed between pairs of elements, so. thateach ZnO element is adjacent to only one beat sink,with the other side being another ZnO element.Based on the above thermal conductivity and heatcapacity, the thermal diffusivity of ZnO is about 8E-6mils. Thus heat diffuses 22 mm in ZnO in about 1minute. The thermal diffusivi ty. of AI is about anorder of magnitude greater so that the heat sinkscome to thermal equilibrium within a few seconds forany reasonable thickness. To consider the IEC TypeTest of Figure 1in greater detail, the first short im-pulse causes adiabatic heating of the ZnO elements.During the one minute relaxation before the nextpulse, the ZnO elements and AI beat sinks approachthermal equilibrium. Since we have two ZnO ele-ments for every heat sink and the volumetric heatcapacity of the two materials is similar, the heat sinksshould absorb roughly one third of the heat dissipatedduring the surge, and in the process, the temperaturerise in the ZnO elements should be reduced by aboutone thud relative to the temperature immediatelyafter the impulse. The second impulse causes asecond, similar temperature rise; however, heat in theZnO can diffuse only about 1 mm in the 0.1 s beforethe AC overvoltage is applied. Thus at the time theAC overvoltage is applied, the temperature in theelements is substantially nonuniform with a largetemperature gradient in the ZnO adjacent to the heatsinks. During the AC overvoltage phase, the powerdissipation is substantial and somewhat nonuniformas a result of the axial temperature gradient in theZnO elements. During the 10 s of the overvoltagephase, the elements do not come to thermal equilih-rium internally or with the AI heat sinks as a result ofthe short time compared to the diffusion time acrossthe elements and the nonuniform power dissipationwithin the elements.

The power dissipation is reduced substantially duringthe maximum rated AC voltage phase, and theelements come close to equilibrium, both axially andradially, with some temperature gradient near theinterface with the beat sink as a result of its largethermal conductivity. However, a minute into themaximum rated AC excitation phase, the axial andradial temperature gradients in the ZnO are only afew degrees unless thermal runaway is imminent.Basically, thermal stability is determined by whetherthe temperature in the ZnO elements after they cometo near thermal equilibrium increases or decreaseswith time. If the elements are in a confined gasspace, as is often the case, the temperature of the gassurrounding the elements can increase over time,which can reduce heat transfer from the elements tothe gas. This can lead to late thermal runaway, asituation where the ZnO elements start to coolslightly during the normal AC excitation phase afterwhich the ZnO element temperature starts to increaseas a result of the increased gas temperature.Obviously, this occurs only at the boundary betweenstable and unstable.Modeling

are

Figure 3 shows the ZnO arrester geometry used in oursimulations, which includes the ZnO elements, heatsinks, insulating gas, and the enclosure. The ZnOelements are cooled through convection from the stacksurface to the gas and radiation from the stack surfaceto the enclosure. To simplify the simulations, weassume a fixed uniform beat t ransfer coefficient HTC)from the stack surface to the gas. The other effects,such as the radiation heat and the gas temperature rise,sidered in this simulation by adjusting the= s? HTC. This enables us to focus on heat

Figure3. ZnOarrester modelemployed inthe presentcomputanons.

sink effects. In other simulations, wehave included both radiation andambient gas temperature.The entire IEC type test is simulatedusing transient nonlinear finiteelement analysis with coupled thermaland electric fields [I]. In the compu-tation, we apply voltage to the ZnOstack to calculate the electric fielddistribution, use the field distributionand known temperature to calculatethe electric conductivity, compute thepower dissipation from the conduc-tivity and electric field, and then usetransient nonlinear finite elementanalysis to compute a new temperaturedistribution from the power dissipa-tion and the known (nonlinear) tber-mal properties.

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AC Overvoltage phaseDuring the overvoltage phase, the ZnO elements areat substantially elevated temperature (around 200 Cat the end of the second impulse) and are subjected tna large electric field which pushes the ZnO slightlyinto the region of nonlinear conduction, as seen inFigure 4. The increase in power density withtemperature is exponential under these conditions.~ ..... 1.0

4 0 5, . \ -a.,o

0 0 8-f o . 5 4.5n o

-1IJ 4.00 5 10 15 20Time (ms)~Figure 4. Power density in a ZnO element at 200 'C during

the AC overvoltage phase. The solid line is applied voltage.The dashed line is the power density. The dotted line is thecumulative power dissipation over one cycle. Most of thepower dissipation is near the peak voltage as a result ofnonlinear conduction in the ZnO.During the overvoltage phase, radial heat transfer ismuch less than axial transfer to the heat sinks, asimmediately after the second impulse and 0.1 srelaxation (without applied voltage), the ZnO is muchhotter than the heat sinks. The simulation resultsindicate that the radial temperature gradient is negli-gible, so we focus on the axial temperature gradient(Figure 5) and corresponding power dissipationgradient (Figure 6). The on-axis uniform axialtemperature and power dissipation as a function oftime for no heat sink case are plotted in Figure 7.Figure 5 shows the on-axis axial temperature gradientin ZnO elements as a function of the distance fromthe heat sink surface at various times during theovervoltage phase and with a HTC of 10W/m*-K andthe heat sink thickness the same as that of ZnOelement 22 mm). This case does not result inthermal runaway. The maximum temperature at thecenter of the ZnO increases less than 1 C andeventually decreases. Through comparison with theon-axis temperature in Figure 7, the case for the sameHTC but no heat sinks, we see that the heat sinks areeffective in reducing ZnO temperature.Figure 6 shows the axial distribution of the cycleaveraged power density with heat sinks during theovervoltage phase. The heat sinks have caused asubstantial reduction in the temperature after thesecond impulse which results much lower power dis-

la .] , -7I . . . . . . . . . . . . . . . . . , . . - .a 5 10 . 5 20Axial Ois lq ce fromHeat SinkSurface(mm)

Figure 5 . The on-axis axial temperamre gradient withiin a ZnOelement as the function of the distance from the heat sink atvarious times during the overvoltage phase. The HTC is 10Wlm2-K, and the heat sink thickness is the same as that ofZnO element.~. . ..-E 7e+Ss.zi; 7e+5:

wstm

P w 5map - 5a 1 .

Axial Distance from Heal Sink Surface(mm)5 10 2

Figure 6. On-axis axial power density within a ZnO elementas the function of the distance from the heat sink surface forvarious times during the overvoltage phase. The HTC is I OW/mz-K, and the heat sink thickness is the same as that ofZnO element.

T i e 1Figure 7.h-axis uniform temperature and power density withno beat sink as a function of time during the overvoltagephase. The gray line is the power density and the black line isthe temperature. The HTC is 10W/m2-K.

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Table 3. Surface temperature 200 s into the IEC ype test withvarious heat sink thickness (1 pu = the thichess of ZnOelement, one heat sink is placed between every two ZnOelements) and heat tmnsfer coefficient (HTC), h (W/m-K).= 0 means no heat sinks employed Numben marked withare for thermally unstable cases.

The final temperature distribution can be seen fromFigure 8 (without heat sink) and 9 (with heat sink).Without heat sink, the heat flows only radially, andthe increased temperature relative to the case for evena thin heat sink results in increased power dissi-pation, especially on axis of the ZnO element whereit is most difficult to remove heat through radial heattransfer. With the heat sink, axial heat flow is greaterthan radial heat flow so that not only the overalltemperature is reduced hut much less power is

Variables ValueHTC: h (W/m-K) IJO 100Thickness: f heatsinWf ZnO 0,0.1,0.5, 1

dissipated near the center of the element as a result ofthe axial temperature gradient.The average surface and center temperatures duringthe tests for various heat sink thicknesses are plotted inFigures 10-12. Without a heat sink, the HTC must heincreased to 100 WIm2-K to maintain thermal stability(Figure 10). When a heat sink with a thickness of 0.1

times of that of ZnO element is disposed between pairsof ZnO elements, a 10 W1m-K HTC will maintainthermal stability (Figure 11). When the heat sink is asthick as the ZnO element, a 1 Wlm-K HTC maintainsthermal stability (Figure 12).s q ~ . . .~ - ,. -1- . 3

Figure 9. Temperature dishibution within a ZnO element at200 s into the IEC type test with a beat sink of thickness 0.1pu and a HTC of 10 W/m*-K. pu thickness is the thicknessof the ZnO element. The heat flows axially towards the heatsink and the system remains stable.Placement of Heat SinksIn previous analyses, one heat sink is place betweenevery two ZnO. Obviously one heat sink betweenevery ZnO element is preferable hut involves morecomplexi ty. We have computed the cases of a heatsink between pairs of ZnO elements with heat sink

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thickness equal to the ZnO element thickness asopposed to a half thickness heat sink between everyZOO elements with a heat transfer coefficient of 1W h - K . As can be seen from Figure 13, the systemwith heat sinks between ZnO elements is stable whilethat with heat sinks between pairs of element is not.This results from the much more rapid cooling of theZnO elements by the heat sinks during thevoltage phase in the former case.~

over

U 1W 150 200TestTime(s)Figure IO Surface and center temperature of a ZnO elementduring the lEC test with no heat sinks as a function of HTC.The black lines are for surface temperature and the gray linesare for center temperature. The system is thermally unstablefor an HTC of 1 and 10W/m2-K, hut stable for 100 W/m'-K.

22

1803 180i 120

c 1w

e 140

806

0 5 W s 200i i m e (s)

Figure 11. Surface and center temperatures of a ZnO elementduring the IEC typ test with heat sinks of 0.1 pu thickness.pu equals the ZnO element thickness. The black lines are forsurface temperature and the gray lines are for central tem-perature. The system is thermally unstable at W/m'-K, butstable at IO and 100 W/m2-K.ConclusionHeat sinks play an important role in reducing powerdissipation and improving heat transfer from ZnOelements. Heat sinks cause a very rapid reduction inZnO temperature after any sudden power dissipationin the ZnO and thereby lower subsequent powerdissipation at normal operating voltage. Heat sinks

ais0 improve thermal transfer from the ZnO- to thesurrounding gas as a result of their large radialthermal conductivity relative to the ZnO. Finally,heat sinks cause an axial temperature gradient in theZnO element which reduces power dissipation nearthe center of the ZnO element by reducing the currentdensity on axis of the element. Thus heat sinks havea beneficial effect on arrester stability.

nw.05m %s f w h f x- * o . s s ~ IOW *

6 --- C e i W . l p h W r , l W ~ K0 50 100 150 200

. . . Time(s)Figure 12. Surface and center temperamre of ZnO elementduring the IEC ype test with various HTC and beat sinks ofvarious thickness. 1 pu thickness equals the ZnO elementthiclmess. The system is thermally unstable for 0.5 puthickness and 1 W/m2-KHTC, but stable for 0.5 pu thicknesswith I O W/mz-K HTC and 1 pu thickness with 1 W/mz-KHTC.

0 5 lob 150 200Time(s)

Figure 13. Surface and center temperature of a ZnO elementdu ri y the IEC type test with 0.5 pu heat sink thickness and 1W/m -K HTC. I pu thickness equals the ZnO elementthickness. The black lines are for surface temperature and thegray lines are for cennal temperature. The solid lines are forthe case when one heat sink is placed between every two ZnOelements. The dotted lines are for the case when half beat sinkis placed between every ZnO elements.References1. Zheng, Zhong and Steven Boggs. Efficient Solution ofTransient Nonlinear Field Problems . Annual Report ofthe IEEE Conference on Electrical Insulation andDielectric Phenomena, October 2002, Cancun, QuintanaRoo, Mexico. pp. 130-133

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