Metal oxide varistors are widely used in many power electronics circuits to protect against transient over voltages. Certain applications are very demanding on the energy handling capability of the varistors. This paper gives an overview of the failure modes of ZnO varistors and investigates their characteristics when subjected to repetitive current pulses. It describes the puncture failure mode caused by melting of a region in the varistor of local current concentration. Experimental tests are performed to evaluate the puncture energy using an infrared imaging camera. A relationship between the energy absorption and the varistor maximum surface temperature is obtained. It is shown that the destructive energy depends strongly on the uniformity of the varistor; the more uniform, the higher the energy handling capability. The paper also presents the results of nondestructive tests using a scanning acoustic microscope to evaluate the uniformity of the varistor

Ahmed, M. M. R.

Penlington, Roger

Putrus, G.A.

Ran, L.

English

Northumbria Research Link

Northumbria Research Link
Citation: Ahmed, M. M. R., Putrus, G.A., Ran, L. and Penlington, Roger (2001) Measuring the energy 
handling  capability  of  metal  oxide  varistors.  In:  16th  International  Conference  and  Exhibition  on 
Electricity Distribution (CIRED 2001), 18th - 21st June 2001, Amsterdam, Netherlands. 
URL: https://doi.org/10.1049%2Fcp%3A20010705 <https://doi.org/10.1049%2Fcp%3A20010705>
This version was downloaded from Northumbria Research Link: http://nrl.northumbria.ac.uk/42591/
Northumbria University has developed Northumbria Research Link (NRL) to enable users to access 
the University’s research output. Copyright © and moral rights for items on NRL are retained by the 
individual author(s) and/or other copyright owners.  Single copies of full items can be reproduced, 
displayed or performed, and given to third parties in any format or medium for personal research or 
study, educational, or not-for-profit purposes without prior permission or charge, provided the authors, 
title and full bibliographic details are given, as well as a hyperlink and/or URL to the original metadata 
page. The content must not be changed in any way. Full items must not be sold commercially in any  
format or medium without formal permission of the copyright holder.  The full policy is available online: 
http://nrl.northumbria.ac.uk/pol  i cies.html  
This  document  may differ  from the  final,  published version of  the research  and has been made 
available online in accordance with publisher policies. To read and/or cite from the published version 
of the research, please visit the publisher’s website (a subscription may be required.)
                        
MEASURING THE ENERGY HANDLING CAPABILITY OF METAL OXIDE VARISTORS
M.M.R. Ahmed           G.A. Putrus            L. Ran R. Penlington
University of Northumbria, at Newcastle, UK
Abstract
Metal oxide varistors are widely used in many power
electronics circuits to protect against transient over
voltages. Certain applications are very demanding on
the energy handling capability of the varistors. This
paper gives an overview of the failure modes of ZnO
varistors and investigates their characteristics when
subjected to repetitive current pulses. It  describes the
puncture failure mode caused by melting of a region in
the varistor of local current concentration. Experimental
tests are performed to evaluate the puncture energy
using an infrared imaging camera. A relationship
between the energy absorption and the varistor
maximum surface temperature is obtained. It is shown
that the destructive energy depends strongly on the
uniformity of the varistor; the more uniform, the higher
the energy handling capability. The paper also presents
the results of nondestructive tests using a scanning
acoustic microscope to evaluate the uniformity of the
varistor.
Keywords: varistors, infrared imaging camera, energy
INTRODUCTION
Metal oxide varistors (MOV) are highly nonohmic
ceramic devices with current-voltage characteristics
similar to those of back-to-back Zener diodes but with
higher energy handling capability [1]. The voltage-
current characteristics of the MOV can be divided into
three regions, as shown in Figure 1. At low voltages, in
the region known as the pre-switched region, only
leakage current occurs. With intermediate voltages in
the switched region, the characteristics are generally
described by the equation:
I =  K Vα
where K  and α  are parametric constants.
At higher voltages in the up turn region, the varistor
current is determined by the resistivity of the ZnO grains
[2].
The basic function of ceramic varistors is to protect
electronic devices and electrical circuits against voltage
surges ,  such as those generated by lightning strikes
and circuit switching transient. Some applications
including fault current limiters [3] and active filters [4],
require the varistors to absorb continuous repetitive
pulses of short duration over a sustained period of time,
say 1 second. Therefore, it is necessary to study the
energy handling capability of the commercial varistors
under these conditions. The research presented in this
paper is a step in this direction. In this paper, the energy
absorption capability of ZnO varistors is first analysed.
Experimental tests on samples of different diameters
and thickness are performed to measure the energy
handling capability with continuous repetitive pulses.
The puncture mechanism caused by current
concentration is analysed and discussed.
Figure 1  Varistor characteristics
ENERGY HANDLING CAPABILITY OF ZnO
VARISTORS
Energy handling capability can be defined as the
amount of energy that the varistor can absorb before it
fails. Thermodynamically, the energy per unit volume
that an ideal varistor is capable of absorbing is
determined by the temperature rise due to the energy
absorption, which is a function of its density and
specific heat [5,6].
J C T T
C
T T
p
p
max ( )= ⋅ ⋅ −ρ
ρ
2 1
2 1
  J / cm
where   =  density,  g / cm
=  specific heat, J / g C
 -  =  temperatue rise,  C
3
3
  
o
o
This theoretical relationship assumes that all of the heat
generated is absorbed by the varistor and that none is
dissipated to the environment. For the MOV, Cp ≈ 0.89
J/g οC at 25 οC and Cp ≈ 1.0 J/g 
οC at 130 οC. The
theoretical density is 5.60 g/cm3. Hence the volumeteric
specific heats are 4.98 and 5.60 J/cm3 οC, at the above
two temperatures, respectively. Thus, a 100οC
temperature rise corresponds to about 500 J/cm3. The
specified energy levels of practical MOV’s fall between
200 and 250 J/cm3.
Switched Region
Up Turn
Region
Leakage
Region
Current
Voltage
According to the above, it is obvious that the theoretical
energy handling capability of an ideal varistor is directly
proportional to its volume.
For MOV to operate without failure or degradation, it
must quickly dissipate the absorbed energy and return to
its pre-pulse design ambient temperature. The duration
of operation, its frequency of occurrence, the geometry
of the device, and the operating environment determine
the rate of heat dissipation.
In practice, the energy dissipated in the varistor is
limited to avoid several types of failures: puncture mode
failure, pulse-induced fracture  and long term degradation
in electrical properties [7,8,9].
The puncture mode failure occurs as a consequence of
current localisation and the associated joule heating that
leads to a hole being melted through the varistor and
shorting the electrodes. The effects of microstructure
inhomogenities, such as variation in grain boundary
potential and grain size, have been explored using
computer modelling. It has been shown that the
microstructure inhomogenities can lead to puncture
mode failure [9].
The second mode of failure is the mechanical fracture of
the varistor when subjected to extremely large but short
time pulses. It has been shown that the fracture failure
mode occurs with short pulses and the puncture mode
with slightly longer pulses [10].
The microstructure nonuniformity of a varistor and its
effects on the energy absorption capability have been
analysed [10]. The nonuniformity of the current
distribution was examined by measuring the current
distribution using dot electrodes distributed on the top
surface of the varistor. Measurements were taken while
the varistor was subjected to currents of different wave
shapes and periods.
In this paper, two methods to measure the structure
nonuniformity of ZnO varistors are analysed: (a)
ultrasonic wave detection and (b) monitoring of the
temperature distribution using an infrared radiation
thermal-camera.
EXPERIMENTAL SET UP
The objective of the tests is to define the maximum
number of repetitive current pulses that the varistor can
conduct without failure or damage. The current pulses
through the varistor are set to be of fixed amplitude,
waveshape and frequency. The varistor temperature is
monitored during operation from a few milliseconds up
to a few seconds. The use of thermocouple was ruled
out as their response is relatively slow and heat
distributions are difficult to measure. An infrared
imaging system was used to measure the temperature
distribution on the varistor surface. The thermal imaging
system consists of a scanner, an infrared detector, a
computer-based processor, a display device and output
instruments (monitor and printer) [11]. Figure 2 shows a
schematic diagram of the experimental set up.
Power circuit. Figure 3 shows the power circuit which,
produces the current pulses to be injected in the varistor
under test. The circuit consists of a d.c. supply (V1), an
IGBT, a freewheeling diode (D1 to protect the IGBT
from reverse voltage), a snubber circuit (R1, C1 and D2
to protect IGBT from high dv/dt), the test varistor (V)
and an RL load bank. Figure 4 shows the experimental
set up where the power circuit components are
assembled on one board in order to minimise the stray
inductance.
Figure 2  The test system set up
Figure 3  A schematic diagram of the power circuit
Figure 4 The power circuit lay out
1
6
2
X1
1
2
2
1
D1
2
2
1
1
3
D2
1
3
R1
3
2
C1
1
5
2
V1
4 41 1 5 5
22
R
1
2
V
1
2
RL
Control
Circuit
Oscilloscope
Current Probe
Current Probe
Power
CircuitDC power
Supply
Control
Circuit
Drive
Circuit
Computer
Image
Processor Printer
Monitor
Load
Voltage Transposer
Varistor under Test
Thermal Imager
varistor
IGBT Switch
Test varistors: Tests were carried out on samples from
two manufacturers with specifications as given in
Table-1.
Table-1 Specifications of test varistors
Specification Product
A
Product
B
Diameter 36 mm 53 mm
Thickness 2 mm 3.56 mm
Clamping voltage 560 V 650 V
Energy rating 170 J 880 J
Test procedure . Normally, the IGBT is kept in the off
state and the supply  voltage (less than the varistor
clamping voltage) appears across the varistor. When the
IGBT is turned on, current starts to flow through the
circuit and energy is stored in the load inductance.
When the IGBT is now turned off, the inductor tries to
maintain the flow of current, as the current through an
inductor cannot change abruptly. Consequently, a high
voltage across the IGBT and varistor is produced which
drives current through the varistor (initially equal to the
operating load current ≈20A). The varistor current then
decays toward zero exponentially with time. Within
each switching cycle, the IGBT is turned on for a period
of 540 µs and is then turned off for another period of
540 µs. The switching operation continues for a
controllable duration, up to a few seconds.
TEST RESULTS
Thermal images obtained at regular time intervals
present a dynamic record of the thermal condition of the
varistor. Figure 5 gives a group of thermal images for
the 36 mm varistor recorded at an interval of 80 ms for a
total recording time of 480 ms. Recording started before
IGBT operation which lasted for about 200 ms. The first
image in Figure 5 shows the varistor still at room
temperature. During the first 240 ms (images 2, 3 and
4), the varistor temperature increased rapidly to more
than 127°C at the hottest spot. Images 5 and 6 describe
the temperature distribution after the switching
operation had stopped. The percentage hot area became
larger indicating heat being conducted from the hotter
parts to a wider area. The images in Figure 5 show heat
localisation with the hottest spot being to the right edge
of the disc. The temperature there was higher than
127°C. In contrast, the coldest part of the varistor
surface was in the region of 30°C. Such a large
difference due to current localisation results in poor
utilisation of the varistor volume.
Similar tests were carried out on a 53 mm varistor.
Figure 6 shows the measured waveforms for the varistor
voltage and current. Figure 7 gives a group of thermal
images of the varistor recorded at an interval of 80 ms
over a period of 480 ms. Continuous repetitive pulses
were applied to the varistor for 400 ms. It is clear from
the images shown that heat distribution is more uniform
than the previous varistor (36 mm). A relatively mild
current concentration spot was identified near the centre
of the disc. Heat was evenly dissipated in all directions.
Figure 5  Temperature distribution of product A
Figure 6  Voltage and current of the product B
Figure 7  Temperature distribution of product B
400 405 410 415 420
0
200
400
600
Waveform of Varistor (53 mm) No. 2
V
ar
is
to
r 
V
ol
ta
ge
400 405 410 415 420
0
10
20
30
V
ar
is
to
r 
cu
rr
en
t
Time in m sec
1 2
3
5 6
4
   1
3
5
2
4
6
MAXIMUM ENERGY HANDLING CAPABILITY
OF THE VARISTOR
In order to define the maximum continuous energy that
a varistor can absorb without any damage, several tests
were performed on three more samples of the 53 mm
type varistor. For each sample, thermal images before,
during and after the switching operation were recorded.
Operation time was varied to change the energy
absorbed by the varistor. In each case, the maximum
surface temperature of the varistor was monitored.
Figure 8 shows, for the three samples, the relationship
between the maximum temperature (Tmax) and the
switching operation time. Figure 9 shows the
relationship between the energy absorption of the
varistors (calculated as ∫vidt) and operation time.
The relationship between varistor temperature versus
input energy is derived from Figures 8 and 9 and is
shown in Figure 10. In all these tests, the maximum
surface temperature of the varistors was raised to about
200°C without causing any damage. Comparing with
the specification in Table 1, it is shown that the varistor
maximum temperature remains under 200°C even when
the total energy absorbed is about twice the “maximum
energy absorption”, 880 J. It is clear that the varistor can
operate with continuous repetitive pulses safely as long
as its thermal stability point is not reached. This is
because the off periods in the pluses allow some heat to
dissipate across the surface of the varistor.
After such tests, the switching time was then increased
to gradually reach the puncture mode. Figure 11 shows
that there are three regions. Firstly there is the thermal
stability region in which the input energy to the varistor
will be proportional to the varistor maximum surface
temperature. Secondly, there is the region “out of
thermal stability”, here a small increase in the input
energy leads to large temperature rise. Finally, there is
the thermal runaway region where the hot spot
temperature increases until local varistor puncture
occurs.
Figure 8  Relationship between varistor temperature and
switching time
Figure 9  Relationship between input energy and
switching time
Figure 10  Relationship between varistor temperature
and input energy
Figure 11  Temperature regions
NONDESTRUCTIVE TEST USING SCANNING
ACOUSTIC MICROSCOPE (SAM).
Ultrasonic is commonly used means of nondestructive
testing. Pulses of compressional or shear pressure waves
at frequencies of 1-100 MHz are generated by a
piezoelectric transducer. The pulse passes through the
specimen and the reflection is picked up by the same
0 200 400 600 800 1000 1200
40
60
80
100
120
140
160
180
200
220
V
ar
is
to
tr
 T
em
pe
ra
tu
re
 in
 C
 (
Tm
ax
)
Time in m sec
V4
V3
V2
0 200 400 600 800 1000 1200
0
200
400
600
800
1000
1200
1400
1600
1800
Time in m sec
E
ne
rg
y 
in
 J
V4
V3
V2
0 200 400 600 800 1000 1200 1400 1600 1800
40
60
80
100
120
140
160
180
200
220
V
ar
is
to
tr
 T
em
pe
ra
tu
re
 in
 C
 (
T
m
ax
)
Energy in J
V4
V3
V2
0 200 400 600 800 1000 1200 1400 1600 1800 2000
0
100
200
300
400
500
600
700
800
Time, ms
Te
m
pe
ra
tu
re
, C
Thermal stability
Out of thermal stability
Thermal runaway
transducer [12]. The pulse is modified by the path taken
and energy is reflected by material discontinuities. The
Ultrasonic transducer cannot generally operate in air.
Water is therefore used as a transparent medium
between the transducer and the specimen to allow
ultrasonic energy to transmit into and out of the
specimen. Varistors to be tested are placed in trays in an
immersion tank and the scanner is positioned to scan
under computer control. Figure 12 shows the images of
a varistor scanned at different depths. The results
obtained for each sample were compared with the
corresponding thermal images of the same sample.
While a vague correspondence could some times be
noticed, no conclusive match could be found. The
ultrasonic scanning microscope is more suitable for
detecting the cracks or larger porosity.
Figure 12  SAM photograph for 53 mm varistor
CONCLUSIONS
The energy handling capability of ZnO varistors has
been examined. The experimental results show that:
1. The infrared imaging camera is a useful mean for
measuring the energy handling capability,
temperature distribution and uniformity of the
microstructure of the varistor.
2. The energy handling capability strongly depends on
the varistor microstructure uniformity; the more
uniform, the higher the energy handling capability.
3. When subjected to continuous repetitive pulses the
varistor can handle a total energy higher than the
rating usually specified for a single pulse (almost
twice).
4. There are three regions before the varistor puncture:
the thermal stability region, the out of thermal
stability region and the thermal runaway region.
5. The acoustic scanning microscopy does not provide
the required resolution for measuring the
uniformity of the varistor microstructure. But it can
be used to find larger defects.
ACKNOWLEDGEMENTS
The authors would like to thank Northern Electric
Distribution Limited and the Regional Center for
Electronic Technology (RECET) for their support to
this work. Thanks are also due to the Government of
Egypt for the scholarship provided to Mr. Ahmed to
study at the University of Northumbria, UK.
This paper describes part of a development work, the
results of which has been submitted in a patent
application.
REFERENCES
1. Lionel M.L., and Herbert R.P., 1986, “Zinc Oxide
Varistor A-Review”, Ceramic Bulletin, Vol. 65,
No. 4, pp. 639-646.
2. Clarke, David R., 1999 “Varistor Ceramics”,
Journal of the American Ceramic Socity, V82, No.
3, pp. 485-502.
3. Ahmad M. and Putrus, G.A., 1999, “Investigation
into Custom Power Technology”, UPEC99, Vol. 2,
pp. 509-512
4. Moran, L.A.; Pastorini, I.; Dixon, J and  Wallace,
R., 1999 “A fault protection scheme for series
active power filters” IEEE Trans. PE., pp. 928-938
5. Bartkowiak M., and Mahan G.D.,1996, “Energy
Handling Capability of ZnO Varistors”, J. Appl.
Phys., Vol. 79, No. 11, pp. 8629-8633
6. Agens, V. and Clarke D.R.,1997, “Electrical-
Impulse–Induced Fracture of Zinc Oxide Varistor
Ceramics”,  J. Am. Ceram. Soc., Vol.80, No. 8, pp.
2086-2092,
7. Mizukoshi A., Ozawa J. and Nakano K., 1983,
“Influence of Uniformity on Energy Absorption
Capability of Zinc Oxide Elements as Applied in
Arresters”:, IEEE. Trans. PAS-102, No. 5, pp.
1384-1390.
8. Bartkowiak M., and Mahan G.D., 1999 “Failure
Modes and Energy Absorption Capability of ZnO.
Varistors”, IEEE Trans. PWRD., Vol. 14, No. 1,
pp. 152-162.
9. Agens V., and Clarke D. R., 1997 “Microstructural
Origin of Current Localisation and Puncture Failure
in Varistor Ceramics”, J. Appl. Phys., Vol. 81, No.
2,  pp. 985-993.
10. Kazuo E., 1984, “Destruction Mechanism of ZnO
Varistors due to High Currents”, J. Appl. Phys.,
Vol. 56, No.10, pp. 2948-2955
11. Land Infrared “Land TI 35sm Thermal Imager
Operation Instructions”, Dronfield UK, 1994.
12. Drinkwater B., 1998 “Sounding out good
adhesion”, Materials World, pp. 149-151.
a
a


Measuring the energy handling capability of metal oxide varistors

Northumbria University Research Portal

Northumbria Research LinkCitation: Ahmed, M. M. R., Putrus, G.A., Ran, L. and Penlington, Roger (2001) Measuring the energy handling  capability  of  metal  oxide  varistors.  In:  16th  International  Conference  and  Exhibition  on Electricity Distribution (CIRED 2001), 18th - 21st June 2001, Amsterdam, Netherlands. URL: https://doi.org/10.1049%2Fcp%3A20010705 <https://doi.org/10.1049%2Fcp%3A20010705>This version was downloaded from Northumbria Research Link: http://nrl.northumbria.ac.uk/42591/Northumbria University has developed Northumbria Research Link (NRL) to enable users to access the University’s research output. Copyright © and moral rights for items on NRL are retained by the individual author(s) and/or other copyright owners.  Single copies of full items can be reproduced, displayed or performed, and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided the authors, title and full bibliographic details are given, as well as a hyperlink and/or URL to the original metadata page. The content must not be changed in any way. Full items must not be sold commercially in any  format or medium without formal permission of the copyright holder.  The full policy is available online: http://nrl.northumbria.ac.uk/pol  i cies.html  This  document  may differ  from the  final,  published version of  the research  and has been made available online in accordance with publisher policies. To read and/or cite from the published version of the research, please visit the publisher’s website (a subscription may be required.)                        MEASURING THE ENERGY HANDLING CAPABILITY OF METAL OXIDE VARISTORSM.M.R. Ahmed           G.A. Putrus            L. Ran R. PenlingtonUniversity of Northumbria, at Newcastle, UKAbstractMetal oxide varistors are widely used in many powerelectronics circuits to protect against transient overvoltages. Certain applications are very demanding onthe energy handling capability of the varistors. Thispaper gives an overview of the failure modes of ZnOvaristors and investigates their characteristics whensubjected to repetitive current pulses. It  describes thepuncture failure mode caused by melting of a region inthe varistor of local current concentration. Experimentaltests are performed to evaluate the puncture energyusing an infrared imaging camera. A relationshipbetween the energy absorption and the varistormaximum surface temperature is obtained. It is shownthat the destructive energy depends strongly on theuniformity of the varistor; the more uniform, the higherthe energy handling capability. The paper also presentsthe results of nondestructive tests using a scanningacoustic microscope to evaluate the uniformity of thevaristor.Keywords: varistors, infrared imaging camera, energyINTRODUCTIONMetal oxide varistors (MOV) are highly nonohmicceramic devices with current-voltage characteristicssimilar to those of back-to-back Zener diodes but withhigher energy handling capability [1]. The voltage-current characteristics of the MOV can be divided intothree regions, as shown in Figure 1. At low voltages, inthe region known as the pre-switched region, onlyleakage current occurs. With intermediate voltages inthe switched region, the characteristics are generallydescribed by the equation:I =  K Vawhere K  and a  are parametric constants.At higher voltages in the up turn region, the varistorcurrent is determined by the resistivity of the ZnO grains[2].The basic function of ceramic varistors is to protectelectronic devices and electrical circuits against voltagesurges ,  such as those generated by lightning strikesand circuit switching transient. Some applicationsincluding fault current limiters [3] and active filters [4],require the varistors to absorb continuous repetitivepulses of short duration over a sustained period of time,say 1 second. Therefore, it is necessary to study theenergy handling capability of the commercial varistorsunder these conditions. The research presented in thispaper is a step in this direction. In this paper, the energyabsorption capability of ZnO varistors is first analysed.Experimental tests on samples of different diametersand thickness are performed to measure the energyhandling capability with continuous repetitive pulses.The puncture mechanism caused by currentconcentration is analysed and discussed.Figure 1  Varistor characteristicsENERGY HANDLING CAPABILITY OF ZnOVARISTORSEnergy handling capability can be defined as theamount of energy that the varistor can absorb before itfails. Thermodynamically, the energy per unit volumethat an ideal varistor is capable of absorbing isdetermined by the temperature rise due to the energyabsorption, which is a function of its density andspecific heat [5,6].J C T TCT Tppmax ( )= × × -rr2 12 1  J / cmwhere   =  density,  g / cm=  specific heat, J / g C -  =  temperatue rise,  C33  ooThis theoretical relationship assumes that all of the heatgenerated is absorbed by the varistor and that none isdissipated to the environment. For the MOV, Cp » 0.89J/g oC at 25 oC and Cp » 1.0 J/g oC at 130 oC. Thetheoretical density is 5.60 g/cm3. Hence the volumetericspecific heats are 4.98 and 5.60 J/cm3 oC, at the abovetwo temperatures, respectively. Thus, a 100oCtemperature rise corresponds to about 500 J/cm3. Thespecified energy levels of practical MOV’s fall between200 and 250 J/cm3.Switched RegionUp TurnRegionLeakageRegionCurrentVoltageAccording to the above, it is obvious that the theoreticalenergy handling capability of an ideal varistor is directlyproportional to its volume.For MOV to operate without failure or degradation, itmust quickly dissipate the absorbed energy and return toits pre-pulse design ambient temperature. The durationof operation, its frequency of occurrence, the geometryof the device, and the operating environment determinethe rate of heat dissipation.In practice, the energy dissipated in the varistor islimited to avoid several types of failures: puncture modefailure, pulse-induced fracture  and long term degradationin electrical properties [7,8,9].The puncture mode failure occurs as a consequence ofcurrent localisation and the associated joule heating thatleads to a hole being melted through the varistor andshorting the electrodes. The effects of microstructureinhomogenities, such as variation in grain boundarypotential and grain size, have been explored usingcomputer modelling. It has been shown that themicrostructure inhomogenities can lead to puncturemode failure [9].The second mode of failure is the mechanical fracture ofthe varistor when subjected to extremely large but shorttime pulses. It has been shown that the fracture failuremode occurs with short pulses and the puncture modewith slightly longer pulses [10].The microstructure nonuniformity of a varistor and itseffects on the energy absorption capability have beenanalysed [10]. The nonuniformity of the currentdistribution was examined by measuring the currentdistribution using dot electrodes distributed on the topsurface of the varistor. Measurements were taken whilethe varistor was subjected to currents of different waveshapes and periods.In this paper, two methods to measure the structurenonuniformity of ZnO varistors are analysed: (a)ultrasonic wave detection and (b) monitoring of thetemperature distribution using an infrared radiationthermal-camera.EXPERIMENTAL SET UPThe objective of the tests is to define the maximumnumber of repetitive current pulses that the varistor canconduct without failure or damage. The current pulsesthrough the varistor are set to be of fixed amplitude,waveshape and frequency. The varistor temperature ismonitored during operation from a few milliseconds upto a few seconds. The use of thermocouple was ruledout as their response is relatively slow and heatdistributions are difficult to measure. An infraredimaging system was used to measure the temperaturedistribution on the varistor surface. The thermal imagingsystem consists of a scanner, an infrared detector, acomputer-based processor, a display device and outputinstruments (monitor and printer) [11]. Figure 2 shows aschematic diagram of the experimental set up.Power circuit. Figure 3 shows the power circuit which,produces the current pulses to be injected in the varistorunder test. The circuit consists of a d.c. supply (V1), anIGBT, a freewheeling diode (D1 to protect the IGBTfrom reverse voltage), a snubber circuit (R1, C1 and D2to protect IGBT from high dv/dt), the test varistor (V)and an RL load bank. Figure 4 shows the experimentalset up where the power circuit components areassembled on one board in order to minimise the strayinductance.Figure 2  The test system set upFigure 3  A schematic diagram of the power circuitFigure 4 The power circuit lay out162X11221D122113D213R132C1152V14 41 1 5 522R12V12RLControlCircuitOscilloscopeCurrent ProbeCurrent ProbePowerCircuitDC powerSupplyControlCircuitDriveCircuitComputerImageProcessor PrinterMonitorLoadVoltage TransposerVaristor under TestThermal ImagervaristorIGBT SwitchTest varistors: Tests were carried out on samples fromtwo manufacturers with specifications as given inTable-1.Table-1 Specifications of test varistorsSpecification ProductAProductBDiameter 36 mm 53 mmThickness 2 mm 3.56 mmClamping voltage 560 V 650 VEnergy rating 170 J 880 JTest procedure . Normally, the IGBT is kept in the offstate and the supply  voltage (less than the varistorclamping voltage) appears across the varistor. When theIGBT is turned on, current starts to flow through thecircuit and energy is stored in the load inductance.When the IGBT is now turned off, the inductor tries tomaintain the flow of current, as the current through aninductor cannot change abruptly. Consequently, a highvoltage across the IGBT and varistor is produced whichdrives current through the varistor (initially equal to theoperating load current »20A). The varistor current thendecays toward zero exponentially with time. Withineach switching cycle, the IGBT is turned on for a periodof 540 ms and is then turned off for another period of540 ms. The switching operation continues for acontrollable duration, up to a few seconds.TEST RESULTSThermal images obtained at regular time intervalspresent a dynamic record of the thermal condition of thevaristor. Figure 5 gives a group of thermal images forthe 36 mm varistor recorded at an interval of 80 ms for atotal recording time of 480 ms. Recording started beforeIGBT operation which lasted for about 200 ms. The firstimage in Figure 5 shows the varistor still at roomtemperature. During the first 240 ms (images 2, 3 and4), the varistor temperature increased rapidly to morethan 127°C at the hottest spot. Images 5 and 6 describethe temperature distribution after the switchingoperation had stopped. The percentage hot area becamelarger indicating heat being conducted from the hotterparts to a wider area. The images in Figure 5 show heatlocalisation with the hottest spot being to the right edgeof the disc. The temperature there was higher than127°C. In contrast, the coldest part of the varistorsurface was in the region of 30°C. Such a largedifference due to current localisation results in poorutilisation of the varistor volume.Similar tests were carried out on a 53 mm varistor.Figure 6 shows the measured waveforms for the varistorvoltage and current. Figure 7 gives a group of thermalimages of the varistor recorded at an interval of 80 msover a period of 480 ms. Continuous repetitive pulseswere applied to the varistor for 400 ms. It is clear fromthe images shown that heat distribution is more uniformthan the previous varistor (36 mm). A relatively mildcurrent concentration spot was identified near the centreof the disc. Heat was evenly dissipated in all directions.Figure 5  Temperature distribution of product AFigure 6  Voltage and current of the product BFigure 7  Temperature distribution of product B400 405 410 415 4200200400600Waveform of Varistor (53 mm) No. 2Varistor Voltage400 405 410 415 4200102030Varistor currentTime in m sec1 235 64   135246MAXIMUM ENERGY HANDLING CAPABILITYOF THE VARISTORIn order to define the maximum continuous energy thata varistor can absorb without any damage, several testswere performed on three more samples of the 53 mmtype varistor. For each sample, thermal images before,during and after the switching operation were recorded.Operation time was varied to change the energyabsorbed by the varistor. In each case, the maximumsurface temperature of the varistor was monitored.Figure 8 shows, for the three samples, the relationshipbetween the maximum temperature (Tmax) and theswitching operation time. Figure 9 shows therelationship between the energy absorption of thevaristors (calculated as òvidt) and operation time.The relationship between varistor temperature versusinput energy is derived from Figures 8 and 9 and isshown in Figure 10. In all these tests, the maximumsurface temperature of the varistors was raised to about200°C without causing any damage. Comparing withthe specification in Table 1, it is shown that the varistormaximum temperature remains under 200°C even whenthe total energy absorbed is about twice the “maximumenergy absorption”, 880 J. It is clear that the varistor canoperate with continuous repetitive pulses safely as longas its thermal stability point is not reached. This isbecause the off periods in the pluses allow some heat todissipate across the surface of the varistor.After such tests, the switching time was then increasedto gradually reach the puncture mode. Figure 11 showsthat there are three regions. Firstly there is the thermalstability region in which the input energy to the varistorwill be proportional to the varistor maximum surfacetemperature. Secondly, there is the region “out ofthermal stability”, here a small increase in the inputenergy leads to large temperature rise. Finally, there isthe thermal runaway region where the hot spottemperature increases until local varistor punctureoccurs.Figure 8  Relationship between varistor temperature andswitching timeFigure 9  Relationship between input energy andswitching timeFigure 10  Relationship between varistor temperatureand input energyFigure 11  Temperature regionsNONDESTRUCTIVE TEST USING SCANNINGACOUSTIC MICROSCOPE (SAM).Ultrasonic is commonly used means of nondestructivetesting. Pulses of compressional or shear pressure wavesat frequencies of 1-100 MHz are generated by apiezoelectric transducer. The pulse passes through thespecimen and the reflection is picked up by the same0 200 400 600 800 1000 1200406080100120140160180200220Varistotr Temperature in C (Tmax)Time in m secV4V3V20 200 400 600 800 1000 1200020040060080010001200140016001800Time in m secEnergy in JV4V3V20 200 400 600 800 1000 1200 1400 1600 1800406080100120140160180200220Varistotr Temperature in C (Tmax)Energy in JV4V3V20 200 400 600 800 1000 1200 1400 1600 1800 20000100200300400500600700800Time, msTemperature, CThermal stabilityOut of thermal stabilityThermal runawaytransducer [12]. The pulse is modified by the path takenand energy is reflected by material discontinuities. TheUltrasonic transducer cannot generally operate in air.Water is therefore used as a transparent mediumbetween the transducer and the specimen to allowultrasonic energy to transmit into and out of thespecimen. Varistors to be tested are placed in trays in animmersion tank and the scanner is positioned to scanunder computer control. Figure 12 shows the images ofa varistor scanned at different depths. The resultsobtained for each sample were compared with thecorresponding thermal images of the same sample.While a vague correspondence could some times benoticed, no conclusive match could be found. Theultrasonic scanning microscope is more suitable fordetecting the cracks or larger porosity.Figure 12  SAM photograph for 53 mm varistorCONCLUSIONSThe energy handling capability of ZnO varistors hasbeen examined. The experimental results show that:1. The infrared imaging camera is a useful mean formeasuring the energy handling capability,temperature distribution and uniformity of themicrostructure of the varistor.2. The energy handling capability strongly depends onthe varistor microstructure uniformity; the moreuniform, the higher the energy handling capability.3. When subjected to continuous repetitive pulses thevaristor can handle a total energy higher than therating usually specified for a single pulse (almosttwice).4. There are three regions before the varistor puncture:the thermal stability region, the out of thermalstability region and the thermal runaway region.5. The acoustic scanning microscopy does not providethe required resolution for measuring theuniformity of the varistor microstructure. But it canbe used to find larger defects.ACKNOWLEDGEMENTSThe authors would like to thank Northern ElectricDistribution Limited and the Regional Center forElectronic Technology (RECET) for their support tothis work. Thanks are also due to the Government ofEgypt for the scholarship provided to Mr. Ahmed tostudy at the University of Northumbria, UK.This paper describes part of a development work, theresults of which has been submitted in a patentapplication.REFERENCES1. Lionel M.L., and Herbert R.P., 1986, “Zinc OxideVaristor A-Review”, Ceramic Bulletin, Vol. 65,No. 4, pp. 639-646.2. Clarke, David R., 1999 “Varistor Ceramics”,Journal of the American Ceramic Socity, V82, No.3, pp. 485-502.3. Ahmad M. and Putrus, G.A., 1999, “Investigationinto Custom Power Technology”, UPEC99, Vol. 2,pp. 509-5124. Moran, L.A.; Pastorini, I.; Dixon, J and  Wallace,R., 1999 “A fault protection scheme for seriesactive power filters” IEEE Trans. PE., pp. 928-9385. Bartkowiak M., and Mahan G.D.,1996, “EnergyHandling Capability of ZnO Varistors”, J. Appl.Phys., Vol. 79, No. 11, pp. 8629-86336. Agens, V. and Clarke D.R.,1997, “Electrical-Impulse–Induced Fracture of Zinc Oxide VaristorCeramics”,  J. Am. Ceram. Soc., Vol.80, No. 8, pp.2086-2092,7. Mizukoshi A., Ozawa J. and Nakano K., 1983,“Influence of Uniformity on Energy AbsorptionCapability of Zinc Oxide Elements as Applied inArresters”:, IEEE. Trans. PAS-102, No. 5, pp.1384-1390.8. Bartkowiak M., and Mahan G.D., 1999 “FailureModes and Energy Absorption Capability of ZnO.Varistors”, IEEE Trans. PWRD., Vol. 14, No. 1,pp. 152-162.9. Agens V., and Clarke D. R., 1997 “MicrostructuralOrigin of Current Localisation and Puncture Failurein Varistor Ceramics”, J. Appl. Phys., Vol. 81, No.2,  pp. 985-993.10. Kazuo E., 1984, “Destruction Mechanism of ZnOVaristors due to High Currents”, J. Appl. Phys.,Vol. 56, No.10, pp. 2948-295511. Land Infrared “Land TI 35sm Thermal ImagerOperation Instructions”, Dronfield UK, 1994.12. Drinkwater B., 1998 “Sounding out goodadhesion”, Materials World, pp. 149-151.aa

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Measuring the energy handling capability of metal oxide varistors

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