The Effect of Thermal Transient on the Leakage
Current of Metal Oxide Arresters
A. Dlamini, P. Bokoro and W. Doorsamy
Department of Electrical and Electronic Engineering
University of Johannesburg
Johannesburg, South Africa
Abstract— The operation of surge arresters depends
on the insulating packaging of zinc- oxide (ZnO) utilized
in arresters. The leakage current within the arrester
increases due to the decay on the insulating property. The
decaying of the insulating property will increase leakage
current within the arrester. The over-voltage that is
generated from surges is clamped by the nonlinear
characteristics of the ZnO block and is neutralized to its
regular level. In this paper, an experiment has been
conducted to monitor the leakage current behaviour at
different thermal and ac low voltage stresses. The metal
oxide varistors (MOVs) were subjected to thermal
degradation under ac field voltage stress. Measured
results from the MOV samples exposed to the electrical
field and thermal stresses are presented and the leakage
current behaviour is analysed. During the time of
measurement, it was observed that the leakage current is
affected by the applied temperature and voltage. The
results obtained illustrate that as soon as the reference
temperature approaches higher temperature a high rise
in leakage current is generated and self-healing
phenomenon is clearly observed during the thermal
transient.
Keywords— Surge Arresters, Leakage Current,
degradation, thermal transient.
I.
INTRODUCTION
In the past few years, the new version of high voltage surge
arresters (metal oxide-based) turned out to be more significant
in surge protection. The reason for that is: compared to classic
SiC surge arresters, metal oxide arresters have significant
favourable properties: for example, a small size which allows
quick reaction for steep discharge current and higher
protective performance. Numerous ways of observing the
condition of a ZnO surge arrester in service have been
introduced before in literature. Most of the strategies depend
on estimating the leakage current of the arrester since the
condition of metal oxide surge arresters is best indicated by its
resistive component of the continuous leakage current [1, 2].
Humidity may cause a significant increase in the continuous
resistive current due to precipitated ageing of the ZnO
arresters, as opposed to a transient rise in the resistive leakage
current brought about by the temporary increase in arrester
temperature [3, 4]. The development of ZnO surge arresters
comprises of a very simple structure [5]. Metal oxide surge
arrester (MOSA) devices are indispensable for clamping
surges in electrical power transmission and distribution
equipment. Recent studies together with field experiment have
shown that monitoring of these devices is carried out where
protection of critical components is required.
This paper presents the impact of thermal transient on the
leakage current of the MOV. The aim of this work is to
investigate the leakage current behaviour at different ac low
voltage and thermal stresses. The results show that as the test
temperature approaches temperature threshold a high leakage
current is obtained and as the temperature is at the initial stage
of holding time high fluctuations are observed and selfhealing is exhibited to help the arrester to conduct to another
thermal stress.
II.
BACKGROUND
A. Thermal characteristics of MOSA’s
The absorption capacity of the thermal energy is
considered as the maximum energy level infused into the
arrester, to allow it to cool down and reach its normal
operating temperature [6]. The electrical power loss due to
continuously applied, power, frequency and voltage are
depending on the temperature [7]. However, the power loss
increases proportionally when the temperature increases. On
the other hand, the arrester gets exposed to heat and it
separates a partial amount of heat into the surrounding area.
Undeniably, the heat flow value also rises with temperature,
however, not the same as the power loss does [7, 8]. During
thermal transient, if the applied temperature exceeds the
reference temperature the arrester explodes protecting the
equipment. The current flowing through the arrester is
thermally activated like in any semiconductor devices. The
overheating risk increases due to high energy short time
lightning strike and long-time overvoltage hazard. As soon as
the temperature increases the leakage current increases too
due to the power of the electrical resistance [8].
B. Leakage current characteristics
The leakage current consists of two components which are
resistive and capacitive currents [9]. The resistive component
of the current consists of the non-linear resistance, it is suitable
for the joule heating within the semiconductor and it is
thermally stimulated which is why it is the most significant in
an arrester [10]. The study shows that the popular method
which can extract the resistive leakage current from the total
current is a third-order harmonic investigation but it has a high
risk of measuring errors and gives high misconceptions[9].
When the MOV is exposed to thermal stresses at a specific
applied voltage, the internal current increases with time. Nonlinear current flows through the terminals of MOV under the
normal operating condition and low leakage current flow in
the arrester. However, overvoltage increases stress on the
arrester and causes high leakage current through the MOV
[7, 8]. The increase of leakage current is also caused by the
weakening of the insulating properties within the arrester [10,
11]. The voltage and temperature applied to the arrester during
the time of measurement influence the leakage current to
increase or decrease [12, 13].
978-1-7281-4162-6/20/$31.00 ©2020 IEEE
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III.
METHODOLOGY
A. Experimental test setup
The test setup is given in figure 1 below. On the 380V AC
supply, the variac was connected to supply the voltage to the
isolating transformer. From the isolating transformer, the end
connectors separating negative and positive respectively were
connected. The fuses were connected in series with the
arresters in case of overshoot the fuse should protect the
arrester. The MOV arresters (S20, K385) were connected
from the thermal chamber in series with the leakage current
data logger. The leakage current was measured in series with
the surge arrester and the supply voltage. The leakage current
depends on applied voltage and temperature at the time of
measurement. The MOV arresters (S20, K385) were
connected in the lab and measurements were taken at different
temperature and voltage.
Fig. 1. Experiment set-up.
B. Thermal Degradation
The leakage current was investigated under thermal
degradation where the heating chamber described in [14] was
supplying thermal stresses to metal oxide surge arrester.
Figure 2 below shows the heating program of temperature
versus time which was programmed on the heat chamber, it
consists of holding time (transition), ramp time(slow heating),
T1 (surface temperature) and T2 (highest set temperature).
The ramp time was set to be constant for 4 minutes whereas
the holding time was 10 minutes for all the experiments. This
figure shows that the high temperature used for this
experiment is 160℃ (T2). The thermal chamber is
independently supplied from a three-phase supply.
C. Surge Arrester parameters
The samples used are based on reasonable measured
values. The experiment was built up in the lab as shown in
figure 1. The MOVs were then subjected to degradation under
different voltages and temperature. Table 1 below shows the
arrester test parameters. The technical specification and size
of MOV samples used in this study have been defined.
TABLE 1: TEST PARAMETERS
Description
Value
Source voltage(V)
380V
(ZnO) surge arresters
5
Reference temperature
85℃
Diameter
20mm
Protection Voltage(Vo)
400V
D. The V-I Characteristic curve
The leakage current, voltage measurements and thermal
degradation test are deliberated. The different MOV samples
were used to conduct thermal degradation to monitor the
leakage current behaviour of the arrester. The 50 Hz ac
thermal degradation test is performed to monitor the long-term
leakage current of MOV varistor samples. The V-I curve is
performed on non-degraded and degraded samples to compare
the effect of thermal degradation. The V-I characteristic
curves obtained for degraded and healthy arresters are
indicated in figure 3. These figures show the rise on the
leakage current, with the degraded MOV having high current
as the temperature was elevated. It is revealed that the more
the arrester is exposed to thermal transients the more it
approaches failure, which is why it needs to be monitored.
Fig. 3. V-I characteristic curve showing the comparison of the measured
leakage current on healthy and degraded MOV arrester.
IV.
Fig. 2. The heating program performed on the Heat Chamber.
RESULTS AND DISCUSSION
The results were analyzed using the MATLAB software
and they are presented below. The experiment was run in the
lab and these results were obtained. The presented results are
only for 140℃ and 160℃ thermal stresses and they are for
different samples of metal oxide varistors.
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The plot on figure 4 below the temperature was set to
160℃ at 360V. As the temperature is applied (stressing the
MOV) the leakage current starts rising. The MOV heats up
heavily due to upper-temperature limit, then the MOV cools
down before the next current stress can be absorbed the
cooling down is interpreted as self-healing of the MOV. This
was at a holding time for a period of 3 hours, but there's not
much of a change on the behaviour as the fluctuations range
from 1.79 × 10−4 A to 1.84 × 10−4 A with an average of
1.82× 10−4 A.
Fig. 6. Comparison of leakage current at different voltages showing large
fluctuations in leakage current at the highest voltage and both having the
same average.
Table 2 below shows the measured leakage current of the
degraded samples after it reached the highest set temperature
which was set to (a)160℃ at 360V and (b) 160℃ at 300V, it
can be seen that the temperature was constant throughout and
the voltage kept on fluctuating due to high transients. At high
voltages, the resistance of the varistor is small, which allows
it to compress transients to safe levels.
Fig. 4. Leakage current measured at the holding time, the fluctuations in
leakage current range from 0.179 to 0.185mA with the average of 0.182mA.
Figure 5 presented below displays the results where the
thermal stress was set to 160℃ and the voltage was set to
380V. This was during the initial period of the transition
(holding) time where thermal transient is being experienced
and the results show larger leakage current fluctuations for this
plot and self-healing phenomenon occurs more often. The
higher oscillations are caused by the high ac thermal stress.
Fig. 5. Leakage Current measured at 160℃ and 380V showing high
fluctuations during thermal transients and self-healing being more visible.
Figure 6 below shows the results of the leakage current
with X1 showing the measured leakage current at 380V and
the red line (X2) indicates the measured leakage current at
300V. The surge arresters were stressed with a temperature of
160℃. The cooling down of the MOV shows that the MOV
was struggling to fail so it exhibits self- healing for it to
conduct to the next stress. During the self-healing process,
there are higher oscillations before reducing to an average.
Higher voltage stresses show larger fluctuations on the
leakage current during thermal transients. On this figure, it is
observed that even though the voltages are different the
leakage current fluctuations under both voltage stresses tend
to similar average being 0.086mA.
TABLE 2: MEASURED VALUES
Time (Min)
Leakage Current
(mA)
Highest set
temperature
(T2)
Voltage
(V)
11:47:30
1.90E-04
160℃
380
11:49:10
1.90E-04
160℃
380
11:50:50
1.91E-04
160℃
380.5
11:52:30
1.91E-04
160℃
380.8
11:54:10
1.90E-04
160℃
380
11:55:50
1.89E-04
160℃
379.9
11:57:30
1.89E-04
160℃
379.7
11:59:10
1.90E-04
160℃
380
12:02:30
1.90E-04
160℃
380
15:00:30
1.34E-04
160℃
300
15:01:00
1.35E-04
160℃
301.2
15:01:30
1.33E-04
160℃
300.5
15:02:00
1.35E-04
160℃
305.2
15:02:30
1.33E-04
160℃
302.89
15:03:00
1.34E-04
160℃
300
15:03:30
1.35E-04
160℃
303.1
Figure 7 below shows the leakage current behaviour where
the MOV was subjected to thermal stresses of 165℃ and
380V. The fluctuations are due to that the MOV was
approaching failure but as a result, it exhibits self- healing in
the process for it to conduct other current stress. But it can be
noticed that the fluctuations are not as big as the one in figure
6, the experiment was conducted on different samples and
different voltages. The results were obtained from holding
time to ramp time before the temperature reached a steady
state. From 0 to 0.58 it was on holding time and from 0.58
minutes upwards it was on ramp time approaching steady
state.
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temperature was set to 80℃ and maximum temperature was
set to 140℃ at 360V. The thermal transient plays a huge role
in the degrading of MOV arresters. As shown that the leakage
current increases rapidly before it reaches holding time.
Fig. 7. Leakage Current behaviour at 160℃ and before it reached steadystate temperature.
Figure 8 below shows the relationship between leakage
current behaviour and time at thermal stress of 160℃ and ac
low voltage of 360V its performance is similar to the one on
figure 4 even though the samples are different and times are
different. It was observed that the voltage only has an effect
on the amplitude of the leakage current but it does not affect
the behaviour. This was at the closing stage of holding time.
Fig. 8. Time Versus Leakage Current at 360V on the closing stage of
holding time.
Figure 9 below interprets the results in milliseconds to
display a closer look at the behaviour of the leakage current
using the same temperature and voltage as figure 8. In this
figure, it was observed that the oscillations were not that fast
compared to other results this was due to the fact that the
experiment was at the end of the holding time. The results
were measured just before the temperature started cooling
down.
Fig. 10. Leakage current measured during the transient temperature (ramp
time) the temperature was still rising from the reference to the highest set
temperature.
V.
The leakage current behaviour on MOV has been
investigated in this paper where the MOV’s were subjected to
thermal degradation at different temperatures and voltages.
Thermal transients increase the leakage current due to the
increase of the ageing level of the metal oxide varistor, it also
rises with time and temperature the moment a voltage is
supplied. This is due to the generated heat of the input power
(RI2) which is radiated from the surface as the temperature
approaches a steady-state. As soon as thermal stress is applied
the higher the rise in temperature, the higher the leakage
current. As the degradation continues, the maximum
continuous voltage (MCOV) is lowered to a level that the
MOV conducts continuously. It was also observed that the
higher the voltage, the higher the fluctuations and the arrester
exhibits self-healing more often.
VI.
REFERENCES
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It is apparent that as soon as the reference temperature
approaches the maximum temperature (T2) a sharp rise in the
nominal leakage current is triggered. Because of the thermally
activated conduction, in the arrester, the heat dissipation is out
of control. Figure 10 is showing how the leakage current was
triggered as the temperature was still rising (ramp time) from
the reference to the highest set temperature. The reference
ACKNOWLEDGMENT
The financial support of the Eskom Power Plant
Engineering Institute is acknowledged. Also, the University of
Johannesburg for availing their calibrated equipment and
facilities is acknowledged.
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Fig. 9. Leakage current measured in milliseconds a few minutes before the
temperature started cooling down to finish the test.
CONCLUSION
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