IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 3, JULY 2006
1273
Onsite MOSA Condition
Assessment—A New Approach
Chandana Karawita, Student Member, IEEE, and M. R. Raghuveer, Senior Member, IEEE
Abstract—A new method to assess the condition of metal-oxide
surge arresters is presented. The fundamental component of the resistive current is used as an indicator. This current is obtained by
exploiting the linear relationship, which exists between the peak
value of the fundamental component of the resistive current and
the phase shift between the fundamental components of the capacitive current and the total arrester current. This relationship is independent of temperature for a given type of arrester. Results are
presented to demonstrate the accuracy and the reliability of the
new technique.
Index Terms—Diagnostic indicator, metal-oxide surge arrester
(MOSA), resistive current.
indicator, is estimated by modeling MOSA as a nonlinear resistor in parallel with a capacitor.
A. Modified Compensation Technique
If the measured waveform of the applied voltage is available,
a compensation technique can be used to derive the resistive current. This method was originally proposed in [3] and modified in
[6] and [7] to take voltage harmonics into account. Although this
is the most desirable method to check the condition of MOSA,
it is not suitable for onsite applications since knowledge of the
applied voltage is required.
B. Probe-Based Method
I. INTRODUCTION
M
ETAL-OXIDE surge arresters (MOSAs) have been employed in power systems for the protection of electrical
equipment since the early 1980s. Their function is to absorb
the energy associated with overvoltages and limit their magnitude to a safe and acceptable value across connected electrical
equipment. With the passage of time in service, MOSAs exhibit
aging due to the cumulative effect of discharging impulse currents, moisture ingress, internal partial discharges, and uneven
heating [1]. The aging manifests itself as an increased component of the resistive arrester current [2], [3], accompanying increased power loss and decreased energy absorption capability
which, in turn, may lead to thermal runaway [4]. Despite the
above factors, it is known that metal-oxide surge arresters have
been functioning well. However, as their end of life approaches,
experience has shown that it is desirable to periodically conduct
diagnostic tests.
Over the years, although several diagnostic methods [5]
based on the measurement of radio interference (RI), partial discharges (PDs), and emitted electromagnetic radiation
(EMR) have been suggested, those based on the measurement
of arrester current have offered the most promise. Some of
the arrester current-based methods are discussed in Section II
and a new technique, which has onsite applicability and better
accuracy is discussed in Section III.
II. ARRESTER CURRENT-BASED DIAGNOSTIC METHODS
In these techniques, arrester current is measured and the resistive current component, which can be used as a diagnostic
Manuscript received February 9, 2005; revised May 12, 2005. This work was
supported in part by Manitoba Hydro and in part by the Natural Sciences and Research Council of Canada (NSERC), Canada. Paper no. TPWRD-00073-2005.
The authors are with the Department of Electrical and Computer Engineering,
University of Manitoba, Winnipeg, MB R3T 5V6, Canada.
Digital Object Identifier 10.1109/TPWRD.2005.860264
A popular onsite diagnostic procedure involves determination of the third harmonic content of the arrester current.
This method gained popularity because it does not involve
measurement of the voltage. Recognizing that the third harmonic component so determined also contains a capacitive
component arising due to the presence of voltage harmonics,
researchers developed a probe-based method [8] to account
for the latter component. A field probe is placed at the bottom
of the arrester and the induced current in the probe is also
measured. The resistive third harmonic current is derived
from the total current, the probe current, and the electric field
pattern around the arrester. This method, however, is not free
of error as has been demonstrated in [6] and [7]. There is an
inherent error associated with this method due to the fact that
the magnitude and phase angle of harmonic voltages influence
the third harmonic component of the resistive current. Operator
error is also present which arises because of probe placement.
Moreover, the use of an approximated field factor introduces
additional error.
C. Resistive Current Waveshape-Based Method
Perhaps the best diagnostic method is one that relies on the
examination of the resistive volt-ampere characteristic. However, in an online situation, one can obtain the resistive component only at the operating voltage. In [9], a method is suggested to obtain the resistive current from the measured total arrester current without resorting to measurement of the voltage.
If the capacitive fundamental component is subtracted from the
arrester current, the remaining current representing the resistive
current component is obtained [9]. According to [9], the phase
shift between the fundamental capacitive component and the remaining current component is constant for a given arrester and
does not change due to its degradation.
The authors of [9] exploit this fact to derive the resistive current. A virtual sinusoidal signal is generated to represent the
fundamental capacitive current and it is phase-shifted until the
0885-8977/$20.00 © 2006 IEEE
1274
IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 3, JULY 2006
phase-shift criterion are satisfied. This current is subtracted from
the total arrester current to find the remaining current. The peak
value of the remaining current is used as the diagnostic indicator.
In this method, only the fundamental component of the capacitive current is subtracted and, therefore, harmonic capacitive currents are embedded in the remaining current. In [9], it is
claimed that the phase-shift criterion are insensitive to the influence of harmonic capacitive currents. However, these harmonic
capacitive currents may affect the peak value of the remaining
current (Section III-D).
III. NEW TECHNIQUE
This section describes a new approach to assess the condition
of MOSA onsite. Only the arrester current is measured and the
fundamental component of the resistive current is determined
without knowledge of the applied voltage. Basic concepts of
the compensation technique and a practical relationship, which
is mentioned in Section III-A, are used to eliminate the voltage
measurement in the process.
In accordance with the modified compensation technique
Fig. 1. Waveshapes of total, capacitive, and resistive currents of an unaged set
of valve elements at its MCOV.
(1)
and
(2)
where , , and are total arrester current, resistive current,
and capacitive current, respectively.
Equations (1) and (2) can be simplified to obtain the peak
value of the fundamental component of the resistive current and
the peak value of the fundamental component of the capacitive
current as given in (3) and (4), respectively
(3)
(4)
,
, and
are the peak
In (3) and (4),
values of the fundamental components of total arrester current,
resistive current, and capacitive current, respectively. The angle
is the phase shift between the fundamental components
of the total arrester current and the capacitive current.
and
through the angle
Equation (3) relates
, which is not known. The only known quantities are measured arrester current and, hence, the derived
. The following illustrates the suggested procedure to find
from
the recorded waveform of the total arrester current.
A.
-
Characteristics of MOSA
It was found experimentally that a unique relationship exists
between the peak value of the fundamental component of the
resistive current and the phase shift between the fundamental
component of the capacitive current and the fundamental com-
Fig. 2.
MCOV.
I
versus
characteristics in the range of 0.8–1.2 p.u. of
ponent of the total arrester current. In this section, the dependence of the relationship on MOSA condition and temperature
is discussed.
Three sets of valve elements were available and were classified as unaged, aged, and very aged depending on their
characteristics. From these characteristics, the peak resistive currents at MCOV were found to be
,
, and
A for unaged, aged, and very aged valve elements, respectively. The aging was accomplished by passage of impulse
currents.
The valve elements were tested under different applied voltages in the range of 0.8–1.2 p.u. of MCOV. The total, capacitive,
and resistive currents of the unaged sample are shown in Fig. 1.
The
characteristics were obtained by applying
the compensation technique. From the results shown in Fig. 2,
it is found that the characteristics are the same for all three samples and, therefore, the relationship is independent of the MOSA
condition.
KARAWITA AND RAGHUVEER: ONSITE MOSA CONDITION ASSESSMENT—A NEW APPROACH
Fig. 3.
I
versus characteristics at 23 C, 50 C, and 100 C.
Fig. 5.
scale).
1275
Method of solving (3) and (5). (Schematic diagram; axes are not to
B. Suggested New Diagnostic Procedure
Fig. 4. I
versus
MCOV of 36.5 kV.
characteristics for two station-type arresters of
The valve elements were also tested at three different temperatures: 23 C, 50 C, and 100 C. The test results (Fig. 3) show
that the change in the characteristics due to temperature change
is negligible in the temperature range of 23 C–100 C. The
characteristics were also derived for complete arrester units of
MCOV 36.5 kV. Results obtained by using two similar arresters
are compared in Fig. 4. This verifies that the characteristics are
unique for a particular arrester type.
Since this relationship involves fundamental components,
voltage harmonics do not affect the characteristics. Therefore,
the relationship is unique for a particular arrester type and it
can be assumed as linear within the operating voltage range. A
least-square linear curve fitting technique was used and the root
mean square (rms) error of the fit was around 3 A. A general
form is given in (5). A and B can be determined by testing an
arrester in a laboratory at few different voltages
(5)
1) Total arrester current is recorded and the fundamental
component is determined.
2) Fundamental component of the resistive current is determined iteratively as follows.
• For a particular voltage and measurement, (3) yields
the
versus
characteristics which are
identified by curve “A” in Fig. 5. It should be noted
that the sinusoidally varying curve “A” is not a
current waveform. Rather, it represents a characteristic relation and only one point on this curve
is of interest. This point represents the operating
point which is found by simultaneous solution of
(3) (curve A) and (5) (line B). The peak value of
the fundamental component of the resistive current
is, therefore, in Fig. 5, at the intersection of curve A
and line B.
• The method of successive substitution is used to
arrive the operating point from an initial approximation of the value of
when
(
from (5)). For this value of
,
a new
is found from (3). The iterative proconverges
cedure is repeated; the solution for
to the operating point. The successive iterations are
interpreted graphically in Fig. 5.
• The same procedure is repeated for few measurements and the average values are used for better accuracy.
3) The fundamental component of the resistive current can
be used as an indicator and the feasibility has been discussed in Section III-C.
C. Diagnostic Indicator
As mentioned in the introduction, the resistive current component of the arrester current reflects the condition of MOSA.
In this method, the peak value of the fundamental component of
the resistive current is used as an indicator. Table I shows the increase in the fundamental component of the resistive current for
aged and very aged valve elements as a percentage of the corresponding values obtained with unaged valve elements at three
1276
IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 3, JULY 2006
TABLE I
COMPARISON OF PEAK VALUES OF I OBTAINED USING A NEW TECHNIQUE
TABLE II
COMPARISON OF POWER LOSS
Fig. 7.
Fig. 6. Variation of I
Variation of I
with voltage for Set 2.
with voltage for Set 1.
selected values of applied voltage. Table II shows similar results
for the associated power loss. A comparison shows that the peak
value of the fundamental component of the resistive current is
well correlated with the power loss which, in turn, reflects the
condition of the valve element.
D. Results and Discussion
Tests were carried out on the following sets of test objects at
room temperature as well as higher temperatures.
Set 1) Unaged and aged samples of valve elements of 3 in.
in diameter and 1.75 in. in height; two in series with
10-kV MCOV.
Set 2) Two new 36.5-kV MCOV station-type arresters.
Set 3) One new 15.3-kV MCOV polymeric distribution-type
arrester.
For Set 1,
characteristics were derived by
using the test results of the unaged sample. The same characteristics were used to implement the new technique for the aged
sample as well. Both samples were tested under voltages in the
range 0.8 to 1.0 p.u. of MCOV. Results derived using the new
technique are presented in Fig. 6. The fundamental component
of the resistive current derived by the new technique increases
more than 30% at the operating voltage while the arrester ages.
The new technique was also applied to the test results of complete arrester units of Set 2.
characteristics were
Fig. 8. Resistive current and fundamental component of resistive current for
Set 3 under different harmonic conditions. (a) Case I. (b) Case II.
obtained by using test results of one of the arresters. Results of
the new technique, which are illustrated in Fig. 7, are almost
the same for both of the arresters. This confirms that the defined
characteristics are unique to a particular arrester type.
Set 3 was tested under different harmonic conditions of the
applied voltage. Fig. 8 shows the resistive current, which is obtained by subtracting only the fundamental component of the
KARAWITA AND RAGHUVEER: ONSITE MOSA CONDITION ASSESSMENT—A NEW APPROACH
1277
By substituting (3) in (6)
TABLE III
COMPARISON OF RESULTS
(7)
from its actual value is very
In this method, deviation of
is less
small. It is found that the maximum deviation of
than 0.4 . Therefore, from (7), the maximum percentage error,
, is less than 8.38%.
which may occur in
capacitive current from the total arrester current, and the fundamental component of the resistive current obtained for Set 3
under two different harmonic conditions of the applied voltage.
Total harmonic distortion (THD) of the voltage in Fig. 8(a) is
1.15% and in Fig. 8(b),it is 0.67%. Although there is considerable difference in the peak values of the resistive currents, the
fundamental components derived by the new technique are almost the same and there is no effect of voltage harmonics on the
new technique.
Finally, in Table III, the results obtained for all three sets at
their MCOV are compared with the results obtained by the use
of the compensation technique. Results obtained by the application of the new technique agree very well with those obtained by
use of the compensation technique. Since only the fundamental
component of the total current is measured, measuring errors
due to noise are negligible and, therefore, the reliability is also
high.
IV. CONCLUSION
The fundamental component of the resistive current can be
used as a diagnostic indicator of aging of MOSA. In order to determine this component, a unique relationship between the peak
value of the fundamental component of the resistive current and
the phase shift between the fundamental component of the total
current and the fundamental component of the capacitive current
is derived. This characteristic, which is central to the suggested
diagnostic procedure, is independent of the condition of MOSA
as well as its temperature up to 100 C. Although it is possible
that the temperature of the valve elements of an arrester can exceed 100 C immediately after energy absorbtion, the suggested
diagnostic procedure is applicable under normal conditions. An
iterative technique is used to obtain the results, which compare
well with the results obtained by use of the compensation technique (Benchmark method). The suggested method yields more
reliable and accurate results. Since this method involves only the
fundamental components, the results are insensitive to voltage
harmonics. If the arrester current is measured with a suitably
designed current monitor, the suggested method has onsite applicability.
ACKNOWLEDGMENT
The authors want to acknowledge D. Hamelin’s assistance in
carrying out the laboratory experiments.
REFERENCES
[1] M. Bartkowiak, M. G. Comber, and G. D. Mahan, “Failure modes and
energy absorbtion capability of ZnO varistors,” IEEE Trans. Power Del.,
vol. 14, no. 1, pp. 152–162, Jan. 1999.
[2] H. Breder and T. Collin, “Supervision of gapless zinc-oxide surge arresters,” in Inst. Elect. Eng. Conf. Lightning Power Syst., Jun. 1984, pp.
91–95.
[3] S. Shirakawa et al., “Maintenance of surge arrester by portable leakage
current detector,” IEEE Trans. Power Del., vol. 3, no. 3, pp. 998–1003,
Jul. 1988.
[4] P. Kirby, C. C. Erven, and O. Nigol, “Long term stability and energy
discharge capacity of metal oxide valve elements,” IEEE Trans. Power
Del., vol. 3, no. 4, pp. 1656–1665, Oct. 1988.
[5] H. R. Philipp and L. M. Levinson, “Low-temperature electrical studies
on metal-oxide varistors—A clue to conduction mechanisms,” J. Appl.
Phys., vol. 48, no. 4, pp. 1621–1627, Apr. 1977.
[6] H. Zhu, “Investigation of on-site diagnostic testing technique for metal
oxide surge arresters,” M.Sc. thesis, Dept. Elect. Comput. Eng., Univ.
Manitoba, Winnipeg, MB, Canada, 2000.
[7] H. Zhu and M. R. Raghuveer, “Influence of representation model and
voltage harmonics on metal oxide surge arresters,” IEEE Trans. Power
Del., vol. 16, no. 4, pp. 599–603, Oct. 2001.
[8] J. Lundquist, L. Stenstrom, A. Schei, and B. Hanson, “New method for
measurement of the resistive leakage currents of metal-oxide surge arresters in service,” IEEE Trans. Power Del., vol. 5, no. 4, pp. 1811–1822,
Nov. 1990.
[9] C. Heinrich and V. Hinrichsen, “Diagnostics and monitoring of metal
oxide surge arresters in high voltage networks—Comparison of existing
and newly developed procedures,” IEEE Trans. Power Del., vol. 16, no.
1, pp. 138–143, Jan. 2001.
Chandana Karawita (S’04) received the B.Sc. (Eng.) degree from the University of Moratuwa, Moratuwa, Sri Lanka, in 2002, and is currently pursuing the
M.Sc. degree at the University of Manitoba, Winnipeg, MB, Canada.
His research interests are online diagnostics of power apparatus, power
system analysis, and stability.
APPENDIX
SENSITIVITY ANALYSIS FOR THE SUGGESTED METHOD
The change in peak value of the fundamental component of
the resistive current for a small change in phase shift between
the fundamental component of the capacitive current and the
fundamental component of the resistive current is given by
(6)
M. R. Raghuveer (SM’83) received the B.Sc. (Hons.) degree in mathematics
from Delhi University in 1960, the B.Eng. degree from the Indian Institute of
Science, Bangalore, India, in 1963, and the Ph.D. degree from the University of
Windsor, Windsor, ON, Canada, in 1972.
Currently, he is a Professor in the Department of Electrical and Computer
Engineering, University of Manitoba, Winnipeg, MB, Canada. He is the author
and coauthor of many technical papers and an electrical engineering textbook.
His research interests are modeling, high-voltage insulation phenomenon, and
diagnostic test techniques.