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Measurement of leakage current for monitoring the performance of outdoor
insulators in polluted environments
Article in IEEE Electrical Insulation Magazine · July 2012
DOI: 10.1109/MEI.2012.6232007
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Isaias Ramirez
Ramiro Hernández Corona
Instituto Nacional de Electricidad y Energias Limpias
Instituto de Investigaciones Electricas
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Gerardo Montoya
Instituto Nacional de Electricidad y Energías Limpias
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Measurement of Leakage Current
for Monitoring the Performance
of Outdoor Insulators
in Polluted Environments
Key words: insulators, pollution, leakage current, diagnostic techniques, flashover risk
Introduction
The measurement of leakage current is used for monitoring
the performance of insulators to minimize system outages attributable to pollution [1]. The technique is applied to transmission
lines exposed to severe and varying conditions of pollution [2].
According to this technique, preventative maintenance is initiated when a critical level of current is reached on polluted insulators. However, the technique requires continuous supervision
to analyze the condition of the monitored insulators.
Important differences in the severity of pollution between
sites can be detected by measuring the leakage current. In some
sites, the severity of pollution can cause high leakage current activity in a short time, whereas in others, a longer time is needed
and monitoring at different sites is necessary to determine the
best insulator profile for each site. With the selection of the best
insulator profile for each site, maintenance can be scheduled accordingly, thereby minimizing pollution flashover outages.
Since 1995, the Mexican Electrical Research Institute (Instituto de Investigaciones Eléctricas), together with the Mexican electric utility (Comision Federal de Electricidad), has had
several projects for online measurement of leakage current that
have been applied to various designs of insulators in an attempt
to minimize pollution flashovers on transmission lines [3]. To
date, the Comision Federal de Electricidad has had 44 leakage
current monitoring systems installed on several transmission
lines [4].
These projects have provided a much broader perspective
on the performance of outdoor insulators for various pollution
zones, and based on laboratory tests, critical leakage current
magnitudes have been determined for various insulator profiles
[5]. Although the maximum permissible levels of leakage current are considerably different for each profile, this does not
necessarily indicate that insulators that permit a lower leakage
current will have the best operating performance because the
July/August — Vol. 28, No. 4
Isaias Ramirez, Ramiro Hernández,
and Gerardo Montoya
Instituto de Investigaciones Eléctricas,
Cuernavaca, Morelos, México 62490
Measurement of leakage current can
be an effective method for minimizing the pollution flashover of insulators on transmission lines, but to
be useful, the technique requires additional information from measurements and analysis of the pollution.
profiles themselves will permit the accumulation of pollutants,
which defines their pollution performance. Furthermore, a leakage current may be sporadic, continuous, periodic, or increasing
with time, all of which are factors in the preventive maintenance
of insulator outages resulting from pollution.
This article presents an analysis of the monitoring of various
types of insulators, namely, porcelain, room temperature vulcanizing silicone (RTV)-coated porcelain, toughened glass, and
nonceramic insulators installed on transmission lines close to
the Villa de Garcia Substation, in the metropolitan area of Monterrey, Mexico. On one occasion during heavy fog, pollution
caused outages on 400- and 230-kV transmission lines, resulting
0883-7554/12/$31/©2012/IEEE
29
Example of Leakage Current Measurements
Table 1. Leakage Current Classifications.
Range
Leakage current magnitude (mA)
1
50–150
2
150–250
3
250–350
4
350–450
5
450–707
in a very serious event in which 6.8 MW of load was dropped for
15 minutes and 5 MW of load was dropped for 1 minute.
Leakage Current Monitoring
The monitoring system classifies leakage current peaks in five
ranges, as shown in Table 1, and the microprocessor monitors all
leakage current pulses in each half cycle (8.33 milliseconds) for
all three phases and provides a data summary every 9 hours. The
data contain the maximum leakage current pulses, have the accumulated peaks classified into each of the five ranges, and have
autonomy for 6 months, after which the data need to be downloaded and the system reset to begin a new monitoring period.
Table 2 outlines the basic parameters of the insulators monitored. The insulators are porcelain, glass, and silicone rubber
nonceramic insulators (NCI). The critical leakage current for
flashover as determined in the laboratory is also shown in the
table. Table 3 shows the transmission line voltage and the string
configurations monitored.
During a period of severe fog lasting 2 days, leakage current activity was monitored on insulator strings; the results for
insulators on tower 59 are shown as examples (Table 4). The
leakage current on nonceramic insulator NCI-2 on phase A,
shown in Figure 1(a), reached peaks of 38 and 44 mA during the
event. During the same event, RTV-coated superfog insulators
on phase C showed higher leakage current activity, reaching levels up to 215 mA. The Vee-string configuration on phase B with
type NCI-2 insulators showed leakage currents of 35 and 60 mA
on each leg of the Vee string, respectively. The NCI on phases A
and B showed lower leakage currents compared with the RTVcoated superfog insulators on phase C on the same tower.
Table 4 summarizes the highest magnitude of leakage current registered by the monitoring systems. No leakage current
levels above the critical ones, as determined in the laboratory
(Table 2), were recorded for the operating conditions of this site.
Therefore, no pollution flashover should have taken place. To
understand the reason for flashovers, several of the flashed insulators were removed from the site for laboratory flashover tests
and chemical analysis of the pollutant.
Laboratory Tests
Insulator strings on line A3450, tower 59, and A3720, tower
58, which showed the highest leakage currents during the 2-day
fog event (Table 4), were removed for flashover tests in a clean
fog chamber. These tests were done following the rapid flashover method in IEC-60507 [6]. The removed strings were made
into shorter strings of four units and tested, obtaining an average
flashover per unit of 10.9 kVrms. Because this flashover is close
to the nominal voltage per insulator (10.5 kVrms) on the transmis-
Table 2. Basic Parameters of the Insulators Monitored.
Leakage
distance per
unit (mm)
Critical leakage
current for
flashover (mA)
270
1 × 292
250
22 × 146
280
1 × 445
350
111
22 × 146
290
1 × 489
450
Superfog
111
22 × 146
321
1 × 612
700
Glass
Superfog
111
22 × 146
321
1 × 612
700
32SV160C
Glass
Fog
160
22 × 171
321
1 × 540
700
NCI-11
Silicone rubber
NCI
130
1 × 3,550
135 major
100 minor
1 × 10,420
NCI-2
Silicone rubber
NCI
130
1 × 1,928
141 major
102 minor
1 × 6,121
NCI-3
Silicone rubber
NCI
133
1 × 3,475
172 major
142 minor
1 × 14,408
Insulator
designation
Rated strength
(kN)
Units × spacing
(mm)
Material
Insulator type
27SV111C
Glass
Standard
111
22 × 146
28SV111C
Glass
Fog
111
29SP111C
Porcelain
Fog
32SP111C
Porcelain
32SV111C
Shed diameter
(mm)
NCI = silicone rubber nonceramic insulator.
1
30
IEEE Electrical Insulation Magazine
Table 3. Transmission Line Designation, Tower Number, Line Voltage, and String Configurations Monitored.1
Transmission line, tower number
and line voltage
Phase A
A3450, tower 67, 400 kV
32SP111C—I string
A3450, tower 98, 400 kV
28SV111C—I string
—
28SV111C—I string
A3D60, tower 5, 400 kV
32SV111C—I string
—
32SV111C—I string
A3450, tower 7, 400 kV
32SV160C—Vee string
32SV160C—Vee string
RTV-coated 29SP111C—I string
NCI-1—I string
93130, tower 59, 230 kV
NCI-2—I string
NCI-2—Vee string
RTV-coated 32SP111CS—I string
A3G20, tower 551, 400 kV
—
NCI-3—Vee string
NCI-3—Vee string
A3720, tower 58, 400 kV
28SV111C—I string
29SP111C—Vee string
RTV-coated 29SP111C—I string
Phase B
Phase C
32SP111C—I string
RTV = room temperature vulcanizing silicone; NCI = silicone rubber nonceramic insulator.
1
sion lines, it was demonstrated that the pollutant on the insulators was indeed the cause of the flashovers.
The pollution level in terms of equivalent salt deposit density
according to IEC 60815 [7] and chemical analysis of the pollutant were determined. These results are shown in Table 5. The
pollutant level corresponds to pollution class III (high pollution)
in IEC 60815.
The pollutant was analyzed by X-ray diffraction using a Siemens D500 diffractometer with filtered Cu radiation. To identify
the pollutant crystalline components, a diffraction pattern file
was used [Mineral Powder Diffraction File, Joint Committee on
Powder Diffraction Standards (JCPDS), Newtown Square, PA,
1980]. The crystalline compounds identified for this pollutant
type and the equivalent salt deposit density values are shown in
Table 5.
As indicated in Table 5, gibbsite was found on insulators at
tower 65 and at tower 14, but gibbsite was not found in an earlier
analysis of the pollutant in this area [8]. It was concluded that the
source of the pollutant must be very near the affected insulators.
Because gibbsite contains aluminum, a review of the manufacturing facilities in the area identified an aluminum automobile
engine block manufacturing facility as the probable emitter of
aluminum, and gibbsite is formed from aluminum under specific
environmental conditions. Under certain conditions of moisture,
gibbsite crystals on insulators can result in sufficient leakage
current for flashover, and this was thought to be the reason for
Table 4. Leakage Current During the Period Analyzed.1
Maximum leakage current during the period analyzed (mA)
Tower
Channel 1
Insulator
Channel 2
Insulator
Channel 3
Insulator
Channel 4
Insulator
67
33
32SP111C
41
32SP111C
—
—
—
—
98
—
—
—
—
1.38
28SV111C
1.38
28SV111C
5
—
—
—
—
16
32SV111C
69
32SV111C
7
13
32SV160C
24
32SV160C
138
RTV-coated
29SP111C
5
NCI-1
59
44
NCI-2
215
RTV-coated
32SP111C
35
NCI-2
60
NCI-2
551
5
NCI-3
2
NCI-3
2
NCI-3
0
NCI-3
58
35
28SV111C
22
29SP111C
13
29SP111C
93
RTV-coated
29SP111C
A dash (—) indicates channel not installed. RTV = room temperature vulcanizing silicone; NCI = silicone rubber nonceramic insulator.
1
July/August — Vol. 28, No. 4
31
Figure 1. Leakage currents recorded on insulators on tower 59 (Table 3) during a period of severe fog: (a) nonceramic insulator
(NCI)-2 on phase A; (b) room temperature vulcanizing silicone (RTV)-coated superfog insulators on phase C; (c) one leg of the
NCI-2 Vee-string insulator on phase B; (d) other leg of the NCI-2 Vee-string insulator.
the high leakage current [up to 215 mA in Figure 1(b)] at tower
59 of the 230-kV 93130 transmission line.
Table 6 shows the elemental analysis by atomic absorption
spectrometry of the solid constituents of the pollutant layer; it
showed a large weight percentage of aluminum, with the top
surface showing 3.5 to 6.4 times higher level than the bottom
surface. This implies that the mechanism of deposition of aluminum on the insulator was likely precipitation. The higher levels
of calcium and sulfates on the bottom of the insulator were associated with the formation of gypsum [9]. Because of its chemical properties and solubility, as well as the shape and design of
the insulator, this pollution tends to accumulate on the bottom
surface of the insulators. Gypsum is a product of the reaction
of calcite-dolomite in the soil and SO2 from the atmosphere,
the latter generated mainly by the combustion of hydrocarbon
fuels. [9]. Dihydrate CaSO4 is a compound with low solubility
that presents a high water absorption capacity, which aids in the
flashover process.
RTV-Coated Insulators
To simulate the field pollutant for laboratory tests, a mixture
of 600 g of kaolin, 200 g of CaSO4, and 5 g of NaCl (common
salt) per liter of water was made and applied to various types of
insulators for clean-fog flashover tests. For these tests, the contaminant layer was applied to the top, bottom, or entire surface
of the insulators [10]. These results showed that when the entire
surface of the insulator was coated, the flashover voltage was
the highest, whereas coating only the bottom surface resulted in
only a slight reduction in the flashover, as illustrated in Figure 2.
Su et al. [11] indicated that RTV-coated insulators could operate longer than 6 years without preventive maintenance in subtropical climates. However, in another study reported by Su et al.
Table 5. Pollutant Equivalent Salt Deposit Density (ESDD) and Crystalline Components.
ESDD (mg/cm2)
Top surface
Bottom surface
String 1, line A3450, tower 14
0.06
String 2, line A3720, tower 65
0.07
Insulator
32
Crystalline components
Top surface
Bottom surface
0.52
Gibbsite [Al(OH)3], quartz (SiO2)
Calcite (CaCO3), quartz (SiO2), gypsum
(CaSO4∙2H2O)
0.43
Calcite (CaCO3), quartz (SiO2), gibbsite
[Al(OH)3]
Calcite (CaCO3), quartz (SiO2)
IEEE Electrical Insulation Magazine
Table 6. Elemental Analysis of the Solid Part of the Contaminant Layer from the Affected Insulators.1
Weight %
Item
Top of insulator C1
Top of insulator C2
Bottom of insulator C1
Bottom of insulator C2
Magnesium (Mg)
0.50
0.55
0.44
0.57
Calcium (Ca)
3.63
7.23
16.95
17.55
Sodium (Na)
0.20
0.24
0.15
0.14
Potassium (K)
0.22
0.36
0.21
0.35
Aluminum (Al)2
9.71
8.20
1.50
2.29
Silicon (Si)
0.26
0.08
0.01
0.01
Iron (Fe)
2.92
3.53
1.84
1.70
Sulfate (SO4=)
10.44
11.80
16.54
16.95
Chlorine (mg/L)
ND
ND
0.14
0.11
Weight of the solid (g)
0.50
0.50
29.0
17.94
Element
The content of metals was analyzed by atomic absorption spectrometry following standard ASTM E-1024-97. Calibration of equipment to determine pH was performed
according to standard ASTM D1293-99 using at least two reference buffer solutions traceable to the National Institute of Standards and Technology. The determination of total
chlorine was carried out according to standard ASTM C 114-2000. Sulfates were obtained following standard NMX-AA-074-1981. Acidity and alkalinity were determined as
specified in standard NMX-AA-036-SCFI-2001. ND = not determined.
2
Please indicate what the boldface line in the table represents.
1
[12] on insulator performance on a 220-kV transmission line in
a coastal environment, in which heavy contamination accumulated when rain was sparse, high leakage current and continuous
partial discharge activity contributed to the drop in hydrophobicity, increasing the likelihood of flashover. However, fully coated
insulators were installed in the Monterrey area, even though the
cost of this preventive solution was considered too high.
According to the leakage current measurements on the 230kV circuit 93130 at tower 59 and flashovers on the 400-kV circuit A3450 at tower 14, RTV-coated fog-type insulators could
operate reliably for 9 years in this environment without flashovers (insulators installed in 1995). As a comparison, nonceramic insulators that were installed at the same time and in the
same location as the RTV-coated fog-type insulators on tower 7
could operate for at least 5 more years, which was estimated by
taking into consideration the very low leakage current and good
condition of the insulators.
Conclusions
Monitoring of leakage currents on various types of insulators,
in the field and in laboratory tests, has provided a much broader
perspective on the performance of outdoor insulators in various
pollution zones. The technique can be effective for corrective
maintenance of transmission lines, provided the type of pollutant and the critical leakage currents are analyzed periodically. A
leakage current that is continuous, periodic, or increasing with
time is one consideration in the performance of insulators in pollution zones, whereas the maximum permissible levels of leakage current at flashover, which is considerably different for each
July/August — Vol. 28, No. 4
Figure 2. Clean-fog flashover tests of room temperature vulcanizing silicone (RTV)-coated insulators for no coating, coated
on the upper side, coated on the lower side, and fully coated.
33
insulator profile, is another consideration. What this means is
that insulators showing lower leakage currents will not necessarily have the best operating performance. The insulator profile
affects the accumulation of pollutants, which defines their pollution performance, and control of the leakage current by the
insulator design depends largely on the type of pollutant. As a
result, there is no unique solution, and leakage current values
with flashover risk from laboratory tests for various types of insulators must be determined by using the type of pollutant in the
field. This combination of strategies will provide an effective
means of ensuring reliable transmission line operation in pollution zones.
Acknowledgment
The authors thank M. C. Georgina Blass Amador, and María
Guadalupe Cruz González, personnel from Comision Federal de
Electricidad-Coordinacion de Transmision y Transformacion,
Mexico, for their invaluable help with this project.
References
[1]
J. Y. Li, C. X., Sun, W. X. Sima, and Q. J. Yang, “Stage pre-warning
based on leakage current characteristics before contamination flashover
of porcelain and glass insulators,” IET Gener., Transm., Distrib., vol. 3,
no. 3, pp. 605–615, 2009.
[2] A. J. Carreira, “Condition detection of non-ceramic insulators for live line
work at electric utilities,” INMR, vol. 17, no. 84, pp. 26–35, 2009.
[3] G. Montoya-Tena, R. Hernández-Corona, and I. Ramírez Vázquez,
“Experiences on pollution level measurement in Mexico,” Electr. Power
Syst. Res., vol. 76, pp. 58–66, 2005.
[4] Montoya, G., Ramirez, I., and Hernandez, R., “The leakage current as a
diagnostic tool for outdoor insulation,” in Proc. IEEE/PES Transmission
and Distribution Conference and Exposition: Latin America, Bogota,
Colombia, Aug. 2008, pp. 1–4.
[5] Isaías Ramírez-Vázquez, José Luis Fierro-Chávez, “Criteria for the
diagnostic of polluted ceramic insulators based on the leakage current
monitoring technique,” in 1999 IEEE Conf. Electrical Insulation and
Dielectric Phenomena, Austin, TX, 1999, pp. 715–718.
[6] Artificial Pollution Tests on High-Voltage Insulators to Be Used on A.C.
Systems, IEC 60507, 2nd ed., Apr. 1991.
[7] Guide for the Selection of Insulators in Respect of Polluted Conditions,
IEC 60815, Ed. 1.0, 2008.
[8] G. N. Ramos, M. T. Campillo, and K. Naito, “A study on the
characteristics of various conductive contaminants accumulated on
high-voltage insulators,” IEEE Trans. Power Del., vol. 8, no. 4, pp.
1842–1850, 1993.
[9] R. T. Campillo, “Characterization of pollutant in outdoor insulators of
the industrial area of Monterrey, N.L.,” Classification IIE/30/31/RP001/5261/1993, Instituto de Investigaciones Eléctricas, Cuernavaca,
Morelos, Mexico, Mar. 3, 1993.
[10] B. Segura Bahena, “Evaluation of electrical Insulators for transmission
lines with pollution problems,” B.S. Thesis, Faculty of Chemical
Sciences and Engineering, Cuernavaca, Morelos, Mexico, Mar. 1995.
[11] H. Su, Z. Jia, Zhicheng Guan, “Durability of RTV-coated Insulators Used
in Subtropical Areas,” IEEE Trans. Dielectr. Electr. Insul., vol. 18, no. 3,
pp. 767–774, June 2011.
[12] Huafeng Su, Zhidong Jia, Zhicheng Guan, and Licheng Li, “Mechanism
of Contaminant Accumulation and Flashover of Insulator in Heavily
Polluted Coastal Area,” IEEE Transactions on Dielectrics and Electrical
Insulation, Vol. 17, No. 5; pp. 1635-1641, October 2010.
34
Isaias Ramirez-Vazquez (S’05) received
the BS degree from the Facultad de Ingeniería Mecánica Eléctrica y Electrónica (FIMEE), Salamanca, Guanajuato, México, in
1990, the MS degree from FIMEE in 1999,
and the PhD in electrical and computer
engineering from the University of Waterloo, Ontario, Canada. He is a researcher at
the Instituto de Investigaciones Eléctricas
(Electrical Research Institute) in Cuernavaca, Morelos, México.
His current research interests include nano materials for outdoor
insulation, insulation coordination, and electromagnetic transients in power systems.
Ramiro Hernández-Corona was born in
Cuernavaca, Morelos, México, on September 17, 1966. He received the BS degree
in electrical engineering from the University of Morelos in 1991. In the same year,
he joined the Instituto de Investigaciones
Eléctricas (Electrical Research Institute)
located in Cuernavaca, Morelos, México.
In 1995, he received an MS degree in electrical engineering from the University of Salford, UK. He is currently a researcher of the Transmission and Distribution Department. He has been working in activities of pollution flashover on
external insulation and of lightning protection in transmission
and distribution lines.
Gerardo Montoya-Tena was born in
México City, México, on April 27, 1962.
He received the BS degree in electronic
and communications engineering from the
Instituto Politécnico Nacional (National
Polytechnic Institute). In 1990, he received
a MS degree with honors in electrical engineering from the Universidad Nacional
Autónoma de México (Autonomy National University of Mexico). In 1986, he joined the Instituto de
InvestigacionesEléctricas (Electrical Research Institute) located
in Cuernavaca, Morelos, México. He is currently a researcher in
the Transmission and Distribution Department. He is author of
several papers about external insulation and overhead transmission lines. He has been working on activities around pollution
flashover on external insulation in transmission and distribution
lines. He is member of specialists committee of the Mexican
utility.
IEEE Electrical Insulation Magazine
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