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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 2, APRIL 2009
Leakage Current Activity on Glass-Type Insulators
of Overhead Transmission Lines in the Northeast
Region of Brazil
Sérgio Campello Oliveira, Student Member, IEEE, Eduardo Fontana, Member, IEEE, and
Fernando José do Monte de Melo Cavalcanti
Abstract—This paper describes a study of the leakage current
activity on glass-type insulators of high-voltage transmission lines
over a three-month period. Three pairs of sensor systems were
installed, one on a 500-kV field tower and the other two on 230-kV
and 500-kV substation towers, all in the northeast region of Brazil.
The locations were chosen due to their particular environment
characteristics and the previous knowledge of pollution severity in
these locations. The correlation of the leakage current attributes
with in situ humidity records, insulator positions on the tower, and
seasonality are discussed.
Index Terms—Flashover, insulators, leakage current, optical-fiber transducers, power transmission lines.
I. INTRODUCTION
I
N the northeast region of Brazil, the deposition of pollutants on the surfaces of glass–type, high-voltage insulators
is responsible for approximately 6% of the line faults [1], but
the records of Companhia Hidro Elétrica do São Francisco
(CHESF), responsible for the generation and distribution of
electrical energy on the northeast region, indicate there are up
to 15.25% line faults that are of unknown cause, which may be
due to pollution deposition problems. When considering only
the 500-kV transmission lines, the pollution caused line faults
to increase to 21.3%, and the line faults due to unknown causes
decrease to 10.1% [1]. On the 500-kV lines, for instance, the
faults caused by pollution deposition are more frequent than
those caused by atmospheric discharges.
Many techniques have been proposed in the literature to determine the pollution deposition severity on high-voltage insulators, including offline techniques, such as the measurement of
the equivalent salt deposit density (ESDD), nonsoluble salt deposit density (NSDD) [2], as well as approaches based on detection of infrared, ultraviolet, or even radio-frequency emissions
Manuscript received October 14, 2007; revised December 10, 2007. Current
version published March 25, 2009. This work was supported by Companhia
Hidro Elétrica do São Francisco (CHESF), under Contract No. CT -I-92.2002.
0960.00. Paper no. TPWRD-00609-2007.
S. C. Oliveira is with the Dep. de Sistemas Computacionais, Universidade de
Pernambuco, Recife PE 50.720-001, Brazil (e-mail: scampello@dsc.upe.br).
E. Fontana is with Grupo de Fotônica, Dep. de Eletrônica e Sistemas,
Universidade Federal de Pernambuco, Recife PE 50.740-530, Brazil (e-mail:
fontana@ufpe.br).
F. J. M. M. Cavalcanti is with Companhia Hidro Elétrica do São Francisco,
Recife PE 50.761-901, Brazil (e-mail: ferrnandj@chesf.gov.br).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPWRD.2008.2002872
caused by partial discharges [3]. More reliable results, however,
can be obtained by direct characterization of the leakage current flowing on the insulator [4]–[12]. CHESF currently uses the
visual inspection approach to evaluate the degree of pollution
deposition from direct nightly observation of the luminescence
produced by partial discharges occurring on the insulators. The
inspectors count the number of lit insulators and qualitatively
evaluate the intensity of the luminescence spots.
In this study, we employ a novel fiber-optic sensor system, recently reported in the literature [12] to quantitatively investigate
the combined effect of humidity and pollution on the leakage
current flowing on glass-type insulator strings of high-voltage
towers located on three distinct states of the northeast region
of Brazil. Six leakage current sensors were installed: two on
a 230-kV tower of the Mossoró substation in the state of Rio
Grande do Norte, two others on a 500-kV tower of the Angelim
substation in the State of Pernambuco, and the remaining two on
a 500-kV tower located on top of the Maranguape Mountain in
the State of Ceará. The correlation of leakage current attributes,
with in situ humidity records, insulator locations, and seasonality are discussed.
II. OPTICAL SENSOR SYSTEMS
A. Substation Sensors
In order to measure the leakage current flowing on the surface of a given insulator string, a fiber-optic sensor, specially designed for leakage current detection on high-voltage lines [12],
is connected in parallel with the insulator closest to the ground
side to guarantee safe operation and installation, as illustrated
in Fig. 1. The fiber-optic sensor detects a sample of the leakage
current waveform and generates a corresponding optical signal
that is guided by the optical-fiber link installed on the substation
underground ducts to the command room. Inside the command
room, the optical signal is detected and analyzed by a processing
module [12], the latter illustrated in the lower left photograph of
Fig. 1. The processing module is controlled by a PIC 16F877A
microcontroller and has a memory capacity of two months of
continuous operation without any human intervention.
Two sensor systems were installed in each substation to allow
a more detailed investigation of the effect of insulator location
on the leakage current activity. Each processing module has a
monitor out connector to allow observation of the leakage current waveform with an oscilloscope inside the command room.
As discussed in the following section, to evaluate the pollution
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OLIVEIRA et al.: LEAKAGE CURRENT ACTIVITY ON GLASS-TYPE INSULATORS
Fig. 1. Substation assembly for online leakage current monitoring and processing.
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Fig. 2. Solar powered field assembly for online leakage current monitoring and
processing.
level on each insulator string, humidity measurement is necessary along with the monitoring of leakage current attributes. To
accomplish this, in substation studies, a humidity sensor is installed externally to the command room and an hourly reading
is taken by the onduty operator for further correlation analysis.
B. Field Sensors
For field applications, the sensor system apparatus illustrated
in Fig. 2 is used [12]. As in the substation configuration, each element from a pair of fiber-optic sensors is connected in parallel
with the corresponding insulator closest to the ground. Since
no low-voltage power supply is available on the field along the
path of the overhead transmission line, the system, consisting
of two processing modules, a rechargeable battery, and control
electronics, is accommodated inside a metal cabinet that protects the electronic parts against rain and the outer environment.
An external solar panel attached to the metal cabinet generates
the power for the electronic components. During the day, the
solar panel supplies energy to the two processing modules and
recharges the battery, whereas at night, the battery supplies energy to the system [12]. A capacitive humidity sensor is connected with one processing module, and the relative humidity
(RH) is recorded every hour and stored together with the leakage
current attributes described next.
C. Leakage Current Signal
During the process of pollution deposition over the insulator’s
surfaces, many ionic and nonionic pollutants adhere to the surface. When the humidity increases, an ionic solution is produced on the surface and electric leakage current is generated.
The Joule effect, associated with the nonuniform electric current
density, evaporates parts of the ionic solution on the surface in a
nonuniform fashion, and in turn, creates dry bands. The nonuniform conductivity of the surface, due to conductive and partially
conductive regions generates charge separations over small gaps
that may lead to partial discharges [13]. The rate and the intensity of the partial discharges increase with the continuous deposition of pollutants and, if no action is taken, a flashover occurs
causing a line fault.
Fig. 3. Typical half cycle of the leakage current waveform showing conduction
and ionization components.
As a result of partial discharges, the leakage current has a sinusoidal component called the “conduction current” and a stochastic component that we refer to as the “ionization current.”
A typical waveform exhibiting these components, measured on
the field on a polluted insulator of a 500-kV field tower, is illustrated in Fig. 3. Only the positive cycle of the leakage current
waveform is shown in Fig. 3 because the fiber-optic sensor only
detects half a cycle of the waveform.
Previous experimental investigations in the field [12] have
shown that the conduction current saturates near 1 mA and is
independent of the pollution severity on the insulator, in turn,
not having any correlation with the partial discharges. Based
on laboratory and field experiments, three levels of ionization
current peaks were elected to classify the frequency of partial
discharges, namely the current levels of 5, 10, and 20 mA, associated, respectively, with level 1, the upper limit for low intensity leakage current, level 2, the upper limit for nonaggressive
leakage current and level 3, the upper limit for highly aggressive leakage current. When the rate of peaks having amplitudes
larger than level 3 starts to increase, maintenance action is required immediately.
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 2, APRIL 2009
Fig. 4. Sensor locations in the northeast region of Brazil.
III. LEAKAGE CURRENT ACTIVITY
In order to study the combined effect of pollution and humidity on high-voltage insulators of CHESF’s transmission-line
system, three towers located across three states in the northeast
region of Brazil were chosen due to their particular environmental characteristics. Two of these towers were within substations, and the other was located on the field. These locations
are shown on the geographical map in Fig. 4. In each substation tower, two sensors were installed—one on a suspension
string and the other on an anchorage string. In the field tower,
located on the Maranguape mountain in the state of Ceará, two
sensors were installed on two anchorage strings on each side
of the tower, with one string having the pin side of each insulator pointing to the littoral and the other having the pin side
of each insulator pointing to the country side, as illustrated in
Fig. 2. This was done in order to study the differences between
the leakage current activities due to preferential wind flow on
either the cap or the pin side of each insulator.
Fig. 5. Leakage current activity of an anchorage string in the Mossoró substation.
A. Mossoró Substation
The 230-kV Mossoró substation is located in the state of Rio
Grande do Norte and is in a region with a semiarid tropical climate with seven to eight rainless months per year. There are
many salt mines just a few kilometers from the Mossoró substation and the salt dust thus produced is carried by the wind to
the substation. The dry climate favors the pollution concentration on the insulator’s surfaces, and many washing procedures
are needed during the year to avoid line faults.
Figs. 5 and 6 show the activity and RH recorded during a
three-month period on the anchorage and suspension insulator
strings, respectively. The sensors were installed a few hours before scheduled washing. The location of anchorage strings, in
general, makes it difficult for the operator to perform a complete
wash of the pin side of the insulator closest to the high-voltage
cable and it is expected that anchorage strings begin to exhibit
partial discharges earlier than suspension strings, because the
Fig. 6. Leakage current activity of a suspension string in the Mossoró substation.
position of the latter in the tower favors a more complete wash
by the operator.
As shown in Fig. 5, it is interesting to note the range of RH
during the measurement period that oscillates between about
30% during the day and 80% at night. Only 15 days after the
insulators had been washed, the anchorage string exhibits partial discharges with ionization current peaks above level 1. The
pollution deposition continues and 20 days after the washing,
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OLIVEIRA et al.: LEAKAGE CURRENT ACTIVITY ON GLASS-TYPE INSULATORS
the first peaks larger than level 2 are registered. Approximately
45 days after being washed, the anchorage begins to exhibit activity above level 3. Without proper action, this process evolved
to a possible flashover event. However, nearly 50 days after the
wash, a heavy rain occurred, providing a natural wash on the
insulators, a fact that was readily detected by the monitoring
system. This rain marks the start of the rainy period of the year
on that location and on the next 40 days of observation, only
isolated activity was registered.
The activity during the same period registered for the suspension string on the same tower is shown in Fig. 6. This string
showed more activity than the anchorage string before washing,
as can be noticed on day zero of the three plots of Fig. 6. It
would then be expected that some time after washing, the pollution deposition on this suspension string started to exhibit a
higher level of leakage current relative to that of the anchorage
string. This did not occur, however, as one can notice from a
comparison of the plots of Figs. 5 and 6, the latter representing a
slower pollution deposition on the suspension string. A number
of factors could cause this modification in behavior, including
wind direction changes occurring prior to the rainy season or
even the more efficient washing that can be performed on the
suspension string, in turn, yielding a slower onset of leakage
current activity as discussed earlier. Near the 50th day of investigation, the heavy rain occurs, and different from the anchorage
string, isolated, but strong partial discharges were registered on
the suspension string. This may be due to the fact that for the
suspension string under rain, at the very beginning of the rain,
water drops fall across the string and salt contaminants dissolve,
producing a conductive water stream along the string, thereby
increasing the probability of partial discharges. The same activity occurs on the light rain event registered a few days later,
but with less intensity.
In Figs. 5 and 6, the RH of the first 60 days of observation
is plotted. On substation environments, the RH acquisition was
not automated and the operators registered it manually. The RH
had regular behavior during the entire period of investigation,
and is characterized by two local maxima—one at 6:00 P.M. and
another near 4:00 A.M. After the 60th day, the onduty operator
at the Mossoró substation stopped registering the RH levels.
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Fig. 7. Leakage current activity of an anchorage string in the Angelim substation.
B. Angelim Substation
The 500-kV Angelim substation is located in the state of Pernambuco within a micro semihumid tropical climate with four to
five dry months during the year. This micro climate is due to the
local altitude and during the humid months, fog is very common
at dawn. In this region, the pollution is mainly composed of dust
from the wind by the agricultural activities around the substation.
The activities of an anchorage and a suspension string from
the Angelim substation are shown in Figs. 7 and 8, respectively.
As shown in Fig. 7, there was no activity on the anchorage string
during the three-month period. Notice in Fig. 7 that, during the
first days, the average RH was low, and with the time passing,
it rose, yielding intermittent condensation points on an almost
daily basis.
The suspension string, on the other hand, registered intense
leakage current activity, but only above levels 1 and 2, as illus-
Fig. 8. Leakage current activity of a suspension string in the Angelim substation.
trated in Fig. 8. There is a lack of activity between the 60th and
the 85th days, probably due to rain that performs a natural wash
on the strings.
Comparing Figs. 7 and 8, it is possible to conclude that there
was a negligible pollution deposition on the anchorage string,
and there was significant pollution deposition on the suspension
string. This difference on deposition degrees between strings is
under investigation but it may be due to the daily water droplet
formation, performing a more efficient natural washing on the
anchorage string relative to that of the suspension string.
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826
IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 2, APRIL 2009
Fig. 9. Leakage current activity of an anchorage string on the Maranguape
mountain. In this string, the wind hits the cap side of the insulators during most
of the year.
C. Maranguape Mountain Tower
The Maranguape Mountain is located near the city of Fortaleza, the capital of Ceará state, and is approximately 800 m
above sea level. It is 40 km from the coast but the strong wind
carries the salt ocean breeze to this location. The tower is located inside a tropical-humid micro climate region with only
one to three dry months per year.
The suspension string on the investigated tower is composed
of polymeric insulators and the two sensors were installed on
two glass-type anchorage strings, as illustrated in Fig. 2. One
of the anchorage strings holds the cable that comes from Sobral
City, located on the countryside, and the other holds the cable
that goes to Fortaleza City, which is on the coast. The string on
the Fortaleza side has the pin side of the insulators exposed to
the preferential wind flow direction, and the string on the Sobral
side has the cap side of each insulator exposed to the preferential
wind direction.
Due to battery malfunction, only 20 days of activity were
registered in the Maranguape mountain tower, as illustrated in
Figs. 9 and 10. A direct comparison of these plots indicates that
the Sobral side anchorage string exhibits slightly more intense
activity than the string of the Fortaleza side. This indicates a
higher degree of pollutants accumulation on insulators having
the cap side placed against the preferential wind flow direction from Fortaleza to Sobral, an effect that may be related to
wind vortices formed on the pin side of insulators under this
wind-flow condition, as observed in previous studies reported
in the literature [2], [4].
It is important to note in the plots of Figs. 9 and 10 that there
are intervals in which the humidity remains at 100% and the
leakage current activities oscillate between high and low values.
This is very likely due to water condensation on the humidity
Fig. 10. Leakage current activity of an anchorage string on the Maranguape
mountain. In this string, the wind hits the pin side of the insulators during most
of the year.
sensor, thus producing a saturated value of the humidity reading,
not reflecting the actual RH of the environment. In fact, it appears that a heavy rain took place some day after the 14th day,
and the malfunction following this event may be due to a short
circuit that caused some damage to the electronic parts within
the metal cabinet.
IV. CONCLUSION
The leakage current activity registered during an approximately 90-day period on six insulator strings located on three
towers of CHESF’s energy distribution system in Northeast
Brazil were studied. The correlation of leakage current peak
rates with RH and seasonal and geographical conditions was
discussed. It was noticed that the pollution deposition depends
strongly on the type of insulator string, on the alignment of the
string relatively to the wind flow, on the location of pollution
sources in the neighborhood of the tower, as well as on the
efficiency of washing. Improvements of the monitoring system
are underway to enable increasing the observation period, thus
allowing more conclusive correlation results to be obtained.
In the present optical sensor system configuration, records of
the leakage current activities are obtained by periodic data transfers from the processing modules to a desktop computer, for the
case of substation systems. For the case of the field-operated optical system, an operator has to exchange processing modules
on a monthly basis to retrieve the records of the leakage current activities. Problems with malfunction of processing modules in the field due to severe weather conditions are being addressed at the moment. In addition, a remote telemetry system
is under development to allow operators in CHESF’s general
headquarters to monitor leakage current activities and humidity
levels on a sensor network deployed across specific points of the
company’s transmission-line system in Northeast Brazil. This
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OLIVEIRA et al.: LEAKAGE CURRENT ACTIVITY ON GLASS-TYPE INSULATORS
telemetry system will allow to better predict when insulator
washing is needed, thus improving maintenance planning and,
in turn, reducing operation costs. Use of the system will also
enable further studies of the leakage current level and attributes
on insulator strings of other materials, including ceramic or even
polymer materials. Further results of this development will be
reported elsewhere.
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827
Sérgio Campello Oliveira (S’08) was born in
Recife, Brazil, in 1977. He received the B.Sc. and
M.Sc. degrees in electrical engineering from the
Federal University of Pernambuco (UFPE), Recife,
Brazil, in 2002 and 2004, respectively.
Currently, he is a Doctorate Student with the
Photonics Group of the Electronics and Systems Department at UFPE and a permanent Faculty Member
of the Department of Computational Systems of
the University of Pernambuco, Recife, Brazil. His
research activities are in the fields of fiber sensors,
optical networks, and DWDM technologies and he is currently engaged in
research and development of optical sensors for high-voltage equipment and
systems as well as optical sensors based on the surface plasmon resonance
effect.
Eduardo Fontana (M’08) was born in Rio de
Janeiro, Brazil in 1957. He received the B.Sc. degree
in electrical engineering and the M.Sc. degree in
physics from the Federal University of Pernambuco,
Recife, Brazil, in 1980 and 1983, respectively, and
the Ph.D. degree in electrical engineering from
Stanford University, Stanford, CA, in 1989.
He has been a permanent Faculty Member with the
Electronics and Systems Department, Federal University of Pernambuco (UFPE), Recife, Brazil, since
1980. From 1991 to 1992, while on leave from UFPE,
he served as a Senior Scientist at Adelphi Technology, Inc., Palo Alto, CA, coordinating research and development activities in the field of surface plasmon
spectroscopy. His past research activities have also included microwave ferrite
devices, low-temperature magnetic materials, magnetic semiconductors, optics,
and free-electron lasers. He is currently a member of the Photonics Group of the
Electronics and Systems Department at UFPE, engaged in research and teaching
activities in the fields of electromagnetic theory and applications, bioelectromagnetism, optical instrumentation, optical-fiber sensors and devices, and surface plasmon spectroscopy applications.
Prof. Fontana is the Financial Director of CAESER (Advanced Center of Engineering and Services of Recife) and is a Research Fellow with the National
Research Council of Brazil (CNPq). He is a member of the following societies:
Optical Society of America (OSA), Brazilian Microwaves and Optoelectronics
Society (SBMO), and Brazilian Biomedical Engineering Society (SBEB).
Fernando José do Monte de Melo Cavalcanti was
born in Recife, Brazil, in 1957. He received the B.Sc.
degree in electrical engineering and the academic and
professional M.Sc. degrees in electrical engineering
from the Federal University of Pernambuco, Recife,
Brazil, in 1980, 2000, and 2004, respectively.
He is a Professional Engineer with Companhia
Hidro Elétrica do São Francisco—CHESF, Recife,
and is currently coordinating CHESF’s program for
widespread access to electrical energy in Northeast
Brazil. He has extensive experience in the maintenance of high-voltage transmission systems and is presently engaged in the
development and deployment of sensor systems for pollution monitoring of
insulator strings of overhead transmission lines.
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