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Investigation of the Influence of Leakage Current on
Wooden Poles for High Voltage Power Distribution Line
M. Al-Dabbagh Sachin Pathak
School of Electrical and School of Electrical and
Computer Science Engineering Computer Science Engineering
RMIT University – City Campus RMIT University - City Campus
Melbourne, Australia. Melbourne, Australia
majid@rmit.edu.au S3093072@student.rmit.edu.au
ABSTRACT
In Australia and many other countries of the world, short rain. Under these conditions, maximum numbers of
Power transmission lines on the distribution level are pole fires were reported by Powercor, Australia [1].
carried mainly on wooden poles. The deposition of Aiming to find the influence of leakage current through
pollution is an unavoidable phenomenon in all overhead the high voltage distribution line insulators on wooden
transmission lines. On a new insulator, which has a high poles, samples of 22kV pin type ceramic insulators and
surface resistivity, the leakage current is usually low and timber made wooden poles already used by powercor
primarily capacitive. When pollution builds up on the and Alinta in their distribution system were used for
surface and the insulator becomes wet by high humidity testing.
or rain, the resistivity reduces and hence leakage current
flows through the high voltage line through the wooden 2. H.V DISTRIBUTION INSULATORS
poles. This paper investigates the influence of leakage
current through high voltage distribution line insulators There are approximately 8 million wooden poles
on the wooden poles. The objective of the study is to throughout Australia that are in service for the power
understand the behaviour of wooden poles and to find distribution. In Victoria, almost 80% of the pole fires
the signature of the leakage current under various due to the leakage current flow through wooden poles
environmental conditions such as humidity, temperature are reported on the 22kV pin type distribution insulators.
and atmospheric pressure in order to establish the Recently, many of the power utilities in Australia are
pattern leading to the deterioration of the wood, which replacing these ceramic insulators with the silicon rubber
ultimately may lead to serious smoke or even a possible (non-ceramic) since silicon rubber offers good
fire. These findings will assist to better understand this performance in contaminated environment due to the
phenomenon for developing a possible early warning hydrophobic nature of silicon. It has been found from
system to avoid any power disruption and hence improve the service and laboratory research that silicon rubber
reliability of power supply. A special pollution chamber maintains its hydrophobicity for much longer time and
is used to conduct this research in collaboration with has the ability to regain its hydrophobicity faster than the
Power distribution companies in Victoria. other polymeric materials [2].
1. INTRODUCTION
For many years, the analysis of the failures in the
network of the various Australian Power utilities has
indicated that a number of these failures in distribution
systems are caused by insulator pollution leading to the
flow of leakage current through the wooden poles which
consequently lead to wooden pole fire. It should be
pointed out that in the past, most of the conclusions on
this subject were drawn from the statistics of outages on
the operating lines rather than from experimental
research. In the last decade, it became obvious that well
known methods used in Australia in order to reduce the
pollution which caused failures, were not satisfactory for
heavy polluted areas. This fact was a consequence of Fig. 1. Contaminated Insulator on wooden cross-arm
some specific conditions of the south-east area due to inside fog chamber in the H.V Research Laboratory at
heavy fog conditions in the morning periods followed by
RMIT.
3. CONVENTIONAL MAINTENANCE
PROCEDURES
Insulator washing is one of the common and effectively Insulator
Voltage to Optical
used practice to maintain the insulting properties of O/V
Protection Frequency
Converter
Transmitter
Connector
insulators. To reduce the frequency of insulator AD 654 HFBR1412
cleanings and therefore cost, second common measure is Copper
Ring
Wooden
Pole Sphere Gap
to increase the leakage distance of the insulation by
either adding additional insulators or by replacing the Optical Transmitter Ckt
insulators with longer leakage path designs. Although R = 100ohm
this measure is a cost effective one, it is one that cannot Optical Fibre Cable
be implemented very often. It is also one that still
requires regular cleaning; albeit, at reduced frequency. Optical Receiver Ckt
The third method is the use of Aerodynamic profiles to
minimize the accumulation of contamination on the
Phase locked Optical
insulator surface as their surfaces are more successfully Data Acquisition
loop LM 565 Receiver
& LM 104 Connector
cleaned by wind and rain which helps in reducing Computer
Card
HFBR2412
contamination flashover but this method eliminates the
under sheds in the insulators which help in increasing the
creepage distance without adding to the overall string Fig. 2. Schematic Representation of Monitoring System
height. The fourth method is the use of glaze in an
attempt to maintain a large dry surface area of insulator
during natural wetting. The main concern with the The frequency of the modulated wave is proportional to
resistive glaze is maintaining the boundary region the RMS value of the leakage current and is fed to the
between the conductive glaze component and the metal HFBR 1412 transmitter connector which communicates
end caps. The most commonly applied remedial measure with the HFBR 2412 receiver connector. In the receiver
remains greasing whereby the insulators are coated with circuit, a phase locked loop (LM 565) is used to
a silicone or hydrocarbon grease. The main problem demodulate the received modulated wave from the
associated with the use of greases is that they become transmitter. The demodulated signal is then amplified by
loaded with pollutant and lose their effectiveness over a operational amplifier (LM108) and is connected to the
period of time. data acquisition card through to the computer. The
sensitive electro-optic circuit require protection against
the occasional flashover of the insulator. This is
4. MONITORING SYSTEM achieved by using a combination of sphere gap of sphere
diameter 25cm and two zener diodes (IN4148).
The monitoring system for transmitting, receiving and
analysing the data is made up of transmitter circuit, A NI PCI-6024E, multifunctional plug-n-play, 12 bit
receiver circuit, optical fibre cable, data acquisition card analog to digital converter with 8 channel input data
and computer. The schematic representation of acquisition card is used for signal acquisition and
monitoring system is shown in fig. 2. A highly effective measurement analysis. This data acquisition card is
optical fibre transmitter and receiver circuit connected directly controlled by the LabVIEW software. A 54624A
by 62.5/125 µm optical fibre cable is developed to Oscilloscope model with a sampling rate of
transmit the signal from the HV lab to the control room 200MSamples/sec was used to view the leakage current
for monitoring the leakage current on the computer. The waveforms. A digital Video camera was also used to
optical fibre communication is used because of its continuously monitor the test sample inside the chamber.
excellent immunity to electromagnetic interference,
noise and provides high optical isolation between HV
equipment and the computer. The high power ST-type 5. EXPERIMENTAL CONFIGURATION
HFBR-1412 transmitter used in the circuit can launch -
12 dBm optical power and is designed to operate with The experimental set-up consists of equipment in control
the ST-type HFBR-2412 receiver. Distances of up to 4.7 room and HV laboratory and is shown in fig. 3. In the
kilometres, and data rate of up to 5 megabaud are control room, voltage control circuit variac, ammeter,
attainable with the chosen fibre optic equipment and voltmeter, multimeter, receiver circuit and PC linked
cable. The leakage current is collected by a copper ring with Data Acquisition Card were used. Ammeter and
fixed at the centre of wooden pole. A 100ohm resistor is Voltmeter were used to measure the very small current
inserted between the wooden pole and ground. The drawn and voltage across the variac respectively. The
voltage drop across the resistor is converted into a train multimeter was connected across the voltage divider to
of frequency modulated square waves using a V/F measure the line voltage through the HV conductor
converter. connected to the insulator. Any over voltage or
breakdown will operate the sphere gap and hence protect drilled through the centre of wooden pole and a copper
the integrated transmitter circuit from damage. sheet is tightly rolled around it with the help of washel
for collecting the leakage current through wood. The
collected current is then connected through the resistor
to the optical transmitter. A well known technique of
CB
A
Conductor inserting a resistor between wooden pole and ground is
Computer
used, to measure a voltage drop across it. This voltage
Resistor Bank T/F
drop is proportional to the leakage current flowing
DAQ through the wooden pole.
Pollution Chamber
V
Vo lta g e D iv id er
Nozzles
1-Phase 415V
variac 22kV Optical
AC
Receiver The measurement results obtained from the tests in the
Ckt
High Voltage Research laboratory at RMIT are shown in
Fig.4 to Fig.6. The graph in Fig.4 corresponds to
Optical
relatively low leakage current flowing through the wood.
Fountain R = 100ohm
Sphere gap Transmitter
Ckt The applied voltage of 21.3kV was measured across the
pump
insulator. For this test, the temperature of 16.56 0 C and
Air Regulator
relative humidity of 93% were recorded by respective
sensors.
Fig. 3. Experimental setup in Control room and HV
laboratory Y- Axis X- Axis
10.0 msec
Time : 15: 11:49
Inside the HV laboratory, a 66kV, 50kVA, single phase 1.0 µA
transformer, voltage divider to measure the line voltage, Leakage Current
Pulses
sphere gap to protect the equipment from breakdown,
resistor bank to limit transformer primary current,
pollution chamber and transmitter circuit were used. A
special cylindrical shaped pollution chamber with
1950mm sides and 1690mm diameter was used to
simulate the natural environmental pollution conditions
in the laboratory in accordance with the IEC 60507 [3].
The ALX-09 type external mix-round brass spray
nozzles were used to inject the fog inside the chamber. A Fig. 4. Leakage Current waveform at 21.3 kV applied
4W fountain pump was used to pressurise the water to voltage
obtain the uniform fog distribution around the whole
length of insulator and wooden pole. The fog is After few minutes, the applied voltage was increased to
developed by eight nozzles located symmetrically 22.5kV, a sudden change in the leakage current
around the chamber and aimed to spray towards the waveform pattern was observed as shown in Fig. 5. This
centre at an angle of 20 0 from the horizontal. A change is attributed to the arcing on the surface of
temperature-humidity sensor was mounted on the wall of insulator to the wooden pole which was recorded by the
fog chamber with the sensor lead going inside the video camera. The relative humidity inside the chamber
chamber to record the temperature and humidity. For the was recorded as 95%.
sphere gap operation, the distance between the spheres is
kept at 1mm. Y-Axis
Div
X- Axis
Div
10.0 msec
2.0 µA Arcing Period
6. TEST PROCEDURE AND RESULTS
Test objects consisted of 22kV pin type ceramic
insulator, wooden crossarm and pole. The contamination
slurry was prepared by mixing 40 g of kaolin to a litre of Leakage Current
Pulses
water and then adding the required amount of sodium
chloride salt [4]. It should be mentioned that wetting
agents were not used in the slurry preparations as
suggested in the IEEE Standard 4 [5]. Prior to the
testing, insulator was dipped in the slurry and then dried
in ambient air for up to 24 hours. It should also be noted Fig. 5. Leakage Current waveform at 22.56 kV applied
that both wooden pole and crossarm were kept in water voltage
for 14 hours to simulate a typical Melbourne weather.
A further change in the pattern of leakage current was
The insulator was mounted on the wooden crossarm of
observed when the applied voltage was increased to
dimensions 950mm×100mm×100mm with the help of
24.1kV and is shown in Fig.6. At this voltage the
bolt of 14mm diameter. Tie wire was used to tighten the
insulator started sparking to the wooden pole. The
HV conductor to the insulators. The centre of crossarm
humidity was recorded as 95.52%.
is fixed to the wooden pole with the help of bolt of
14.5mm diameter. Another bolt of same diameter was
[3] “Artificial pollution tests on High Voltage Insulators
Scales
to be used in the ac systems”’ Publication IEC
Div Div
Y- Axis X-Axis 60507.
Time: 15:19:14
Leakage Current
10.0 msec
Pulses [4] P.J. Lambeth and H.M. Scheider, “Final report on
1.0 µA
the clean fog test for HVAC insulators”, IEEE
Transaction on Power Delivery, Vol. 2 No. 4, pp
1317-1326, 1987.
[5] “IEEE standard technique for high voltage testing”
Std 4, 1995.
Fig. 6. Leakage current waveform at 24.1kV applied
voltage
7. CONCLUSION
The investigations of wooden poles used in the
distribution systems indicate that the problem needs
further studies. From the preliminary analysis and
experimental work under 22 kV voltage conditions, it is
obvious that under certain humidity (close to 90%) and
temperature, the insulator can no longer remain in
perfect isolation from the grounded wooden pole. It was
observed that as the voltage increase within the limits of
voltage regulation, different levels and waveforms for
leakage current were observed. Preliminary recorded
waveforms of these results are shown in Fig.4 to Fig.6.
These preliminary results show that when sparks occur
on the insulator to the wooden surface, the phenomenon
produces spikes in the leakage current waveform.
It was concluded that further detailed investigations are
to be conducted using the developed experimental set-up
for insulators on wooden crossarm to understand their
behaviour under different conditions., in order to have
some understanding of the signature of the leakage
current for possible prevention of further deterioration.
8. ACKNOWLEDGEMENTS
We would like to express our deep appreciation and
gratitude to Powercor, United Energy and Citipower for
supplying test samples for the project. We would also
like to thank David Welch, Ivan Kiss and Sinisa
Gavrilovic for assistance during the development of the
experimental work.
9. REFERENCES
[1] “Annual report on Distribution networks”, Powercor
Australia, 2004
[2] A. De La O, R. S. Gorur and J. Chang, “AC clean
fog tests on non-ceramic insulating materials and a
comparison with porcelain”, IEEE Transaction on
Power Delivery, Vol.9, No. 4, October 1994.
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