Investigation of the Influence of Leakage Current on Wooden by kvw36946


									       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

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].


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

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
insulators. To reduce the frequency of insulator                                                                            AD 654           HFBR1412

cleanings and therefore cost, second common measure is         Copper
                                                                           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

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

                                                                                                              Conductor                            inserting a resistor between wooden pole and ground is
                                                                                                                                                   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
                                                            Vo lta g e D iv id er
     1-Phase                            415V
      variac                                         22kV                                                                                Optical
                                                                                                                                        Receiver   The measurement results obtained from the tests in the
                                                                                                                                                   High Voltage Research laboratory at RMIT are shown in
                                                                                                                                                   Fig.4 to Fig.6. The graph in Fig.4 corresponds to
                                                                                                                                                   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
                                                                                                                                                   insulator. For this test, the temperature of 16.56 0 C and
                                                                                              Air Regulator
                                                                                                                                                   relative humidity of 93% were recorded by respective
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

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
                                                                                                                                                                                                                        X- Axis

                                                                                                                                                                              10.0 msec

                                                                                                                                                                  2.0 µA                                    Arcing Period


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

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
                                                                                  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

 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.

To top