Investigation of the Influence of Leakage Current on Wooden
Shared 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 email@example.com 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 . 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 . 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 . 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 . It should be mentioned that wetting agents were not used in the slurry preparations as suggested in the IEEE Standard 4 . 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  “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  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.  “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  “Annual report on Distribution networks”, Powercor Australia, 2004  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.