AC

HV Resistor

VoltageSupply

50kVATransformer

Voltage Divider1000:1 Metal Cap

Cross Arm

Aluminium Plate

Wooden Pole

King Bolt

Ammeter

Figure 1. Experimental set up

Thermographic Study of Stainless Steel Cross-arm on Overhead Distribution System

M. F. Rahmat School of Electrical and Computer Engineering

RMIT University Melbourne, Australia

s3172130@student.rmit.edu.au

K. L. Wong School of Electrical and Computer Engineering

RMIT University Melbourne, Australia

alan.wong@rmit.edu.au

Abstract Many new innovations in the pole top equipment were brought onto the market in recent years. A good example of that is the stainless steel cross-arm that is designed to replace the conventional wooden cross-arm for pole-top fire mitigation. The main objective of this study is to compare the thermal characteristics on both the stainless steel cross-arm and the wooden cross-arm at different leakage current levels. Our results show that the stainless steel arm has similar heat dissipation property at low current level of 10mA compare with wooden cross-arm. However, at a current level of 90mA, the build-up of heat causes the king bolt junction to heat up dramatically and smouldering of the wooden pole was observed. The time-lapse thermographic images of the heating process are presented. This paper also presents a low-cost solution of which the leakage current at the king bolt junction can be reduced.

Keywords-cross-arm, leakage current, wooden pole, thermal images

I. INTRODUCTION Wooden cross-arm is being used around the world as the

preferred support structure for insulators in the electrical distribution networks. Wooden cross-arm offers excellent electrical and mechanical performance and is known to have a service life span of more than 20 years [1]. In recent years, many of the existing wooden cross-arms were replaced by stainless steel cross-arm or fiber glass cross-arm across the world. This is due to the fact that wooden cross-arm is prone to pocket burning or fire in areas that are subjected to high level of pollution, dust or salt spray. In the coastal area where heavy salt spray from the sea is high, the contamination layer on the insulator surface degrades the surface resistivity. It is reported that the leakage current flow due to surface contamination can reach hundreds of mili-amperes [2-5] and causes the spot burning and pole-top fire.

Stainless steel cross-arm is often installed in systems where the mechanical stresses on the line are too high for any other alternatives [6]. In comparison to wooden cross-arm, the stainless steel has more superior mechanical strength but offers lower impulse level and insulation performance. In areas that suffer from heavy pollution level, the stainless steel has been a good alternative to wood.

The main objective of this paper is to investigate the thermal and electrical properties of stainless steel cross-arm on wooden pole. This paper will show the study of thermal

distribution in stainless steel cross-arm and comparing it with the conventional wooden cross-arm. The thermographic study is performed using the Thermovision A320 thermal camera measuring the spot temperature at the pole-cross-arm attachment point (king bolt junction) and the adjacent surfaces for 60 minutes duration. Our results show that both types of cross-arms have similar heat dissipation property at low current level of 10mA. However, when the leakage current level reached 90mA, the build-up of heat causes the king bolt junction to heat up dramatically and smouldering of the wooden pole was observed. The time-lapse thermographic images of the heating process are presented. As part of this study, the thermographic properties of the wooden cross-arm and a low-cost current diverter that successfully reduce the leakage current at the king bolt junction are also presented.

II. EXPERIMENTAL SYSTEM In this paper, the leakage current produced by the resistive

components of the insulator surface is simulated using high voltage resistor connected to a high voltage power supply. As depicted in Figure 1, a high voltage supply of 11kV is

Figure 2. Wooden cross-arm

Figure 3. Steel cross-arm

connected in series with a 2M and a 100k resistor to simulate polluted insulators that suffered from medium and severe surface pollutions. The current was measured at the primary side of the transformer and by using the transformers transformation ratio, the leakage current level of 10mA and 90mA, which represent the current at the secondary side was calculated.

The Thermovision A320 thermal camera was used to measure the spot temperature on the surface of the king bolt, the cross-arm and the pole. The A320 is designed to deliver accurate thermographic imaging and repeatable temperature measurements in a wide range of applications. Each thermal image is built from 76,800 individual picture elements that are sampled by the camera's on-board electronics and software to measure temperature. The thermal camera is installed at a one meter distance from the test object and the near-real-time 16-bit 320x240 image data are recorded and stored at 10 minutes interval during the test.

In this paper, a two-meter length hardwood utility pole was used in the experimental study. As shown in Figure 2, the wooden pole has a diameter of 220mm. The pole under test is a CCA-treated hardwood pole to prevent termites from attacking. Two different types of cross-arms were used in this study. The wooden cross-arm is an untreated hardwood of 2m x 0.1m x 0.1m. This type of wooden cross-arm is commonly found in power distribution network of 11kV to 22kV around Australia.

The stainless steel cross-arm which is depicted in Figure 3 is made of a 3mm thick galvanised steel. There are 5 holes that are pre-drilled for insulators and king bolt attachment.

As part of this study, a low-cost current diverter was introduced to the experimental set-up. The purpose of the current diverter is to provide a low resistive path for the leakage current to flow from the source of leakage current (e.g. metal pin of the insulator) to the middle section of the pole. A specially designed metallic band is installed on the surface of wooden pole. Its acts as an earthing that diverts the leakage current away from the king bolt junction and also as a heat sink.

III. RESULTS There are many studies in the past which focused on the

leakage current level on the high voltage insulators. However, there is a lack of knowledge in the heating process of the supporting structure due to high level of leakage current. Many past researches suggested that the heating process was

(a) 0 minute (b) 10 minutes

(c) 20 minutes (d) 30 minutes

(e) 40 minutes (f) 50 minutes

(g) 60 minutes

Figure 4. Thermographic images for wooden cross-arm at 10mA

(a) 0 minute (b) 10 minutes

(c) 20 minutes (d) 30 minutes

(e) 40 minutes (f) 50 minutes

(g) 60 minutes

Figure 5. Thermographic images for wooden cross-arm at 90mA

mainly contributed by the micro-arc that took place at the metal-wood interface in the insert. The following results are the time-lapse images taken by the thermal camera for both cross-arms at 10mA and 90mA.

A. Wooden cross-arm with leakage current of 10mA and 90mA Figure 4 and Figure 5 illustrate the heating process at the

wooden cross-arm. The spot temperatures that are shown in these figures represent the temperature at the metallic surface of the king bolt (spot 1), temperature at the wood surface (spot 2), temperature at the cross-arm and pole interface (spot 3) and temperature at surface of the cross-arm (spot 4). Temperature measurement at spot 1 is the most critical in this study. The initial temperature at spot 1 was 23.7oC before the 10mA current is being applied. When the 11kV high voltage supply is switched on, the temperature at spot 1 increased to 25.2oC in a period of 10 minutes and finally reaches 29.4oC at the end of the test period. On the other hand, the temperatures at spot 2, 3 and 4 did not experience the same level of temperature rise and

the final temperature readings are 25.8oC, 23.1oC and 23.1oC respectively. A maximum change of 5.7oC was observed at spot 1 after 60 minutes test.

In the case of 90mA current, the temperature at spot 1 increases from the initial value of 23.4oC to 35.0oC after a 10 minutes period. The temperature continues to rise sharply to 50.1oC after 20 minutes and finally a maximum temperature of 75.1oC was recorded at spot 1 at the end of the test period. The temperatures at spot 2, 3 and 4 also show some changes as depicted in Figure 5. A maximum change of 51.7oC was observed at spot 1 after the 60 minutes test.

B. Stainless steel cross-arm with leakage current of 10mA and 90mA Figure 6 and Figure 7 depict the time-lapse thermographic

images of the heating process of stainless steel cross-arm at 10mA and 90mA. A similar 2M and 100k current limiter were used with 11kV voltage supply to simulate the leakage current level. The temperature at spot 1 increases from 23.5oC

(a) 0 minute (b) 10 minutes

(c) 20 minutes (d) 30 minutes

(e) 40 minutes (f) 50 minutes

(g) 60 minutes

Figure 6. Thermographic images for steel cross-arm at 10mA

at 0 minute to 25.4oC in a 10 minute period. This spot temperature grows gradually to 31.1oC in the 60 minutes testing period. A maximum change of 7.6oC was observed at spot 1 after 60 minutes test. In contrast, for the leakage current level of 90mA, the temperature reading at spot 1 increased sharply from an initial temperature of 23.8oC to 47.9oC after 10 minutes. The temperature of the king bolt changes rapidly to 62.3oC after 20 minutes and reaches 76.3oC at the end of the 60 minutes test. A maximum change of 52.5oC was observed at spot 1 after the 60 minutes test.

IV. LOW-COST CURRENT DIVERTER SYSTEM A low cost current diverter system for reducing the heating

effect of the king bolt is presented . The current diverter system was installed on 6.5 meter hardwood pole CCA treated as shown in Figure 8. The current diverter system consists of insulated cables and a specially designed metallic band that is installed at the middle section of a pole. The metallic band is made of an alluminium sheet that has 21 fins pointing outwards for heat dissipation purpose and it is located at 5m from the top section of the wooden pole. Nails were applied to achieve maximum physical contact between the pole and the metallic band.

Figure 9 shows the heating process for the steel cross-arm over a two hour period. The temperature readings were taken from spot 1 and it is clearly demonstrated that the low cost current diverter reduced the temperature by 0.6 oC. As shown in the figure, the king bolt temperature increased from 23.0oC to 24.4oC for the set-up without current diverter system during the two hour test period. This is equivalent to a change of 1.4oC. In contrast, the king bolt temperature raised slowly from 23.3oC to 23.8oC and a change of 0.5 oC is observed. The current diverter system successfully reduced the heating effect at the king bolt junction.

V. DISCUSSIONS This study allows us to compare the performance of a

wooden cross-arm and a stainless steel cross-arm at different leakage current levels. When the pole is subjected to low leakage current level of 10mA, both types of cross-arms show similar heating process. From our results, it is clearly shown

Figure 8. Low-cost current diverter system

(a) 0 minute (b) 10 minutes

(c) 20 minutes (d) 30 minutes

(e) 40 minutes (f) 50 minutes

(g) 60 minutes

Figure 7. Thermographic images for steel cross-arm at 90mA

0 20 40 60 80 100 12023

23.2

23.4

23.6

23.8

24

24.2

24.4

Time (Minutes)

Tem

pera

ture

(Ce

lciu

s)

nobypassbypass at 5m

Figure 9. The thermographic images for steel cross-arm

with 100k current limiter

Figure 10. King bolt temperature for steel cross-arm and wooden cross-arm

that the metallic king bolt experienced the highest temperature increase during the test period. This is due to the fact that the king bolt has a higher thermal conductivity which conducts more heat compared to wood. On the other hand, the two cross-arms have gone through a very different heating process when the pole is subjected to a higher leakage current. The heating rate of the king bolt at steel cross-arm is much higher compared to a wooden cross-arm as shown in Figure 10. In term of reducing the risk of pocket burning or pole-top fire, our results has shown that the steel cross-arm has no obvious benefits to wooden cross-arm under medium or severe leakage current conditions. During the two hour test, smoldering of wood at the king bolt junction was observed when the temperature reached more than 70 oC. At this level, a scent of wood burning could be detected within the laboratory. As part of this study, we also demonstrated the effectiveness of the current diverter system. The low cost diverter managed to reduce the heating effect at the king bolt.

VI. CONCLUSION This paper investigated the thermal characteristic of the

steel cross-arm and the wooden cross-arm at different leakage current levels using highly accurate thermal camera. The leakage current flow due to the surface contamination on insulator increased the temperature at the king bolt to a

hazardous level on both cross-arms. High leakage current caused the king bolt to heat up at a more rapid rate than the low leakage current. The recorded thermographic images provide better accurate spot temperature reading of the heating process on the king bolt. In conclusion, the results in this study offers new insights into the initial stage of pole-top burning at the king bolt junction. The results suggest that the continuous monitoring of king bolt temperature could be included as a part of regular wooden pole inspection and asset management program. The proposed current diverter system also has a huge potential to reduce the heating effect of king bolt.

ACKNOWLEDGMENT We would like to acknowledge the assistance of our

technical staff, Ivan Kiss and Sinisa Gavrilovic for setting up the poles and cross-arms in HV laboratory.

REFERENCES

[1] R.A.C Altafim, J.F.R. Silva, H.C. Basso, C. Calil Junior, J.C. Sartori and C.R. Murakami, Study of timber cross-arms coated with castor oil-based polyurethane resin: electrical and mechanical tests, Conference Record of the 2004 IEEE International Symposium on Electrical Insulation 2004, pp. 556-559, Sept. 2004.

[2] C. Jehn-Yih and R.J. Chang, Field experience with overhead distribution equipment under severe contamination, IEEE Transaction on Power Delivery, vol.11, no.3, pp. 1640-1645, July 1996.

[3] E.A. Cherney, R. Hackam and S.H. Kim, Porcelain insulator maintenance with RTV silicone rubber coatings, IEEE Transaction on Power Delivery, vol.6, no.3, pp.1177-181, July 1991.

[4] S.A. Sokolovsky and V.G. Santotsky, Environmental impact on the insulation of 10kV distribution power lines, Conference on Electrical Insulation and Dielectric Phenomena, vol.1, pp.56-59, Oct. 1998.

[5] K. L. Wong and M. F. Rahmat, Study of Leakage Current Distribution in Wooden Pole using Ladder Network Model, to be appear on IEEE Transaction on Power Delivery.

[6] Anthony J. Pansini, Electrical Distribution Engineering, 3rd edition, Fairmont Press, 2006.

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