N. Qaddoumi et al.: Detecting Defects in Outdoor Non-ceramic Insulators using Near-field Microwave Non-destructive Testing

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ABSTRACT This paper presents a novel near-field microwave nondestructive testing technique for defect detection in non-ceramic insulators (NCI). In this work, distribution class 33 kV NCI samples with no defects, air voids in silicone rubber, cracks in the fiberglass core and small metallic inclusion between the fiber core and shank were inspected. The microwave inspection system utilizes an open-ended rectangular waveguide sensor operating in the near-field at a frequency of 24 GHz. The used inspection system is simple, safe and relatively inexpensive. A data acquisition system was used to record the measured data. The results showed that all defects were repeatedly detected with high sensitivity. Line scans of the samples were obtained revealing the presence of different defects and their location. The technique also demonstrated ability to detect thickness variations in the silicon rubber shank.

Index Terms – Non-ceramic insulators, near-field microwave NDT.

1. INTRODUCTIONNON-CERAMIC insulators (NCI), particularly those

made of silicone rubber (SIR), are increasingly being used in both distribution and transmission systems. They exhibit several advantages over ceramic insulators. The low surface energy of SIR insulators maintains a hydrophobic surface that facilitates a good performance in polluted outdoor service conditions. Moreover, these SIR insulators cost less and weigh less than the heavy ceramic insulators. However, non-ceramic insulators still have certain disadvantages like aging, their unknown long-term reliability, and the difficulty in detecting defective insulators. Aging in SIR insulators has been studied extensively in the literature under both natural and accelerated aging conditions. Effect of chemical composition and insulator profile on the aging performance of SIR insulators has been thoroughly investigated [1-4]. In previous studies on NCI, little attention has been given to developing an effective technique to detect defects in this type of insulators. The presence of defects in such structures may lead to catastrophic failures in power industry. Mishandling of NCI by untrained workers may cause cracks in the fiber reinforced polymer (FRP) rod which may lead to disastrous failures in the overhead lines [5]. Another possible cause of

cracks in the FRP core is the over stress in the end metal fittings [6]. Also, the presence of conducting impurities in the rod or rod shed interface may cause flashover and lead to faults in the power system network. Moreover, internal macroscopic interfacial problems have been reported between the FRP rod or tube and the elastomeric shielding material leading to electrical problems in composite insulation [7]. Due to the nature and structure of NCI, their testing presents quite a challenge to the field of nondestructive testing and evaluation. The possibility of using different techniques to detect damage in NCI like elevated voltage application, insulation resistance, partial discharge and leakage current measurements have been examined [8]. All the previously mentioned tests suffer from sensitivity to any disturbance, limited to some defects and do not provide reliable results in the case of air voids and cracks in the FRG rod [ 8]. Infrared (IR) imaging has been used to detect damages in non-ceramic insulators using remote inspection [9]. However, IR technique can be applied only when the insulator is energized. Furthermore, the technology is susceptible to external influences such as wind and solar flux [10]. Defects in polymer surge arrestors leading partial discharge could be detected using electromagnetic radiation measurement system [10]. Similarly to what has been mentioned in [9], this technique is only suitable for on-line detection. In this paper a novel near-field microwave non-destructive testing technique (NDT) will be employed to detect different

Detecting Defects in Outdoor Non-ceramic Insulators using Near-field Microwave Non-destructive Testing

Nasser Qaddoumi, Ayman H. El-Hag, Mohamed Al Hosani, Ibrahim Al Mansouri and Hamad Al Ghufli

Department of Electrical Engineering American University of Sharjah, Sharjah, UAE

Manuscript received on 12 May 2009, in final form 30 June 2009.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 2; April 2010 403

defects in NCI. The ability of microwaves to penetrate deeply inside dielectric materials makes microwave NDT techniques very attractive for interrogating such materials.

2. THEORETICAL BACKGROUND

Microwave NDT techniques are based on measuring properties that would reveal information about the material dielectric properties and physical dimensions. Microwave techniques are based on transmitting a microwave signal into a dielectric structure and using the magnitude and/or phase information of the transmitted and/or the reflected signal to either create a two dimensional image of an object, get line scans or get a single point measurement. This can be done in a contact or non-contact fashion and either in the near-field or the far-field. Near-field approach uses simple probes such as open-ended waveguides and coaxial lines, whereas far-field approach requires an antenna for focusing the microwave energy. Furthermore, far-field testing approaches do not offer good spatial resolution since the footprint is relatively large. Therefore, focusing lenses are often used to remedy this problem. Measurements are conducted using either phase or amplitude information. Far-field techniques generally use amplitude information. Near-field testing is more versatile since phase information is easily produced and often contains more or complimentary information to amplitude information. Additionally, in near-field testing the foot print of a sensor is close to its physical aperture (e.g. waveguide opening). Generally, microwave NDT techniques have the following advantages:

• they are fast, reliable and relatively inexpensive, • the sensor may or may not be in contact with the surface

under examination , • the technique may be applied to curved and other

complicated surfaces, • the dimensions of a defect can be closely estimated, • defect orientation, and location can be determined, • no special operator skills, in the fields of microwaves or

signal interpretation, are needed for successful defect detection,

• the technique is environmentally compliant and operator friendly and safe,

• the required microwave power is in low milliwatt range, • such a system may be battery operated and portable, • the results are obtained in real-time, • the technique is neither a source of electromagnetic noise

pollution (interference), nor sensitive to external electromagnetic sources of interference.

An open-ended rectangular waveguide probe radiating into a stratified dielectric composite made of any number of layers backed by air is shown in Figure 1. For a generally lossy material, the dielectric property is represented by a complex parameter whose real part (permittivity) indicates the ability of the material to store microwave energy and the imaginary part (loss factor) indicates its ability to absorb microwave energy. Relative to the permittivity of free-space, the complex relative dielectric property of a material is denoted by ( rrr j ''' ).

b

Z = 0

Waveguide

oZ1

r1 rN

xz

y

d1

d2

dN-1 ZN-1

r2 rN-1

Figure 1. Cross-section of a rectangular waveguide radiating into a layered media terminated into an infinite half-space.

Open-ended rectangular waveguide probes have been used in most of the investigations that deal with nondestructive noninvasive inspections of materials. The mode of operation of a rectangular waveguide is dictated by the frequency of operation. Waveguides are usually used at frequencies allowing only the dominant mode to propagate. Considering the dominant TE10 mode incident on the rectangular waveguide aperture, the terminating admittance of the waveguide can be written as [11-16]:

dxdyyxeyxE

dxdyayxWyxEjBGY

2

apertureo

aperturez

0

00

),().,,(

ˆ.),,(),,( (1)

Where

on aperturennn ddeEyxhY

yxHyxW

0

0

),().,,(),(

),,(),( (2)

where E(x, y, 0) and H(x, y, 0) are the electric and magnetic aperture field distributions, respectively. The admittance expression is constructed using transverse vector mode functions and their orthogonal properties. The nth vector mode functions are en, and hn, and Yn is the characteristic admittance of the waveguide for the nth mode. a and b are the broad and narrow dimensions of the waveguide, respectively. Hence, the TE10 mode aperture field distribution is given by

apertureyx(0

apertureyx(ax

ab2

yxe0yxE 0y

),

),)cos(

),(),,((3)

The complex reflection coefficient at the aperture of the waveguide, is related to the terminating admittance of the waveguide, Y, by

(4) 11

YYe j

N. Qaddoumi et al.: Detecting Defects in Outdoor Non-ceramic Insulators using Near-field Microwave Non-destructive Testing404

The complex reflection coefficient is a complex quantity whose phase and magnitude variations can be calculated and measured for testing and assessment. The higher the variation in the phase and/or magnitude of the reflection coefficient due to the presence of a defect in the structure, the easier it is to detect it.

3. MATERIALS AND METHODS

Microwave nondestructive inspection techniques possess the ability to detect minute thickness variations in dielectric layers as well as slight dielectric property variations in stratified dielectric composites [11-16]. The presence of a disbond or delamination may be considered as an additional new thin layer of material in a structure. Microwave signals can penetrate inside low-loss dielectric materials and interact with their inner structure without suffering from high attenuation. They are also sensitive to changes associated with boundary interfaces, which makes them very attractive for thickness variation detection as well as disbond and delamination detection. Microwave NDT techniques do not require a couplant and may be applied either in-contact or non-contact fashion. When used in a non-contact fashion and in the near-field of a probe, the standoff distance may be used as an optimizing parameter to achieve increased sensitivity to the presence of defects (i.e., disbonds, delaminations, excess porosity, etc.). Microwave nondestructive inspection methods can also provide for on-line and real-time detection of disbond and delamination as well as other similar applications. Open-ended rectangular waveguide sensors have shown to be effective tools in inspecting stratified dielectric composite structures [11]. In this work, distribution class 33kV NCI samples with no defects and with defects including, air void in the silicone rubber, inclusion of small metallic particles and crack in the fiber glass core were inspected. The used microwave inspection system utilizes an open-ended rectangular waveguide operating in the near field region at a frequency of 24 GHz. The footprint of the sensor is equal to the opening of the waveguide (10.7 mm x 4.3 mm). A data acquisition system was used to record the measured data which is a voltage related to the reflection coefficient at the aperture of the waveguide. Measurements were conducted in contact and without contact between the sensor and the sample under inspection. The block diagram of the experimental setup is shown in Figure 2. A dc voltage is used to trigger a Gunn oscillator to produce a signal at 24 GHz that travels through a waveguide and interrogates the insulator sample. The reflected signal is picked up using the same waveguide and a diode detector is used to convert it to dc voltage that is amplified before it goes to the acquisition card.

4. RESULTS AND DISCUSSION To conduct the experimental study, NCI samples with and without defects were used. The open-ended rectangular waveguide based system transmits a microwave signal towards the specimen under investigation.

Figure 2. Block diagram of the experimental setup.

As the signal reaches the specimen a certain reflection, dictated by the dielectric properties of the material in front of the waveguide, takes place. The presence of defects changes the dielectric properties of the region within the sensing region and that causes a different reflection to occur. Thus, monitoring these variations reveals the presence of defects. To obtain line scans of the samples under investigation, the samples were mechanically rotated in front of the waveguide and data was recorded. The measurements reported in this paper were conducted at a frequency of 24 GHz and at a standoff distance of 2 mm. Figure 3 shows the measured dc voltage (related to the reflection coefficient at the waveguide’s aperture) as a function of position around the circumference of a non defective NCI. The gradual variation of the signal level is due to the variation of the thickness of the rubber around the rod. This signal was correlated with the physical dimensions of the rubber and the thicker the rubber became the higher the voltage measured. This measurement was repeated and was found to be very consistent for different samples. For the same sample, 30 readings for the thickness of rubber shank were taken using a caliper (a reading every 12o). Figure 4 shows a plot of shank rubber thickness measurement along its circumference. This result demonstrates that this technique may be used as a tool to monitor the thickness variation of the rubber shank or any other sheet of a dielectric material.

To mimic the presence of a metallic inclusion in a specimen, a thin metallic inclusion (1.5 mm wide) was inserted between the rod and the rubber casing in a sample at one location. This sample was scanned and the presence of the metallic inclusion was detected using the measured reflected voltage as shown in Figure 5. The sample was rotated twice to demonstrate the repeatability of the measurement. As shown on the figure a drastic change in both the shape and magnitude of the detected signal are observed. The presence and location of the defect are clear. As the waveguide scans over the edge of the metallic inclusion a drop occurs first then as it lines up in front of the waveguide maximum reflections occur resulting in the large peak.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 2; April 2010 405

Figure 3. Line scan of a sample with no defect at a standoff distance of 2 mm at 24 GHz.

Figure 4. Rubber thickness variations versus the shank circumference.

The influence of the rubber thickness variation may still be observed. Additionally, the lower values of the signal at 60 mm and 150 mm indicate that the metallic inclusion is seen at the opposite side of the sample. This clearly suggests that any variation in the structure throughout the NCI may be detected.

Another type of defects that may exist in NCI is an air void in the rubber shank. A sample with a 2 mm diameter air void in the rubber shank was tested. The measured voltage is shown in Figure 6. The presence and location of the air void may be clearly obtained from the recorded data. The results shown in Figures 5 and 6 suggest also that defect classification and shape are also possible using this technique since the signatures of the different defects vary.

The 50 Hz electric field along the surface of 33 kV line insulator with and without air void has been calculated using a finite element solver. The equipotential lines and the location of the air void are shown in Figure 7. The electric field 0.01 mm from the insulator surface has been calculated with and without air void and depicted in Figure 8. It is evident from Figure 8 that the existence of the air void has altered the electric field distribution along the insulator surface near the void location.

Figure 5. Line scan of a sample with a metallic inclusion.

Figure 6. Line scan of a sample with an air void.

This confirms that electric field measurement along the insulator surface could be used to detect such type of defects in NCI. However, to locate the air void using the near field detection method is easier than electric field measurement as the change of the signal near the void is more apparent, Figure 6, than the change in the electric field near the defect. Moreover, measurement of the electric field requires energizing the insulator which is not required if the near-field method will be used.

A very important defect that might exist in non-ceramic insulators is a crack in the mechanical load bearing part, i.e. the fiber glass core. A crack was introduced in the fiber glass core covered with silicone rubber by applying intensive mechanical load to it. The scanning of the insulator surface is shown in Figure 9. Unlike the defects shown in Figures 5 and 6, more irregular pattern of the scanned signal is evident for crack type defect. This could be due to the presence of several cracks around the scanned area. A 3-D image for a length of 15 cm of the damaged fiber glass rod is shown in Figure 10. The figure clearly shows the location of the crack which is around 5 cm from the insulator end.

N. Qaddoumi et al.: Detecting Defects in Outdoor Non-ceramic Insulators using Near-field Microwave Non-destructive Testing406

Figure 7. Equipotential lines of 33 kV insulator.

Figure 8. Electric field calculations 0.01 mm away from the insulator surface.

Figure 9. line scan of a sample with cracks in the fiber glass core.

Figure 10. 3-D scan of a fiber glass core with internal crack.

5 CONCLUSION

The ability of microwaves to penetrate dielectrics makes microwave techniques attractive to inspect the presence of voids and damage in NCI. The results show that detection of different types of defects may be conducted utilizing the suggested near-field approach. Simple line scans revealed the potential of this technique as a tool to inspect the integrity of NCI’s. Defect location was demonstrated to be an easy task to achieve using the suggested technique. In addition to detecting and locating defects, thickness variations in the silicone rubber shank may be monitored using the same technique.

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Nasser N. Qaddoumi was born on 24 January 1966. He received the B.S. degree in electrical engineering from the United Arabs Emirates University in 1988 and the M.S. and Ph.D. degrees from Colorado State University in 1993 and 1998, respectively. From 1998-1999 he was a postdoctoral fellow at the Applied Microwave Nondestructive Testing Laboratory at Colorado State University. Since 1999 he has been with the Electrical Engineering department at the American University of Sharjah (AUS), Sharjah, UAE.

He is currently the director of the Microwave Imaging & Nondestructive Evaluation Laboratory (MINEL), College of Engineering at AUS. His fields of interest are microwave theory, microwave nondestructive testing, microwave/RF circuit design and system development.

Ayman H. El-Hag (S’99-M’04- SM’08) received the B.S. and M.S. degrees from King Fahd University of Petroleum and Minerals and the Ph.D. degree from the University of Waterloo in 1993, 1998 and 2003, respectively. He joined the Saudi Transformer Co. as a Quality Control Engineer from 1993 until 1999. In 2004 Dr. El Hag worked as a Postdoctoral fellow at the University of Waterloo, Canada.

He was a Postdoctoral fellow at the University of Toronto, Canada from 2004 until 2006. Currently Dr. El-Hag is an Assistant professor in the Electrical Engineering Department at the American University of Sharjah. Dr. El-Hag main areas of interest are condition monitoring and diagnostics of electrical insulation and pulse power applications in biological systems.

Mohamed Al Hosani received the B.Sc. degree from the American University of Sharjah, Sharjah, U.A.E., in 2008. He is working toward his Ph.D. in electrical engineering at the University of Central Florida, Orlando, U.S.A. His current interests include anti-islanding algorithm and micro-grid stability analysis.

Ibrahim Sultan Al Mansouri received the B.Sc. degree from the American University of Sharjah, Sharjah, U.A.E., in 2008. He is working toward his Master degree in electrical engineering at the University of Freiburg, Germany in the area of Renewable Energy Management with specialization of Photovoltaic.

Hamad Buti Al Ghufli received the B.Sc. degree from the American University of Sharjah, Sharjah, U.A.E., in 2008. Currently Mr. Hamad is working with Abu Dhabi Distribution Company (ADDC) as a control engineer.