Detecting Damages 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

American University of Sharjah, Sharjah, UAE 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 33kV NCI samples with no defects, air voids in silicone rubber and small metallic particles 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 silicone rubber shank.

I. INTRODUCTION Non-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.

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 will lead to disastrous failures in the overhead lines [1]. Another possible cause of cracks in the FRP core is the over stress in the end metal fittings [2]. 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 [3].

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 damages in NCI like elevated voltage application, insulation resistance, partial discharge and leakage current measurements have been examined [3]. 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 [3]. In this paper a novel near field microwave non-destructive technique (NDT) will be employed to detect different damages to NCI. The ability of microwaves to penetrate deeply inside dielectric materials makes microwave NDT technique very attractive for interrogating such materials.

II. 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 [5-10]. 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 [5].

In this work, distribution class 33kV NCI samples with no defects and with defects including, air void in the silicone rubber and small metallic particles 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 (1.07 cm x 0.43 cm). 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

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inspection. The block diagram is the experimental setup is shown in Fig. 1.

Fig. 1: Block diagram of the experimental setup

III. 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. 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. Fig. 2 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 obtained. 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 caliper (a reading every 12o). Fig. 3 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.

Fig. 2: Line scan of a sample with no defect at a standoff distance

of 2 mm at 24 GHz.

Fig. 3: Rubber thickness variations along the shank circumference

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 raw data shown in Fig. 4. 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. The influence of thickness variation may still be observed. Additionally at the lower values 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.

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Fig. 4: Line scan of a sample with a metallic inclusion.

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 Fig. 5. The presence and location of the air void may be clearly obtained from the recorded raw data. Additionally the signature of the air void as a defect is different from that of the metallic inclusion. This indicates that classifying different defects may be achieved using the measured signal.

Fig. 5: Line scan of a sample with an air void.

IV. CONCLUSIONS

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. Images of NCI’s may be obtained if necessary.

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