Regular articleAging characterization of 500-kV field-serviced silicone rubber composite insulators with self-normalized photothermal radiometry
Introduction
Silicone rubber composite insulators are increasingly used for outdoor insulation applications in high-voltage transmission lines, compared to porcelain and glass insulators, due to their excellent hydrophobicity, light weight, robust structure, and pollution resistivity [1], [2], [3], [4]. However, unlike the porcelain and glass insulators, composite insulators are vulnerable to corona discharge [5], [6], dry band arcing [7], ultraviolet radiation [8], humidity etc., which cause the degradation of electrical performance. This performance degradation, usually referred to as aging, seriously threatens the safety of power transmission systems. Thus, it is necessary to evaluate the aging degree of the composite insulators and replace the severely aged composite insulators with fresh ones in time. Silicone rubber composite insulators are usually made of silicone polymer and fillers such as ATH and silica particles. Adding fillers to the polymer matrix results in a more compact packing structure. However, the aging causes the destruction of the compact packing structures and bond breaking of the polymer matrix, which increases the interfacial thermal resistance within the polymer matrix and therefore reduces the effective thermal diffusivity. The aging of composite insulators is a dynamic process starting from the surface, forming an aged surface layer whose microstructure was irreversibly changed. Sorqvist et al. found significant differences in atomic concentrations and IR spectra between surface and bulk of aged silicone rubber composite insulators [9]. Vlastos et al. observed that the structural changes of the composite insulators after long-term exposure to adverse environment were only within a surface thin layer, leaving the bulk properties unaffected [10].
Various electrical, physical, chemical and structural properties of the composite insulators were employed to characterize the aging degree via different approaches. For example, due to the high correlation between the electrical performance and hydrophobicity of the composite insulators, hydrophobicity class (HC) [11], [12], [13] and contact angle [14] were used to evaluate the aging effects and aging state. Leakage current [15], [16], which altered when insulator surface was long-termly exposed to heavily polluted and/or humid environments, as well as thermally stimulated current (TSC) [17], [18], [19], which characterized the trapped charge and the trap level of the composite insulators, were utilized to assess the aging degree. In recent years, microscopic characteristics of the composite insulators were found to be connected with the aging processes. Images of surface microstructures (acquired by scanning electron microscopy, SEM) [20], [21], infrared absorption spectra of the functional groups of Si-CH3 and Si-O-Si (measured by Fourier transform infrared spectroscopy, FTIR) [22], [23], [24], and elemental composition differences of the composite insulators (measured by x-ray photoelectron spectroscopy, XPS) [25], [26], [27] were demonstrated to be good indicators in characterizing the aging degree of the composite insulators. As to the thermal properties, the reports on the thermal performance of aged composite insulators were rarely found in literature, except that a few researchers studied the internal defects and local temperature rise of aged composite insulators [28], [29], [30]. Actually, thermal properties are usually connected to the material microstructure. Using photothermal radiometry (PTR), an anti-correlation between thermal diffusivity and micro-hardness of steel products was established and the hardness case depth profiling was reconstructed [31], [32], [33]. In addition, Huan et al obtained a direct correspondence of thermal diffusivity to mechanical strain of aerospace materials in the process of stretching the material from the free (unstressed) state to fracture [34], [35]. Very recently, we found a significant difference of the thermal diffusivities between fresh and aged silicone rubber composite insulators using PTR [36] and proposed to use the thermal diffusivity of aged composite insulators as a characteristic parameter to evaluate the aging degree of the composite insulators as we observed the thermal diffusivity of the composite insulators was highly sensitive to the aging degree. From the PTR measurements, it was found that the aging occurred mostly at the surface of the composite insulators and influence the thermophysical properties of only a shallow depth (less than 100 μm) with the bulk mostly unaffected. Therefore, to extract the thermophysical properties of the aged layer, we developed a two-layer PTR theoretical model to determine simultaneously the effective thermal diffusivity and thickness of the aged layer of the aged composite insulators [37]. It is easily understandable that the thermal diffusivity of the aged layer of the field-serviced composite insulators is highly related to the thermal diffusivity of the corresponding fresh composite insulators when they were initially deployed. Unfortunately, the initial thermal diffusivities of the silicone rubber composite insulators been in field service in high-voltage transmission lines were usually not known and were varied in a wide range as these composite insulators were from different manufacturers and manufactured with different manufacturing processes and formulas such as material type and amount of fillers [38]. For example, Sim et al found that the addition of Al2O3 or ZnO fillers increased the thermal conductivity of the silicone rubber [39], while Zhou found that the use of hybrid size particles increased the thermal conductivity of the composite insulators as the matches of different sizes of particles constructed a more compact packing structure in silicone rubber matrix [40]. This variation of the initial thermal diffusivities of different composite insulators may make the evaluation of the aging degree with the absolute thermal diffusivity value not very reliable as the thermal diffusivities of aged layers of composite insulators with different initial thermal diffusivities might be different even following a same aging process.
In this paper, based on a two-layer model which divides the aged composite insulator into an effective aged surface layer and a substrate layer, a self-normalized PTR method is proposed to evaluate the aging degree of the field-serviced composite insulators. In this self-normalized PTR, the thermal diffusivity and thickness of the aged surface layer are determined by fitting the experimental frequency dependences of PTR amplitude and phase to the two-layer PTR model, while the thermal diffusivity of the substrate layer is determined separately via PTR with the aged layer totally removed. The thermal diffusivity ratio of the aged layer to the substrate layer is introduced as the characteristic parameter to describe the aging degree of the field-serviced composite insulators. By doing so the influence of the initial thermal diffusivity variation of composite insulators on the aging degree evaluation is expected to be eliminated. In addition, it is found that the aged layer thickness is also sensitive to the aging process and can be used as a parameter to describe the aging degree. Once the correlations between the thermal diffusivity ratio as well as aged layer thickness and the aging degree as well as electrical insulation performance of the field-serviced composite insulators are experimentally established, corresponding thresholds of both parameters (i.e. thermal diffusivity ratio and aged layer thickness) could be determined which are expected to be valuable to decision-making on when a replacement of field-serviced composite insulators is in order.
Section snippets
Sample preparation
Four 500-kV composite insulators with different field service years and one composite insulator without being used in wild field are used as samples in our experiment. The field-serviced composite insulators, numbered as No.2 to 5, were produced by different manufacturers, and were in field service in four cities (Chengdu, Nanchong, Mianyang, and Ya’an) in Sichuan province, China. The unused composite insulator, numbered as No.1, is an in-stock one also for 500-KV applications. Table 1 lists
Results and discussions
As an example, Fig. 4 shows the experimental frequency dependences of PTR amplitude and phase, as well as the corresponding theoretical best-fits for the intact and polished regions of Samples 3 and 5, respectively. It’s obvious that the PTR experimental data and corresponding theoretical best-fits are in good agreements for both regions. Meanwhile, significant differences of the PTR measurements between the polished region and intact region are observed, indicating that aging of the composite
Conclusions
Thermophysical and structural properties of five silicone rubber composite insulators with different degree of aging (via different years of field service) were investigated by PTR. A two-layer PTR theoretical model was employed to extract the thermal diffusivity and thickness of an aged layer which was formed at the surface of the composite insulator due to the aging effect. Hydrophobicity classes of the five samples were also performed for comparison. Experimental results showed that both
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (No. 51706036). The authors are also grateful for samples support from the State Grid Sichuan Electric Power Company of China.
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