1. Introduction
Insulators play an important role in the electrical insulation and mechanical stress of high-voltage power transmission lines. The insulator is composed of three parts: the shed, the core rod, and the metal fittings. The shed is often made of silicone rubber, glass, or ceramic. Due to their superior electrical and mechanical qualities, as well as resistance to fouling, SRI materials have largely replaced other insulation materials in power grids [
1,
2,
3]. SRI materials in long-term outdoor operations are subject to strong ultraviolet light, strong electric fields, fouling, and other factors, which will inevitably lead to serious aging phenomena such as chalking, hardening, hydrophobic deterioration, and other aging phenomena. These aging phenomena lead to transmission line failures, and threaten the safety of power systems [
4,
5,
6,
7]. Therefore, a quantitative analysis of the aging status and aging rule of SRI materials is extremely important.
At present, the detection methods for the aging of the SRI shed at home and abroad mainly include the direct observation method, the HC (hydrophobicity class) classification method [
8,
9,
10,
11,
12], the contact angle method [
11,
13], the leakage current method [
14], the thermally stimulated current method [
13,
15,
16], Fourier transform infrared spectrometry [
17,
18], and so on. The direct observation method and the HC classification method are directly observed with the naked eye, but their accuracy is poor and they are subject to several inaccuracies, due to the O&M staff’s subjective judgments. The leakage current method estimates the operating condition of SRIs by measuring the leakage current. The measurement of leakage current may show in real time how several dynamic factors, such as voltage, temperature, and fouling, affect the operating conditions of SRIs [
14,
19], but this method is limited by the randomness of the measurement and the uncertainty of the measuring standard. The contact angle method, the thermally stimulated current method and Fourier transform infrared spectrometry could be interfered with by environmental elements, when measuring. Generally, these measurements can only be carried out under laboratory conditions. In addition, these are destructive testing methods for the SRI sheds that need to be cut.
The power engineers’ ideal method for determining the SRIs’ aging condition should be quick, easy, and inexpensive to implement. It should also be able to assess the silicone rubber’s aging status at the microscopic level and be widely applicable as an engineering measurement method. Due to its quick, practical, and quantitative nondestructive measurements, magnetic resonance technology has been used in materials science to measure the crosslink density of polymeric materials, regulate the quality of rubber production processes, and detect the aging of rubber and polymer materials [
20,
21]. Commercial MR equipment, however, is expensive, cumbersome, and unable to carry out non-destructive on-site measurements. In recent years, unilateral magnetic resonance (UMR) has gained popularity, due to its portability, affordability, and capacity to take non-destructive measurements [
22].
This study describes the design, fabrication, and performance testing of a portable unilateral magnetic resonance sensor employing the unilateral magnetic resonance technique. With the help of this sensor, the effective transverse relaxation times (
T2eff) of new, 2-year-old, and 5-year-old composite insulators were measured at various depths, and on the upper and lower surfaces of the shed, respectively (
Figure 1). These measurements can accurately reflect the degree of aging of SRIs. The results of the magnetic resonance measurements were also compared with those of the static contact angle method; both methods produced the same conclusions, and the magnetic resonance method was more effective at detecting alterations in distinctive parameters brought on by age.
4. Conclusions
In this paper, a portable magnetic resonance non-destructive testing method is proposed for the aging of SRI materials. The method is not only able to measure the aging of the surface of SRI materials, but also to detect the aging of their internal conditions. From the experimental results, it can be concluded that: (1) the longer the composite insulator is in service, the more severe the aging, and that its corresponding T2eff decreases; (2) the insulator sheds from the surface to the inside, T2eff gradually increases, and the aging state phenomenon gradually is reduced; (3) compared with the static contact angle method, the characteristic parameters of the magnetic resonance measurement method change more obviously with the aging of the material. This study is of significance for a better understanding of the aging law of SRIs in the longitudinal direction, a scientific assessment of their service life, and a reasonable evaluation of their quality.
Although this study initially demonstrated the feasibility of this new method, it is limited by the number of samples that can be tested. More samples need to be selected and more data need to be collected, to derive a rule for assessing the aging status of composite insulators.