1. Introduction
The trend of using composite material in electrical applications is increasing since the last decades. Fiber-reinforced plastic (FRP) composites have been utilized for manufacturing pylons, wind turbine blades, insulators, crossarms, etc. [
1,
2,
3,
4]. In general, glass fibers have been widely used as reinforcement where the E-glass fibers are more dominant compared to other types of glass fiber e.g., ECR-glass, R-glass, T-glass and S-glass due to their large production rate, relatively low cost, and good electrical as well as mechanical strength [
5,
6]. The concept of using composite material is not only due to the mechanical strength, but also the good insulation properties [
7]. The combination of these two characteristics is a significant advantage for designing a compact and reliable transmission system where Right-of-Way (ROW) has been an issue [
5].
History shows that composites were first used to manufacture insulators in the 1960s in Europe and the USA before they are gradually gained acceptance for a large range of voltage levels worldwide [
5,
8,
9,
10,
11]. Meanwhile, the Japanese have introduced composite crossarms for a 66 kV transmission tower in which a simple structure of bare FRP bars has been used. However, the design is vulnerable to surface discharges, especially when contaminated [
12]. Nowadays, glass fiber reinforced polymer (GFRP) composite has specifically been used as the core structure for an insulating crossarm used on compact transmission lines [
13,
14].
Meanwhile, in Malaysia, most of the 275 kV transmission towers are equipped with the GFRP crossarm. It appears to be the best alternative to the wooden crossarm, which is now being replaced due to a limited source of high-quality timber [
15,
16]. The GFRP crossarm constitutes a hollow square tube of bare GFRP filled with closed-cell foam. Malaysia experiences high lightning activities throughout the year. It has been reported that lightning overvoltages have contributed to more than 50% of the total outages, since higher cases have been reported in 275 kV lines [
17,
18]. The combination of the harsh tropical environment and high lightning activities could progressively affect the insulating material of the composite crossarm.
In the literature, many studies have been carried out to investigate the insulation performance of GFRP material. A previous study presents the breakdown voltage of GFRP sheets, referred to as G10 at different thicknesses, in which the breakdown strength varies from 21 kV/mm to 45 kV/mm [
19].
The influence of thickness on the electrical strength and rigidity of the composite has been highlighted in [
3], where the GFRP composite with more lamination was found to have a lower breakdown voltage. According to [
2], the occurrence of micro-voids which normally exist within the laminations of the pultruded product might promote partial discharges under sufficient electrical stress. Eventually, the insulation strength would be affected. Such agreements were also reported in the earlier study, where the breakdown strength of GFRP composite sandwiched between parallel planes decreased with the thickness [
19].
Some researchers have dedicated their work to investigating the damage modes of GFRP, concerning the electrical stresses caused by the electrical breakdown. In [
2], a failed GFRP composite due to the electrical arc has been inspected, whereby the micro-fractures within the layers of the composite are correlated to the visible burnt area. Another study suggested that the degradation of GFRP rods caused by electrical stress under AC voltage and wet conditions can be divided into four stages, namely the inception stage, hydrolysis stage, carbonization stage and breakdown stage [
20]. The investigation reveals that the epoxy resin matrix of the composites has oxidized and carbonized when subjected to sufficient electrical stress. In this case, an increase of leakage current was expected as the resistance of the degraded GFRP gradually decreased until the breakdown stage occurred [
20]. To some extent, finding the cause of electrical failure on the GFRP can be challenging, as the extreme heat and flame produced by the electrical arc will typically eradicate any sign of the failure mode [
2]. Thus, numerical approaches have been utilized to locate the potential threats through field prediction at different parts of the crossarm [
21,
22,
23,
24].
In the acceptance or type test, the performance of the GFRP crossarm is evaluated through some sets of tests, which mainly cover the structural and mechanical aspects [
25,
26,
27,
28]. Meanwhile, for electrical aspects, an inclined plane test is normally adopted to evaluate the tracking and erosion resistance of the material due to electrical stress where the methods suggested by standard ASTM D2303 and IEC 60587 are the most preferred [
29,
30,
31]. Meanwhile, IEC 60060-1 and IEC 60383-2 have been adopted for the voltage withstand test concerning lightning impulse voltage and power frequency (AC) voltage [
32,
33]. From a research perspective, critical flashover (CFO) and AC flashover voltages have been widely used to describe the insulation strength of insulating equipment against lightning impulse and AC voltage, respectively. According to the series of investigations conducted in the earlier studies, the expected CFO and AC flashover voltages of solid cylindrical GFRP crossarms at various lengths and conditions have been determined [
34,
35,
36].
Figure 1 illustrates the CFO voltage of the crossarm in dry and wet conditions for which the CFO gradient has been determined at 0.700 kV/mm and 0.605 kV/mm respectively.
To date, there are challenges in designing good composite crossarms. One of the challenges is to select the most suitable GFRP material for the application. With limited studies available, particularly on the square tube GFRP crossarm, this work intends to understand the behavior of GFRP under different conditions. In this case, the performance of the square tube GFRP crossarm was investigated by determining the insulation characteristic for both internal and external insulation, considering the lightning impulse voltage (LIV) stress.
4. Conclusions
In this paper, the behavior of typical 275 kV crossarm samples used in Malaysia was investigated. The insulation performance under the LIV condition was considered, where the breakdown strength and CFO voltages were evaluated.
The study revealed that the breakdown of the GFRP material is approximately 7.2 kV/mm. The breakdown strength under the LIV condition was much lower than that obtained under the AC voltage presented in previous studies.
In addition, the CFO voltage was highly influenced by the length of the crossarm, where the increasing trends have been witnessed. Based on the trends, CFO gradients were determined, and thus the CFO of any crossarm length can be estimated.
A significant effect of polarity was found in dry conditions compared to wet conditions. Simultaneously, it was realized that the CFO voltages for negative impulses are greater than that for positive impulses, marking a difference up to 18.2% and 6.4% for dry and wet conditions, respectively.
The surface conditions have distinctly influenced the flashover paths during the lightning test. The dry condition governs the flashover path across the air gaps on the surface, whereas during the wet conditions, flashover took place across the dry-band while bridging the water droplets.
The use of a FEM-based simulation was proven to particularly locate the stresses along the crossarm surfaces under LIV. The 3D simulation sufficiently predicted and located the maximum E-field localization immediately before the breakdown. It was found that the different length of crossarm samples suffers approximate similar field stress, with an average field at 2.45 × 106 V/m. A similar field magnitude was also predicted for a longer crossarm.