Analysis of Failures and Protective Measures for Core Rods in Composite Long-Rod Insulators of Transmission Lines

Abstract

Composite insulators are deployed globally for outdoor insulation owing to their light weight, excellent pollution resistance, good mechanical strength, ease of installation, and low maintenance costs. The core rod in composite long-rod insulators plays a critical role in both mechanical load-bearing and internal insulation for overhead transmission lines, and its performance directly affects the overall operational condition of the insulator. However, it remains susceptible to failures induced by complex actions of mechanical, electrical, thermal, and environmental stresses. This paper systematically reviews the major failure modes of core rods, including mechanical failures (normal fracture, brittle fracture, and decay-like fracture) and electrical failures (flashunder and abnormal heating of the core rod). Through analysis of extensive field data and research findings, key failure mechanisms are identified. Preventive strategies encompassing material modification (such as superhydrophobic coatings, self-diagnostic materials, and self-healing epoxy resin), structural optimization (like the optimization of grading rings), and advanced inspection methods (such as IRT detection, Terahertz (THz) detection, X-ray computed tomography (XCT)) are proposed. Furthermore, the limitations of current technologies are discussed, emphasizing the need for in-depth studies on deterioration mechanisms, materials innovation, and defect detection technologies to enhance the long-term reliability of composite insulators in transmission networks.

1. Introduction

1.1. Application Status of Composite Insulators in Transmission Lines

The electric power industry serves as a cornerstone of national economic development, exerting a direct impact on both economic growth and societal progress [1]. By the end of 2024, China’s cumulative installed power generation capacity reached approximately 3.35 terawatts (TW), marking a 14.6% year-on-year increase and ranking first in the world [2]. An obvious spatial mismatch exists between the distribution of energy resources and major load centers in China: while energy resources are primarily located in the northwest and southwest regions, electricity demand is concentrated in the economically advanced eastern coastal areas. Projections indicate that by 2050, roughly 75% of the nation’s electricity consumption will remain centered in the central and eastern regions in China [3]. This geographic disparity in energy supply and demand necessitates the development of long-distance, high-capacity transmission technologies, with ultra-high-voltage (UHV) overhead transmission lines serving as a critical solution [4]. As of 2024, China had constructed 41 UHV transmission lines, achieving continuous enhancements in voltage levels, network scale, and national power resource allocation capacity.

In high-voltage transmission systems, HV insulators represent a specialized category of power transmission and transformation equipment, characterized by their large quantity and widespread distribution. Compared to porcelain and glass insulators, composite insulators offer notable advantages, including a stronger pollution resistance, lighter weight, ease of transportation and installation, lower manufacturing energy consumption, and shorter production cycles. These merits have driven the global shift in HV external insulation technologies towards organic materials [5,6]. At present, composite insulators account for more than 75% of the insulators used in China’s UHV transmission lines, and the number of composite insulators in substations is rapidly increasing, which position China as the first country worldwide to mainly adopt organic external insulation in UHV networks [7]. By May 2021, the total length of AC/DC transmission lines (66 kV and above) had reached 940,000 km, with over 10.05 million composite insulators installed, representing 38.7% of the total insulator population. In practical operations, composite insulators are exposed to complex service conditions, requiring long-term endurance against the combined stresses of electrical, mechanical, and environmental factors [8].

1.2. The Critical Role of the Core Rod in Composite Insulators

The typical structure of a composite insulator is pictured in Figure 1 [9,10], comprising high-temperature-vulcanized (HTV) silicone rubber (SIR) housings, a glass-fiber-reinforced plastic (GFRP) core rod, and metallic end fittings. All components demonstrate well-defined functionalities and exhibit synergistic cooperation:

(1)

External Insulation

The silicone rubber housings exhibit excellent hydrophobicity and weather resistance. The shed structure not only provides the required creepage distance but also forms a protective barrier that effectively shields the internal components from environmental stresses.

(2)

Mechanical Load Bearing

The core rod, manufactured via a pultrusion process, consists of axially aligned, electrical-corrosion-resistant (ECR), boron-free glass fibers embedded in the epoxy resin matrix. The resin material is usually either polyester or epoxy, with polyester commonly used for distribution-level composite insulators and epoxy for those applied in HV transmission lines [5]. In terms of its long-term performance, the core rod must maintain chemical stability to ensure that its electrical and mechanical properties remain unchanged over time. Specifically:

• ECR glass fibers provide excellent resistance to acid corrosion;
• The epoxy resin matrix ensures the effective stress transfer and structural integrity of the fibers;
• The composite system simultaneously fulfills the dual functions of mechanical load-bearing and internal insulation.

(3)

Mechanical Connection

The end fittings employ crimping technology to establish a reliable connection with the core rod, ensuring effective mechanical load transfer.

1.3. Failure Modes of Composite Insulator Core Rods in Transmission Lines

During long-term outdoor service, composite insulators are subjected to the combined effects of intense electric fields, high temperatures, acid corrosion, corona discharge, water immersion, and mechanical loads, which can induce various failures, including loss of hydrophobicity, sheath erosion, bird pecking damage, flashover, brittle fracture, and abnormal heating. These failures pose serious threats to the operation of transmission lines [11,12]. This study focuses on the main failure forms associated with the core rods of composite insulators. Based on the consensus of domestic and international research, failure is defined herein as an irreversible state in which a composite insulator loses its rated electrical insulation performance or mechanical load-bearing capability.

According to failure cases reported in the literature, the failure modes are categorized into three types: mechanical failure, electrical failure, and material failure. However, since material degradation of the core rod primarily manifests as interfacial defects between the core rod and SIR housings—originating from either manufacturing flaws or operational stresses, its final consequences are exhibited as electrical failures. Therefore, these material-related failures are classified under the electrical category.

A novel abnormal fracture phenomenon, known as decay-like fracture, has been increasingly observed in transmission lines recently. This fracture mode is characterized by the pulverization of the core rod, complete loss of mechanical strength and electrical insulation properties, and the presence of lateral puncture holes in the sheath. Notably, the mechanical load on the insulator at the time of failure often remains below the designed maximum load [13]. In addition, the flashunder, as defined by the International Council on Large Electric Systems (CIGRÉ) and the Electric Power Research Institute (EPRI) (shown in Figure 2), refers to flashover events initiated by carbonized tracking along or through the core rod [6,14]. This phenomenon shares the same characteristics as the so-called internal breakdown reported in China and is classified as a typical type of electrical failure.

1.4. Failure Data of Composite Insulator Core Rods in Transmission Lines

1.4.1. China

Statistical data collected by the end of 2006 (Figure 3) [15] showed that among 2.2 million installed composite insulators surveyed in China, 58 failure cases were reported. The distribution characteristics and causes of these failures show remarkable similarity with those in the United States. Although the cumulative failure rate in China was only 0.0026%, it is noteworthy that over 50% of the reported incidents were directly associated with core rod failures. Further analysis revealed that material aging and brittle fracture were the two main factors contributing to insulator failures.

Lu et al. [16] conducted a nationwide survey of composite insulator failures from 2010 to 2020, yielding 59 valid failure records. The statistical results are summarized in

Table 1. Statistics of composite insulator failures nationwide from 2010 to 2020 (Data from [16]).

Reason for Failure	Normal Fracture	Brittle Fracture	Decay-like Fracture	Internal Breakdown
Occurrences	2	23	22	12
Proportion (%)	3.4	39.0	37.3	20.3

, and fracture morphologies of the three fracture types are presented in Figure 4.

As shown in Table 1, the occurrence rates of brittle fracture and decay-like fracture are much higher than those of normal fractures and internal breakdowns. Brittle fractures mainly occur in high-humidity regions and on transmission lines rated at 220 kV and above. No significant correlation with service life has been observed. Notably, since the adoption of acid-resistant core rods, the incidence of brittle fractures has substantially decreased [17]. Decay-like fractures mainly happen in insulators with 7–12 years of service and are likely to occur under humid environments. Currently, this failure type has only been reported on 500 kV transmission lines. Internal breakdowns are generally found in insulators with service lives of 5–13 years, mainly in humid regions (no cases have been reported in arid areas), and commonly occur in lines operating at 110 kV and 220 kV.

Moreover, abnormal temperature rise represents another characteristic failure category in composite insulators [18]. According to a 2023 survey conducted by the State Grid Corporation of China (SGCC), among 1.14 million I-string composite insulators investigated, 85 instances of abnormal heating were identified (incidence rate: 0.74‰). By comparison, 75 cases were reported among 260,000 jumper-string insulators (incidence rate: 2.88‰), and 26 cases were observed among 440,000 V-string insulators (incidence rate: 0.59‰). These results clearly indicate that the abnormal heating rate of jumper-string insulators is much higher than that of other string types. The distribution of abnormal temperature rise across different string types is shown in Figure 5.

1.4.2. International

EPRI provides failure statistics for composite insulators in the USA, as shown in Figure 6 [19]. The data reveal that, out of nearly 3 million installed composite insulators, 315 failures have been reported, yielding a cumulative failure rate of 0.0105%. Analysis indicates that brittle fracture and flashunder (characterized by complete or partial separation at the rod–sheath interface) are generally initiated by other direct factors, such as sheath damage that exposes the core rod. This dataset encompasses all historical failure data since the introduction of composite insulators, which explains the high incidence of brittle fractures, reflecting the limitations of early designs, particularly those E-glass core rods.

As of December 2021, EPRI has documented 430 failure cases of composite insulators, with 89.5% of these occurring in North America (385 cases). Figure 7 presents the distribution of failure types [6], showing that brittle fracture and flashunder are the principal failure types. Other failure modes, such as destruction of the rod by discharge and mechanical damage, occur with a lower frequency, while rod pullout and unknown failures are comparatively rare.

A global survey on composite insulator failures, conducted by CIGRE in 2000 [20], highlighted core rod-related failures as the most common failure type. Importantly, a significant proportion of these failures resulted from improper handling during transportation and installation, leading to defects. The detailed results are shown in

Table 2. Global composite insulator failure incident statistics from 2000 (Data from [20]).

Failure Mode	Electrical Damage		Mechanical Damage			Total
	Flashover	Interface Breakdown	End Fitting	Rod Slip-Off	Rod Fracture
U < 200 kV	25	51	2	4	23	105
200 ≤ U < 300 kV	8	10	0	2	8	28
300 ≤ U < 500 kV	0	6	0	0	101	107
U ≥ 500 kV	0	2	0	0	1	3
Total	33	69	2	6	133	243
Failure rate (%)	0.015		0.02

.

In 2011, CIGRE conducted its third evaluation on the service experience of line insulators, integrating both CIGRE and EPRI’s updated data [19]. This study confirmed that brittle fracture and flashunder remain the primary failure modes. The statistical evaluation suggests that the overall annual failure rate range for composite insulators is 1 × 10⁻⁵~1 × 10⁻⁴, with mechanical failures being the most frequent among the various failures.

Figure 8 provides an overview of the main failure categories of composite insulators in power networks. Five major failure types were identified: flashunder, flashover, surface degradation, brittle fracture, and bird damage. In-depth analysis demonstrates that the failure mechanism of flashunder is primarily linked to inadequate bonding strength at the rod–sheath interface, leading to moisture infiltration channels. Moisture migration occurs via two pathways: diffusion penetration through the sheath (commonly resulting in condensation at the interface), and direct penetration due to poor sealing (lower probability occurrence).

2. Mechanical Failures

Mechanical failures and potential hazards in composite insulators mainly involve slippage or pullout at the end fitting–core rod interface, as well as core rod fractures. The latter can be classified into three types: normal fracture, brittle fracture, and decay-like fracture [21]. Although fracture incidents are statistically rare, their consequences are extremely serious, including irreversible power outages, reclosing failures, and potential secondary accidents. Figure 9 shows cases of insulator fractures observed in operational transmission lines. Notably, the implementation of improved crimping techniques and acid-resistant core rods has led to a marked reduction in fracture-related failures caused by brittle and normal fractures [7]. However, field data indicate that decay-like fracture has emerged as the major fracture type in current composite insulators [13].

2.1. Normal Fracture

The normal fracture of composite insulator core rods is a typical mechanical overload failure, occurring when the tensile load applied exceeds the mechanical strength limit of the core rod. Macroscopically, the most notable feature of this kind of fracture is the irregular, jagged fracture surface. The fracture points of glass fibers are not aligned on the same plane and exhibit clear misalignments with the fracture surface of the epoxy resin matrix, as exhibited in Figure 10 [22]. This fracture pattern reflects the strength failure of the material under the ultimate load.

The microscopic features of normal fracture are shown in Figure 11 and Figure 12 [21], and are characterized as follows: (1) Multiple failure modes of glass fiber bundles, including fiber–matrix debonding, fiber pull-out, and fiber breakage. (2) Different traces of separation at the fiber–matrix interface. (3) Extensive fragmentation of the epoxy resin matrix, with numerous fragments adhering to fiber surfaces [23]. The fractured fibers display a disordered arrangement, and individual broken fibers exhibit a cross-section that can be divided into three typical regions: mirror zone, mist zone, and hackle zone, with the mirror zone occupying a small proportion. Furthermore, no chemical alteration occurs in the GFRP rods after normal fracture; their chemical composition remains consistent with that of the original material.

Under tensile loads, due to variations in fiber diameters and strengths, fibers with higher stress concentrations and a lower strength tend to break first [24]. The influence of a single fiber’s fracture on adjacent fibers depends on the core rod’s fracture behavior and tensile strength properties. The subsequent propagation of fiber breakage depends on the tensile behavior, mechanical properties, and toughness of the fiber–matrix interface [25]. Two different kinds of normal fracture are observed:

(1)

Stepped Fracture

In this mode, the fiber–matrix interfacial strength is moderate, and the matrix is rigid. Stress concentration at the fractured fiber tip initiates matrix cracking, propagating towards neighboring fibers and causing their failures [25]. However, when encountering high-strength fibers, crack propagation is arrested, thus localizing the initial damage zone. As tensile load increases, subsequent fiber fractures occur at other locations, with each new damage zone remaining constrained and randomly distributed. With continued loading, numerous damage zones develop until the core rod’s load-bearing capacity is exceeded. Shear failure occurs along fiber–matrix interfaces between different damage zones, leading to longitudinal delamination. As damage accumulates, the rod ultimately fails. The fracture surface often forms at approximately 45° to the rod axis and exhibits a characteristic jagged appearance [26].

(2)

Splitting Fracture

Here, the fiber–matrix bonding is weak, and the matrix lacks sufficient toughness. Upon initial fiber failure, the large shear-stress-induced interfacial damage causes limited stress transfer to adjacent fibers, resulting in localized fiber breakage. With continued tensile loading, multiple fiber bundles fracture accompanied by extensive longitudinal delamination, culminating in the final fracture of the rod [27].

The fracture of composite insulator core rods generally follows two mechanisms: short-term tensile fracture and long-term creep fracture. Short-term tensile fracture occurs when the axial tensile force exceeds the rated breaking load, characterized by broom-like fracture surfaces with the extensive separation of glass fibers from the epoxy resin matrix, and random axial distribution of fiber break points [28]. Long-term creep fracture results from material degradation caused by design defects or improper operation under sustained loads [29]. Given the similarities in the fracture morphology between these two modes, they are collectively referred to as normal fracture. In actual service conditions, the normal load borne by composite insulators (such as conductor weight) is around 10% of the rated rod capacity. With proper selection of rod materials and crimping technology, the axial tensile strength and long-term creep resistance of GFRP rods have been sufficient to meet operational demands [30].

2.2. Brittle Fracture

2.2.1. Definition

Brittle fracture refers to an abnormal failure of the core rod under rated mechanical loads during normal service conditions, triggered by the synergistic effects of mechanical stress and chemical attack [31]. In such incidents, the fracture load ranges from only 10% to 20% of the rated mechanical load, and the service life before failure is generally less than 10 years [32]. The macroscopic hallmark of brittle fracture is a largely smooth and flat fracture surface perpendicular to the rod axis, accompanied by a small region exhibiting broom-like roughness. Additionally, no organic residues are observed on the fracture surface [33], as depicted in Figure 13.

Figure 14 and Figure 15 present the microscopic morphology and Fourier-transform infrared (FTIR) results of the brittle fracture, respectively [34,35]. The X-ray photoelectron spectroscopy (XPS) sampling locations are marked in Figure 16 [34]: Sites 1 and 2 correspond to the central regions of sub-surfaces 1 and 2, while Site 3 is aligned with the axial split. For comparative analysis, original rod samples were also analyzed. The elemental compositions, quantified from XPS peak areas, are listed in

Table 3. Atomic composition of the GFRP measured by XPS [34]. (Licensed under CC BY-ND 3.0).

Sample	C, %	O, %	Si, %	Ca, %	Al, %	B, %	N, %	S, %
Site 1	50.67	33.21	11.89	1.21	1.06	-	1.96	-
Site 2	49.03	34.82	11.66	0.83	1.42	-	2.24	-
Site 3	59.34	24.81	8.52	2.53	1.93	1.08	1.79	-
Original	58.29	26.34	9.01	1.59	2.26	1.33	1.18	-

[34], with relative abundances normalized to the carbon content and detailed in

Table 4. Binding energy (eV) and relative intensity (%) of a single component involved in the high-resolution C peak fitting signals [34]. (Licensed under CC BY-ND 3.0).

Carbon, C 1s [%]
Chemical state	C-C, C-H	C-O	C=O	O-C=O
Binding energy	284.8 eV	286.7 eV	287.1 eV	288.6 eV
Site 1	48.6	29.1	7.6	14.7
Site 2	54.3	21.0	7.0	17.7
Site 3	74.5	5.9	7.0	12.7
Original	74.2	7.0	6.2	12.6

[34].

2.2.2. Fracture Characteristics

The brittle fracture characteristics of the core rod are described from three perspectives: macroscopic morphology, microscopic morphology, and chemical composition.

From a macroscopic perspective, brittle fracture mainly occurs within the HV end fitting or at a short distance above it. Key features include fracture surfaces comprising one or multiple smooth, flat fracture planes oriented perpendicular to the rod axis, interconnected by axial cracks. These flat planes generally occupy 30–90% of the total cross-sectional area [36]. The remaining portion of the fracture resembles that of normal fractures, resulting from the rod’s inability to sustain mechanical loads after significant damage propagation. In some cases, sheath surface damage is observed near the fracture site, while in others, the sheath and sealing around the fittings remain intact. Additionally, most failed rods exhibit poor adhesion at the rod–sheath interface. It is critical to emphasize that a smooth and flat macroscopic fracture surface alone cannot be solely used to diagnose a brittle fracture, as similarly smooth surfaces may also appear under short-term bending stresses, such as uneven ice loading or wind-induced galloping [31].

Microscopically, the fractured glass fibers exhibit stepped fracture surfaces with five smooth sub-surfaces perpendicular to the rod axis. The morphology of individual fractured fibers typically displays three zones: a mirror zone, mist zone, and hackle zone [33,34]. The mirror zone, representing the smoothest region, is governed by mechanical stress, chemical exposure (like acid concentration), and distance from the fracture initiation point. The hackle zone, characterized by rough, radiating ridge-and-valley structures, arises primarily from mechanical forces. The mist zone serves as a transitional region between the mirror and hackle zones.

From a chemical composition perspective, clear signs of acidic erosion are observed at fracture surfaces. Both FTIR and XPS analyses [37,38] confirm that the leading acid source is nitric acid (HNO₃), generated through partial discharge within cracks in the presence of oxygen and moisture. Surface discharges during brittle fracture promote the oxidation of the silicate network structures of the glass fibers and epoxy matrix. Consequently, silicon (Si)’s atomic concentration increases, while calcium (Ca), aluminum oxide (Al₂O₃), and boron (B) concentrations decline on the fractured surface, indicative of non-silicate phase leaching and the formation of high-silica glass fibers. Notably, boron elements leach more readily than calcium and aluminum from the silicate matrix [34].

2.2.3. Mechanism Studies

Several hypotheses have been proposed to explain the mechanism of brittle fracture, including the mechanical fatigue hypothesis, thermal fatigue hypothesis, and stress corrosion cracking (SCC) hypothesis [33]. Experimental evidence indicates that only the SCC hypothesis successfully reproduces the brittle fracture phenomena. This hypothesis posits that brittle fracture arises from stress corrosion cracking under the combined influence of mechanical stress and acidic environments [38,39,40,41]. Based on this framework, researchers have explored the SCC behaviors of rod materials, focusing on crack propagation kinetics, material susceptibility to SCC, key influencing factors, and microscopic corrosion mechanisms.

Currently, there is a broad consensus that brittle fracture results from stress corrosion [34,35,37,38,39,40,41,42,43,44,45,46], although different models have been proposed regarding the detailed fracture process. Four main hypotheses exist:

(1)

Water-Induced Stress Corrosion Hypothesis

This proposes that brittle fracture may occur under the combined effects of water and mechanical stress, with water (rather than acid) as the primary driver [41]. However, experiments confirm that water alone under no strong electric field cannot cause fracture [42].

(2)

Acidification by Curing Agent Residue Hypothesis

This attributes fracture to acids formed from residual curing agents under mechanical tension [43]. Nevertheless, it does not explain why fractures mainly occur at the HV end [39].

(3)

Oxalic Acid Corrosion Hypothesis

This believes that oxalic acid generated by corona discharges are the causative agent [44], yet no direct evidence of oxalic acid has been found in actual failed samples [39].

(4)

Nitric Acid Corrosion Hypothesis

This is currently the prevailing model, supported by experimental validation and field sample analysis [34,35,37,38,39,40,42,45,46]. FTIR detections confirm the presence of nitric acid produced by discharges [35,37,38]. In addition, sulfuric acid from acid rain may also contribute to field degradation [38].

Synthesizing available evidence, nitric acid generated from discharges in humid environments is considered the most credible initiator of brittle fracture. An overview of brittle fracture research is provided in Figure 17.

Moreover, extensive studies have been conducted on other critical factors influencing brittle fracture. Regarding fracture mechanisms, studies demonstrate that stress concentration effects induced by microcracks on the core rod surfaces can explain the random distribution of brittle fracture events [47]. In terms of load characteristics, [36] compared SCC behaviors under static versus cyclic loading, revealing that low-frequency and low-amplitude vibrations superimposed on tensile loads accelerate fracture propagation and result in more complex crack morphologies compared to purely static loads. A considerable proportion of field-failed insulators show intact sheaths near fracture positions [48], prompting in-depth investigations into nitric acid transport mechanisms. Experiments indicate that nitric acid can chemically react with silicone rubber to form hydrophilic carboxyl groups and dissolve aluminum hydroxide, thereby creating permeable pathways through intact sheaths that allow nitric acid (HNO₃) to reach the core rod surface and trigger brittle fracture [37,48]. In fracture morphology, Kumosa et al. [45,49] highlighted that fracture morphologies under mechanical stress and stress corrosion differ greatly. Fractographic analyses can trace crack propagation paths and confirm that nitric acid generated by discharges act as primary initiators.

2.2.4. Mitigation Strategies

Currently, the prevention of brittle fracture in composite insulators mainly involves two approaches: first, enhancing sheath sealing to prevent moisture ingress; and second, improving the stress corrosion resistance of core rod materials. This includes the use of acid-resistant ECR glass fibers, which outperform conventional E-glass fibers in SCC resistance, and modifications to the epoxy matrix [44,50,51]. Additionally, optimizing end fitting design and grading ring configurations can effectively reduce stress concentration near the end fittings. Field applications indicate that these measures have truly mitigated the issue of brittle fracture.

2.3. Decay-like Fracture

2.3.1. Definition

In recent years, under the synergistic effects of high temperature, high humidity, strong electric fields, and mechanical stress, a novel failure termed decay-like fracture has been observed in composite insulators. This fracture, characterized by surface pulverization of the core rod, fiber–matrix debonding, and a brittle, wood-like texture at the fracture site, was first identified and named by Liang et al. [52]. They defined decay-like fracture as an abnormal core rod failure induced by the combined moisture, acidic media, partial discharges, leakage currents, and mechanical tension. Crucially, the degree of epoxy matrix degradation serves as a key criterion for distinguishing decay-like fractures from normal and brittle fractures. Accident cases and subsequent findings (as shown in Figure 18, Figure 19 and Figure 20) confirm that decay-like fractures exhibit different morphological and physicochemical characteristics compared to other fractures.

Since the first reported incident of decay-like fracture in China in 1998, nearly 50 failures have been documented domestically [53,54,55,56,57]. Besides China, similar cases have been found in Mexico, South Korea, and South Africa [13,58]. A statistical analysis of decay-like fracture incidents within SGCC during 2010–2020 by Lu et al. [55] derived several findings: (1) Geographically, incidents are concentrated in the eastern coastal and southern China. (2) The 500 kV AC lines exhibit the highest incidence rates. (3) Compact V-string and jumper-string insulator assemblies, which are more susceptible to bending loads, show a higher prevalence of decay-like fractures due to their electric field distributions.

Internationally, similar mechanical failures have been reported [6,14], attributed, respectively, to thermal core decomposition and rod destruction due to discharge activity, as depicted in Figure 21.

2.3.2. Fracture Characteristics

Figure 22 presents the representative microscopic morphologies of decay-like fractured rods. Figure 23 compares the thermogravimetric analysis (TGA) profiles between decay-like fractured and intact core rods. Comparative FTIR spectra of internal regions from decay-like fractured rods versus intact rods are displayed in Figure 24.

Table 5. Binding energy (eV) and relative intensity (%) of a single component involved in the high-resolution C1s peak fitting signals for GFRP.

Carbon, C 1s [%]
Chemical state	C-C, C-H	C-O	C=O	(C=O)-N	O-C=O
Binding energy	284.8 eV	286.7 eV	287.1 eV	287.9 eV	288.6 eV
Intact GFRP	84.3%	6.2%	5.5%	-	4.0%
Interior region	58.3%	18.5%	6.4%	5.1%	11.7%
External region	69.8%	10.7%	5.2%	7.6%	6.7%

quantifies the elemental composition through fractional areas relative to the carbon content.

Table 6. Binding energy (eV) and relative intensity (%) of single components involved in the high-resolution N ls peak fitting signals for GFRP.

Nitrogen, N 1s [%]
Chemical state	-(C=O)-NH-	CH2-NH2	Nitrite
Binding energy	399.4 eV	400.4 eV	407.8 eV
Intact GFRP	-	-	-
Interior region	18.3%	44.9%	36.8%
External region	14.5%	29.3%	56.2%

further details the component percentages relative to nitrogen, derived from integrated peak areas.

Unlike brittle fractures, decay-like fractures expose damage not only within the core rod but also at the rod–sheath interface and within the housing, indicating a more complex failure mechanism. Based on systematic analyses of failed samples, the typical features of decay-like fractures are summarized as follows [52,53,54,59,60,61,62,63]:

(1)

Macroscopic Characteristics

• Significant degradation of the rod, with rough, wood-like fracture surfaces and noticeable discoloration (whitening or yellowing).
• Presence of powdered or flocculent debris on the core rod surface, and visible carbonization observed in some cases.
• Detachment of rod debris adhering to the inner sheath surface, accompanied by loss of sheath toughness.
• Multiple transverse perforations extending outward from the sheath, occurring from the HV end towards the fracture location and beyond.
• Complete failure of adhesion at the rod–sheath interface near the fracture, with visible hydrolysis or carbonization pathways.
• Fracture location typically found within several shed distances from the HV end.
• Abnormally increasing electrical conductivity and dielectric constant in the degraded rod regions.

(2)

Microscopic Morphology

The epoxy resin matrix exhibits great degradation, including ablation, porosity, and bubble formation. The glass fibers show signs of fracture and surface erosion, and debonding at the fiber–matrix interface is widespread.

(3)

FTIR Characteristics

Compared to intact core rods, decay-like rods show a decrease in aromatic structures (benzene rings), aliphatic structures, Si-O bonds in Si-O-Si groups, and C-H bonds in Si-CH₃ groups, methyl, and ester groups. Meanwhile, hydroxyl content increases.

(4)

TGA Characteristics

TGA results reveal that decay-like fractured rods exhibit smaller mass loss during thermal decomposition, indicating oxidation and depletion of the epoxy resin matrix.

(5)

XPS Characteristics

XPS shows an increase in silicon (Si) and oxygen (O) content and a decrease in carbon (C) content, with increases in nitrogen (N) content compared to undamaged rods.

2.3.3. Research Progress

Extensive investigations employing various analytical techniques have truly advanced the understanding of decay-like fractures in composite insulators. Key findings include the following: B. Lutz et al. [53] revealed that poor interfacial bonding combined with moisture intrusion leads to fiber–matrix interface debonding, hydrolysis of glass fibers, ion exchange reactions, and subsequent partial discharges that degrade the epoxy matrix. Wang et al. [54] proposed that initial defects within the insulator trigger partial discharges and leakage currents, with moisture acting as a critical accelerant in the decay-like fracture process. Gao Y.F. et al. [59] identified the failure of the rod–sheath interface as a major cause of core degradation, with moisture being a necessary catalyst for the aging process. They also explained the formation of pores and bubbles in the epoxy matrix. Liang et al. [52,60] suggested that decay-like fractures result from synergistic mechanisms: discharges and currents cause epoxy matrix erosion, discharges generate acidic media under humid conditions, and the combined effects of acid attack and mechanical stress “cut” the glass fibers. Lu et al. [61] emphasized the key role of partial discharge caused by manufacturing defects and the resulting nitric acid and thermal effects. Yuan et al. [63] clarified that epoxy degradation, fiber fracture, and resin–fiber interfacial failure led to the formation of erosion gaps and a decrease in GFRP rod translucency. Reddy et al. [64,65] discussed the synergistic damage process involving discharges, moisture, and acid corrosion, concluding that the arc heating effect and ohmic losses in the presence of moisture accelerate the chemical degradation of the epoxy resin. Gao C. et al. [66] considered internal discharges at the rod–sheath interface as the principal causes of thermal degradation and decay-like aging in composite insulators. Liu et al. [67] argued that interface failure between the sheath and core rod is the main cause of fracture. Partial discharges at the HV end, combined with weak fiber–resin bonding, facilitate the formation of degradation channels. Surface pollution and powdering of the sheath also contribute.

Collectively, these studies indicate that moisture penetration, partial discharge initiated by interfacial defects (accompanied by nitric acid generation and thermal effects), and subsequent degradation currents are the critical drivers of decay-like fractures [52,53,54,59,60,61,62,63,64,65,66,67].

Based on these hypotheses, numerous laboratory simulations have been conducted to reproduce decay-like fracture features under different conditions. Extensive studies have been conducted on the artificially accelerated aging of GFRP materials under various conditions, including thermal aging [68,69], acid–thermal aging [70], hygrothermal aging [71,72,73,74], and mechanical stress aging [75]. Wan et al. [69] employed pyrolysis–gas chromatography–mass spectrometry (PY-GC/MS) to analyze the thermal decomposition behavior of epoxy/anhydride thermosetting resins at temperatures ranging from 200 to 1000 °C. Research has shown that high temperatures greatly accelerate the corrosion of core rods by nitric acid [70], whereas hygrothermal conditions primarily lead to ester bond hydrolysis and molecular chain scission within the epoxy resin, resulting in disordered fiber alignment [71,72,73,74]. Furthermore, [75] established a multi-stress coupled aging platform integrating humidity, heat, and mechanical loads to simulate the aging behavior of composite insulators under alternating bending loads. Nevertheless, these tests could not fully replicate the properties observed in naturally fractured rods due to the absence of electrical stresses.

The degradation of core rods under electrical stresses has also been extensively studied [76,77,78,79,80,81,82,83,84], covering influences such as electric field stress [76], current–acid mist coupling [77], current–salt mist coupling [78,79,80,81], multiple stresses [82,83,84]. Current–salt mist experiments [78,79,80,81] divided the degradation process into four continuous stages: degradation initiation, hydrolysis, carbonization, and breakdown. These experiments confirmed that surface currents highly accelerate core rod hydrolysis [79], with partial discharges and overheating being the leading contributors to epoxy matrix degradation [65,80]. In terms of multi-stress coupling studies [82,83,84], experiments combining humidity–electric field–alternating mechanical load [82], high temperature–acid exposure–strong field strength [83], and high temperature/humidity–electric field–tensile load [84] demonstrated that water and nitric acid erode the epoxy resin matrix, while strong electric fields further exacerbate its degradation [82]. Particularly, [84] emphasized the role of moisture-induced interface discharge as the primary aging stress, identifying epoxy decomposition as the key degradation mechanism. In addition, studies on moisture ingress into the interface [85], interfacial defects [86,87], and electric field simulations [88] have deepened the understanding of decay-like fractures.

Given that the bisphenol-A epoxy resin is the main material of core rods, its degradation is recognized as a core mechanism in decay-like fractures. However, there remains ongoing debate regarding the dominant degradation factors—whether discharge/current, nitric acid attack, hydrolysis, or thermal decomposition plays the key role. In particular, the microscopic degradation mechanisms induced by partial discharge products remain insufficiently understood. While current experimental evidence provides critical insights, a complete replication of the field deterioration processes remains a big challenge.

2.3.4. Mechanism Studies

Decay-like fractures in composite insulators are recognized as the result of the synergistic interaction of multiple factors, primarily moisture (hygrothermal), electrical stress, and mechanical stress. The degradation usually initiates at the rod–sheath interface and progressively propagates inward, leading to core rod deterioration, mechanical property decline, and eventual fracture. Given the complexity of the different factors, a clear elucidation of the specific roles and mechanisms of each factor is crucial for fully understanding the decay-like process.

At the microscopic level, decay-like fracture is manifested by the degradation and deterioration of the epoxy resin matrix within the core rod. The widely accepted mechanisms can be summarized as follows:

(1)

Liquid Infiltration at the Rod–Sheath Interface

Hydrolysis of coupling agents reduces bonding strength at the rod–sheath interface, leading to interfacial degradation and distortion of the local electric field.

(2)

Erosion Induced by Discharge and Current

Under the combined effects of humidity and strong electric fields, partial discharges and interfacial currents occur on the core rod, causing the epoxy matrix to undergo erosion, softening, melting, and even evaporation.

(3)

Fiber–Matrix Interfacial Degradation

Under the influence of moisture, currents, and mechanical stress, partial discharges generate nitric acid and heat, which accelerates ion exchange and the hydrolysis of glass fibers, ultimately causing debonding at the fiber–resin interface.

Nevertheless, several critical issues remain unresolved:

• Whether the current model can account for all observed decay-like fracture phenomena requires further experimental validation;
• The specific action modes and targets of different degradation factors (moisture, heat, electrical stress, mechanical stress) need to be clearly delineated;
• Since not all insulators under mechanical, electrical, and environmental stresses develop decay-like fractures, refinements and corrections to the current mechanistic models are necessary.

Based on current research, decay-like fracture is attributable to the synergistic effects of hygrothermal aging, partial discharges, and mechanical loading, as shown in Figure 25, and detailed as follows:

(1)

Hygrothermal Aging

Moisture ingress pathways include the following:

• Poor end-sealing, allowing moisture intrusion from the end fittings;
• Sheath damage (due to manufacturing defects, external impact, or aging), enabling direct moisture ingress;
• Moisture permeation through the silicone rubber sheath by diffusion.

Once infiltrated, moisture induces degradation via the following:

• Hydrolysis of core rod materials and the fracture of glass fibers under an acidic environment (produced by discharges);
• Thermal–oxidative aging, leading to oxidation at the fiber–matrix interface and a thermal expansion mismatch, resulting in interfacial debonding and microcrack formation, which further accelerates moisture ingress;
• Thermal decomposition of the epoxy matrix and glass fibers under high temperatures, exposing fibers to acidic conditions and promoting further degradation.

(2)

Partial discharge

Partial discharges include the following:

• Manufacturing defects (poor adhesion, voids, pores) causing local electric field distortion;
• Interface weakening due to long-term operational stresses (humidity, vibration), leading to micro-defects and subsequent debonding.

The degradation effects of discharges manifest as the following:

• Formation of electrical erosion holes: The sheath is perforated from the inside out;
• Epoxy matrix ablation: Melting, gasification, and carbonization leading to pores and bubbles;
• Chemical degradation: Discharges facilitate reactions between the matrix and ambient gases (oxygen, nitrogen), forming compounds and accelerating resin breakdown.

(3)

Mechanical Load Effects

While the precise role of mechanical load remains somewhat ambiguous, research indicates the following:

• Long-term vibration or fatigue promotes the formation of interfacial gaps, facilitating moisture ingress and erosion;
• Combined action of acid and mechanical stress may accelerate the fracture of glass fibers.

However, some experiments (e.g., by Kumosa et al. [39]) have shown that mechanical prestress alone has a limited impact on moisture absorption and leakage currents, suggesting that the role of mechanical load predominantly occurs through synergistic coupling with other factors.

Current challenges in understanding decay-like fracture include the following:

• Further studies are needed to explain the interactive effects of moisture, heat, electrical stress, and mechanical stress and to quantify their respective contributions.
• Advanced experimental techniques are essential to reveal the deterioration processes of resin, fibers, and interfaces.
• Improving coupling agents, enhancing sealing, and boosting the hydrolysis resistance of the core rod materials are vital to enhancing the long-term reliability of composite insulators.

3. Electrical Failures

Electrical failures in composite insulators primarily stem from material degradation or aging, often resulting from immature material formulations in early stages. Under the combined influence of environmental and electrical stress, composite insulators exhibit various aging phenomena, including surface tracking and electrical erosion, loss or reduction in hydrophobicity, powdering, discoloration, cracking, flashover, and fracture. Potential defects at the rod–sheath interface are particularly critical, as they can induce partial discharges under electric field stress, leading to the development of electrical trees, water trees, or even localized conductive channels. In severe cases, this can cause interfacial breakdown or sheath cracking. When the sheath is compromised, moisture ingress accelerates the deterioration process, further exacerbating electrical failure.

3.1. Flashunder (Or Internal Breakdown)

3.1.1. Definition

Flashunder (internal breakdown) is a representative electrical failure of composite insulators, referring to the formation of a carbonized conductive path (tracking) inside the core rod or along the rod–sheath interface due to partial discharges. This eventually results in interfacial field strength beyond the dielectric strength of the sheath, leading to breakdown [6,14]. Figure 26 shows several insulators that failed due to flashunder.

Studies have shown that such failures generally initiate at the end fittings and subsequently propagate along the interface or resin-enriched regions of the core rod, finally forming puncture holes in the weakened areas of the sheath. As residual insulation becomes insufficient, flashover occurs [89,90,91].

Similarly to decay-like fracture, flashunder also exhibits features such as interface failure, sheath puncture, and abnormal heating [92,93]. However, flashunder is usually detected during its electrical failure stage before complete mechanical fracture occurs. Current research mainly focuses on online detection and electric field analysis [88,94], while the understanding of the microscopic mechanisms behind carbonized path formation remains limited. Identical interfacial damage has been observed in a composite post insulator in Sweden, where a semi-conductive path developed at the interface, accompanied by interface failure, core rod ablation, and sheath punctures [95]. These findings not only reveal the key characteristics of flashunder but also provide important insights into its relationship with decay-like fracture.

3.1.2. Failure Characteristics

Flashunder in composite insulators exhibits obvious multi-scale damage characteristics.

At the macroscopic level, the following occur:

(1)

Discharge channels originate from the HV end fitting, penetrating the interface along the axial direction of the core rod and causing severe surface carbonization;

(2)

A big gap forms at the rod–sheath interface due to complete interfacial bonding failure;

(3)

Electrical erosion holes are observed on the sheath surface;

(4)

Intense short-circuit currents cause severe thermal effects, resulting in blocky spalling of the silicone rubber housings, exposing fractured glass fibers aligned longitudinally.

At the microscopic level, the following occur:

(1)

Complete interfacial debonding is evident;

(2)

Broken fibers and silicone rubber debris mix and adhere to the sheath.

Mechanistically, poor interfacial bonding is the key factor that promotes the development of internal carbonization channels. Once the carbonized path extends to a critical length, the remaining insulation distance becomes insufficient to withstand the operating voltage. Unlike decay-like fractures, flashunder primarily involves the loss of electrical insulation while maintaining relative mechanical integrity, offering a clear characteristic for field diagnosis.

There is another case of purely electrical failure [96], where flashunder is triggered by the sharp edges of metallic fittings or voids within the sheath, leading to enhanced local electric fields and discharge initiation.

3.1.3. Mechanism Studies

Flashunder represents a typical electrical failure in composite insulators, caused by poor adhesion at the rod–sheath interface.

The general mechanism is as follows:

(1)

Deficiencies in interfacial adhesion combined with moisture ingress lead to partial discharge;

(2)

These discharges form carbonized conductive paths (tracking) either within or on the core rod surface;

(3)

As the carbonized channel grows, the residual insulation distance decreases;

(4)

When the critical threshold is reached, flashover occurs.

Flashunder exhibits three notable features:

(1)

Expansion of carbonized channels and abrupt flashover, while the mechanical structure remains largely intact.

(2)

Discharge traces at the HV end, core rod carbonization, and interfacial separation.

(3)

High concealment, rapid development, and severe consequences.

The interface failure mechanism includes the following factors:

(1)

Moisture Ingress and Accumulation

Over time, voids at the rod–sheath interface are filled with condensed water. Poor sealing accelerates moisture diffusion through the sheath. Although vapor usually has minimal impact on qualified outdoor composite insulators due to bidirectional diffusion, condensed water at the interface considerably increases the risk.

(2)

Moisture-Induced Electrical Changes

Water condensation (with a relative dielectric constant as high as 81) markedly increases interfacial conductivity and can dissolve pollutants and impurities, further enhancing conductivity and facilitating internal discharges.

(3)

Partial Discharge Effects

In free air, discharges tend to lift away due to buoyancy, but within constrained interfacial spaces, thermal effects act directly on the core rod and internal sheath surfaces. This accelerates the growth of conductive layers (tracking) or causes other damages under electric field forces.

The development process of rod–sheath interface defects is summarized in Figure 27.

During service, composite insulators are subjected to dynamic mechanical loads such as wind-induced vibration and conductor galloping. These stresses may lead to material fatigue and interfacial damage, especially in the presence of design or manufacturing defects [97]. Regarding the relationship between moisture and interface defects, studies have confirmed that moisture can accumulate at non-adhered points along composite insulator interfaces, and external moisture can penetrate the sheath to reach these points [98,99]. Research further indicates that two major factors contribute to interface defects: (1) residual mold-release agents during manufacturing, which precipitate under high temperatures and create weak interfacial zones [100,101]; and (2) moisture infiltration during service through the sheath into the rod–sheath interface [71]. The development of such defects exhibits a “from inside to outside” progression: initially, interfacial damage occurs within the sheath interior, followed by moisture diffusion along glass fibers, and ultimately accelerates erosion under partial discharge. Interface defects not only distort electric fields and trigger partial discharges [102], but also severely compromise mechanical strength, potentially leading to insulation breakdown or insulator detachment.

3.1.4. Relationship with Decay-like Fractures

Research by Lu et al. [103] indicates that flashunder and decay-like fractures share a common failure mechanism, both originating from aging processes triggered by rod–sheath interface defects. Their study revealed that the core rod deterioration simultaneously degrades both the electrical and mechanical properties of the insulator. Whether the insulator finally suffers electrical failure (flashunder) or mechanical failure (decay-like fracture) depends on the ratio between the insulator’s minimum dry arc distance and the core rod diameter. This supposition is validated by field data: flashunder mainly occurs in composite insulators on 110 kV and 220 kV transmission lines, while decay-like fractures have so far been observed mainly on 500 kV lines. The reason is that in lower voltage insulators, the shorter string length results in faster axial aging than radial aging during rod degradation, thus leading more frequently to electrical failure. Conversely, in higher voltage insulators, longer string lengths make radial degradation more significant, eventually causing mechanical fracture. Figure 28 illustrates the relationship and developmental pathways between decay-like fracture and flashunder.

3.2. Abnormal Heating in Composite Insulators

Nowadays, core rods of overhead line composite insulators primarily face three critical failure types: abnormal heating, decay-like fracture, and internal breakdown [82]. Field experience confirms that composite insulators are usually detected with abnormal heating before decay-like fracture. Field investigations of abnormal heating reveal four root causes, such as aged sheath, decay-like core rods, internal defects, and pollution accumulation [104]. The causes can be broadly categorized as follows: heating induced by moisture after sheath aging, heating caused by partial discharge resulting from decay-like aging or defects in core rods, and heating caused by increased leakage current due to surface pollution. Power utilities have implemented infrared thermography for preventive maintenance [105]. Different types of abnormal temperature rise in operating lines are shown in Figure 29.

According to the heating range, heating shape, and temperature rise values, the above abnormal heating types are summarized in

Table 7. The characteristics of different types of abnormal heating.

Heating Type	Shape	Position	ΔT	Factors
Sheath-aged [106,107]	Point	Surface layer of the HV end sheath	Low	Aging, humidity, and field strength
Decay-like [54,108] Lantern-type [109]	Segment	Multi-region	High	Internal defects
Contaminated [110]	Point or Segment	HV end, other areas (severe pollution)	Low	Pollution, humidity

.

Several hypotheses have been proposed to explain the mechanisms underlying abnormal temperature rises in composite insulators [104,105,106,107,108,109,110]: (1) Partial discharges at defects along the rod–sheath interface lead to heat generation. (2) Resistive losses caused by deterioration of insulation resistance result in heating. (3) Polarization losses induced by moisture ingress into the sheath and rod–sheath interface contribute to heating. (4) Surface contamination under high humidity causes resistive heating of the sheath. Comparative studies on decay-like fractures indicate that moisture penetration and material degradation may be common triggering factors for both decay-like fractures and abnormal heating. External insulation factors primarily involve sheath aging, surface contamination, and improper grading ring configurations. Internal insulation factors are mainly associated with defects in the core rod and the rod–sheath interface.

(1)

Partial Discharge Induced by Interface Defects

Wang et al. [54] found that rod–sheath interface defects in decay-like insulators can trigger partial discharges, leading to abnormal temperature rises. The resistive currents associated with core rod degradation also contribute to heating. Lu [61] proposed that internal defects within the insulator cause partial discharges, manifesting externally as localized heating near the HV end. Zhang et al. [111] confirmed that moisture infiltration reduces the electric field strength within unbonded regions while increasing it at the edges, enhancing localized heating. The extent of temperature rise depends on the length of the unbonded interface.

(2)

Resistive Losses Due to Insulation Resistance Degradation

Tu et al. [112,113] investigated the abnormal heating of aged sheaths under varying humidity, measuring dielectric constants and loss factors at the HV end. Their results indicated that sheath aging was the intrinsic cause of abnormal heating, with high environmental humidity acting as an external accelerant. Aged sheaths exhibit heating primarily at the HV end, correlated with surface aging, ambient humidity, and local electric field intensity. Ma et al. [114] studied the effects of wind speed and ambient temperature on infrared temperature measurements and proposed a temperature compensation model. Their work emphasized that dielectric losses resulting from sheath aging and moisture absorption are the fundamental causes of abnormal heating in field-deployed insulators. Cheng et al. [115] also pointed out that long-term environmental exposure leads to sheath aging and subsequent insulation resistance decline, which is a primary driver of abnormal heating.

(3)

Polarization Losses Induced by Moisture Ingress

Zhang et al. [107] observed that micro-porosity in the sheath enhances moisture absorption and local electric field strength, leading to increased dielectric losses and heating, especially concentrated near the HV end. Zhong et al. [116] developed equivalent circuit models for both short samples and 500 kV decay-like insulators to analyze the contributions of leakage conductance and polarization losses to temperature rise. Wang et al. [117] attributed internal heating primarily to polarization losses (orientational and interfacial polarization) induced by moisture ingress at the rod–sheath interface. Similarly, Wang et al. [118] demonstrated through improved water diffusion experiments that increased leakage currents at the rod–sheath interface cause heating preceding decay-like fracture.

(4)

Resistive Losses Due to Surface Contamination

Wang et al. [110] found that surface contamination greatly exacerbates abnormal heating at the HV end, with heating intensity increasing alongside contamination severity and environmental humidity. Edson G. et al. [119] also observed that surface pollution leads to abnormal heating in composite insulators.

(5)

Combined Factors

Zeng et al. [108] noted that abnormal heating in decay-like insulators results from a combination of factors, including polarization losses, partial discharges, and resistive losses, with partial discharges playing a dominant role. Yuan et al. [120] reported that decay-like insulators exhibit significant heating under both high- and low-humidity conditions, with temperature rises markedly greater than those of aged but non-decay-like sheaths. Their findings suggest that, under high humidity, polarization losses dominate heating behavior, while under low humidity, both polarization and conductive losses contribute. Cheng [121] proposed that under high humidity, heating arises from the polarization of water molecules, polar groups, and interfacial layers, whereas under low humidity, partial discharges become the primary cause.

Several studies have examined the morphological features of heated regions within core rods [61,109,122,123,124], as illustrated in Figure 30.

Figure 30a reveals that the core rod surface displays non-uniform coloration with visible signs of ablation and discoloration, ranging from grayish-white to dark patches. From Figure 30b, localized material degradation is evident at the heated regions, including multiple erosion pits on the epoxy matrix, fiber damage, and silicone rubber erosion. Partial discharges cause localized high temperatures, leading to the reduction in Al(OH)₃ fillers; an increase in -CH₃, Si-CH₃, and -OH groups; and extensive molecular chain scission in the epoxy matrix. Carbon atoms are oxidized and released as CO₂, thereby reducing carbon content. Additionally, nitrogen oxides (NO_x) generated during discharge combine with moisture to form nitric acid, which aggressively attacks the core rod, resulting in increased nitrogen content. These findings elucidate the synergistic electro-thermal–chemical degradation mechanisms underlying abnormal heating at the microscopic level.

4. Preventive Measures

4.1. Material Modification

4.1.1. Superhydrophobic Materials

To address mechanical damage and electrical failures encountered during the operation of composite insulators, material modification has recently emerged as a key research focus for enhancing insulator performance. The introduction of functional materials, such as superhydrophobic coatings [125,126,127,128,129], self-diagnostic materials [130,131,132,133,134], and self-healing materials [135,136,137,138,139,140], markedly improves the environmental adaptability and long-term reliability of composite insulators. As previous findings suggest, moisture ingress plays a critical role in the degradation of composite insulators [42,43,44,45,46,52,53,54,107,108,109,110,111]. Superhydrophobic coatings can effectively block moisture penetration, thereby greatly extending the service life [125]. Liu et al. [126] developed a multifunctional SiO₂/PDMS/EP nano-coating exhibiting excellent properties (contact angle > 160°, rolling angle ≈ 0°), endowing the insulator with outstanding self-cleaning capabilities. Moreover, emerging fabrication techniques such as nanosecond fiber laser processing [127,128] have provided additional avenues for the development of superhydrophobic coatings. Reference [129] reviewed the performance advantages (self-cleaning, anti-icing, and electrical properties) and future prospects of outdoor superhydrophobic insulating materials, offering valuable guidance for engineering applications. These material innovations enhance environmental resilience, providing a new pathway for the long-term reliable operation of composite insulators.

4.1.2. Self-Diagnostic Materials

Self-diagnostic materials introduce innovative solutions for composite insulators by enabling the early, real-time visualization of insulation defects through intelligent response mechanisms. Three main categories of self-diagnostic technologies have been developed: (1) Fluorescence-responsive materials, based on electroluminescence principles [130,131]. For instance, the intrinsic fluorescence of silicone gel under electrical stress enables the 3D imaging of damage locations. Additionally, sensor-responsive materials (SRM) inspired by bioluminescent organisms can accurately map defects and electric field distributions. (2) Chromatic-responsive materials [132]. These use molecular indicators whose color changes provide autonomous warnings of electrical degradation. (3) Mechanically responsive materials [133,134]. These monitor mechanical strain and damage progression in real time through mechanochromic effects or embedded sensor networks. These technologies offer high-precision, non-destructive solutions for the intelligent monitoring of engineering materials.

4.1.3. Self-Healing Epoxy Resin

Self-healing dielectric polymers represent a cutting-edge solution for restoring electrical insulation after damage. Magnetically guided healing systems [135,136] utilize superparamagnetic nanoparticles to guide the healing of electrical treeing under magnetic fields, enabling the repeated restoration of dielectric properties. Multifunctional microcapsule systems include magnetically targeted UV-responsive types [137,138], magneto-UV dual-responsive types [139], and UV/moisture/magnetic triple-responsive types [140]. These technologies offer three key advantages: intelligent repair triggered by in situ electroluminescence; a low additive concentration (<5 wt%) maintaining the base material performance; and efficient multiple repairs, significantly enhancing the durability and lifespan of insulating materials without compromising their original properties.

Reference [141] discussed the potential mechanisms of bulk breakdown and surface flashover in polymeric insulation and proposed strategies to enhance material performance. For improving bulk breakdown strength, methods such as molecular design, free volume reduction, trap introduction, and suppression of electrical breakdown propagation are recommended. For surface flashover, mitigation strategies depend on usage conditions: (1) Surface charge accumulation is critical for internal insulation. (2) Surface contamination is a major concern for external insulation. (3) Secondary electron emission is a key issue under vacuum conditions.

4.2. Structural Design

Previous studies have confirmed that moisture ingress into the rod–sheath interface is a key factor leading to interface degradation and insulation failures in composite insulators [42,43,44,45,46,52,53,54,107,108,109,110,111]. The accumulation of moisture at the interface not only triggers partial discharges but also accelerates material aging, finally causing insulation deterioration or even breakdowns. To address these challenges, preventive measures are proposed:

(1)

Strict quality control of core rods, ensuring excellent moisture resistance;

(2)

Development of rigid insulators to enhance interfacial sealing, effectively preventing moisture ingress;

(3)

Optimization of grading ring designs to improve electric field distributions.

These approaches work synergistically across three dimensions—material quality assurance, structural enhancement, and electric field optimization—substantially improving the interfacial moisture resistance and operational reliability of composite insulators.

4.2.1. Core Rod Void Detection

Voids within the core rod are a manufacturing defect, but not all voids can be detected through standard dye penetration tests. Some undetected voids may lead to abnormal temperature rises during service. Zhang et al. [142] classified voids into two categories based on their structural characteristics: dependent and independent voids. Dependent voids are formed by multiple interconnected adjacent voids, resulting in extended permeation paths that conventional dye tests may fail to detect effectively. To enhance detection accuracy, the use of ethanol-vapor-assisted dye penetration testing has been recommended. Furthermore, Darcy’s law was employed to analyze the fiber infiltration behavior in core rods, leading to the development of an improved manufacturing process aimed at reducing porosity [143]. Key measures include maintaining the fiber volume fraction above 70% and regularly replacing the wetting agents during processing. Moreover, a 300 h water diffusion test with a current threshold of 50 μA is suggested as an effective method to assess core rod quality.

4.2.2. Optimization of Grading Rings

The structural configuration of composite insulators often results in an extremely non-uniform electric field distribution along their surface. Under conditions of high humidity or condensation, this non-uniformity promotes the formation of water droplet corona discharges, which in turn accelerate the housing aging, and potentially lead to flashover. The proper deployment of grading rings has been shown to effectively mitigate localized peak electric fields, thereby improving both the operational stability and dielectric reliability of composite insulators [144,145,146].

4.2.3. Cycloaliphatic Epoxy Resin (CEP) Insulator

Cycloaliphatic epoxy resins have garnered widespread application in insulation materials due to their superior electrical properties [147]. Notably, the novel hydrophobic cycloaliphatic epoxy resin (HCEP) materials not only exhibit excellent hydrophobicity, hydrophobicity transfer, and recovery properties, but also outstanding arc and tracking resistance [148]. These attributes render HCEP an ideal candidate for use in the fabrication of insulator housings. Studies have demonstrated that using HCEP greatly suppresses moisture ingress through the sheath, thereby reinforcing the protective function of the housings and reducing the likelihood of moisture-induced failures in the core rod [149].

4.3. Operation and Maintenance

This section presents an overview of diagnostic methods for assessing mechanical and electrical faults in composite insulators, with a particular emphasis on the identification of internal defects. During long-term service, composite insulators are susceptible to develop internal defects—such as rod–sheath debonding, discharge-induced erosion, and surface carbonization of the core rod—caused by material aging, environmental stress, and electrochemical interactions. Unlike external defects such as bird pecking or surface cracks, these internal defects are difficult to detect through visual inspection alone. Consequently, they are prone to be missed or misjudged during routine assessments. If unaddressed, such hidden defects may decrease mechanical integrity or dielectric strength, eventually resulting in fractures, flashovers, or even major power outages, and posing a serious threat to the safe and reliable operation of transmission lines.

Therefore, the application of effective detection techniques is crucial for evaluating the operational status of composite insulators, preventing severe incidents, and ensuring grid reliability [150]. The integrated application of multiple diagnostic methods provides multi-dimensional assessments, facilitating the early detection of hidden defects and supporting the formulation of targeted maintenance strategies.

4.3.1. Line Inspections

(1)

Infrared (IR) Thermography

IR thermography is commonly employed to identify internal defects by detecting abnormal heating during operation. This technique is advantageous for its non-contact nature and applicability over long distances, and has already been applied in field diagnostics [114,116,151]. Nevertheless, its accuracy is easily affected by weather conditions, testing distance, and the surface condition of the insulator. Moreover, the method struggles to distinguish temperature rises caused by internal defects, surface aging, or contamination, and has limited effectiveness for detecting surface cracks and interfacial voids [152,153]. Thus, it is recommended to optimize inspection strategies and combine infrared thermography with other methods to improve its diagnostic accuracy and avoid unnecessary insulator replacements.

(2)

Ultraviolet (UV) Imaging

UV imaging identifies defects by capturing stable corona discharges at different positions on the insulator surface, and is particularly effective in detecting carbonization paths and erosion damage [153,154]. However, discharges resulting from surface cracks on damp insulators may be confused with pollution-induced wet flashovers, complicating fault interpretation. While UV diagnostics offer long-range detection applicability, their effectiveness is largely constrained to corona-related phenomena and remains highly susceptible to environmental variables such as ambient lighting and relative humidity [119,155]. Thus, UV imaging is primarily used as a supplementary diagnostic method.

(3)

Electric Field Detection

Electric field measurement analyzes axial electric field distributions along the insulator to identify conductive defects [153,156]. When defects are present, the field distribution exhibits distortions. By comparing the field profiles of defective and normal insulators, defect locations and lengths can be determined with high sensitivity and positional accuracy, especially for HV insulators. However, this method has several limitations: (1) A reduced effectiveness for low-voltage insulators without grading rings; (2) it often requires tower-climbing; and (3) there are challenges in obtaining direct surface field data. To address these issues, non-contact electro-optical probes [157,158] have been introduced, enhancing operational safety and measurement convenience.

Currently, unmanned aerial vehicle (UAV)-based inspection is the most common method. UAVs can be equipped with various detection devices, such as infrared cameras, visible light cameras, and ultraviolet detectors. Helicopter or fixed-wing aircraft inspection is suitable for UHV and extra-high-voltage (EHV) transmission lines, especially for rapid large-scale surveys across regions. There are also traditional manual ground inspections and tower-climbing checks, which are used for the positions that UAVs and helicopters cannot accurately assess or which require close-up observation and testing.

4.3.2. Diagnostic Methods for Internal Defects

(1)

Ultrasonic Inspection

Ultrasonic inspection identifies internal defects in insulators by analyzing the propagation characteristics of ultrasonic waves within the material. Reflections and refractions at interfaces between different media generate echo signals, whose waveforms, amplitudes, and phase changes can be analyzed to locate and characterize flaws. Liang et al. [159] monitored the growth of brittle cracks in core rods using an oblique incidence ultrasonic longitudinal wave technique and established a correlation between crack growth and echo amplitude. Xie et al. [160] utilized phased array ultrasonic technology to detect interfacial air gaps and impurities, achieving a minimum detectable size of 0.5 mm. Chen et al. [161] developed a flexible coupling technique that significantly improved the adaptability and efficiency of the inspection process. Wang et al. [162] established acoustic and bilinear stiffness models for the progressive micro-debonding at the rod–sheath interface, revealing that the nonlinear distortion induced by high-power ultrasound could effectively diagnose early-stage micro-debonding defects (1–20 μm) in composite insulator interfaces.

(2)

Radiographic Inspection

Radiographic inspection employs the attenuation characteristics of X-rays or γ-rays as they traverse materials to perform qualitative and quantitative analysis for internal defect detection [163]. Although X-ray imaging excels at identifying larger defects and offers intuitive visualization of internal structures, it remains limited in detecting micro- or concealed flaws. For debonding between the sheath and the core rod, detection is only feasible when the gap reaches a considerable size [164].

(3)

Microwave Inspection

Microwave inspection utilizes the difference in reflection coefficients at material interfaces to detect defects. By analyzing the spatial distribution of reflected signals, the type and location of defects in insulating equipment can be inferred. Studies have demonstrated that microwave techniques can detect interfacial defects as small as 0.4 mm [165,166]. However, due to the weak penetration ability of microwaves through metals, the method is less effective for detecting flaws in end fittings and crimped joints.

(4)

Terahertz (THz) Inspection

Currently, terahertz techniques recognize internal defects such as voids and fractures by analyzing parameters such as amplitude, phase, time delay, and energy in the time and frequency domains. Research has shown that terahertz imaging can effectively detect voids and fractures; however, it exhibits limited differentiation between impurities and debonding defects. Moreover, the imaging precision is susceptible to scattering, making it challenging to accurately image irregular objects [167,168].

(5)

Optical Fiber Sensor Inspection

Optical fiber sensor inspection involves embedding Fiber Bragg Grating (FBG) sensors within the core rod to achieve real-time monitoring of the insulator’s state, capitalizing on the sensitivity of the reflected wavelength to stress and temperature variations. Damage-induced changes in stress and temperature cause shifts in the Bragg wavelength, thereby indicating the presence of defects. This method offers high sensitivity and effectively monitors internal temperature changes and prevents brittle fracture; however, its accuracy can be affected by environmental factors such as icing and wind-induced displacement [169,170].

In addition to the methods described above, research has also been conducted on steep-front impulse tests [171], electroluminescence techniques [172], X-ray computed tomography (XCT) [66,84,111,173], and acoustic emission (AE) detection [174,175]. The steep-front impulse method applies high-magnitude voltage impulses to the insulator, inducing significant internal voltage stress without causing external flashover, thereby facilitating defect identification based on breakdown characteristics at different voltage levels. AE signals generated by polymers can be categorized: one associated with material fractures such as crack propagation and structural damage, and the other with electrical discharges, including partial discharges and corona discharges [176].

5. Conclusions and Outlook

5.1. Research Summary

This study focused on the failure mechanisms of core rods in the composite long-rod insulators utilized in transmission lines. Statistical data from both China and international sources were reviewed. Preventive measures involving material modification, structural optimization, and operational maintenance were proposed, along with discussions on future research directions. The main findings are summarized as follows:

(1)

Mechanical Failures

a.

Normal Fracture

Caused by mechanical overload, including short-term tensile fractures and long-term creep fractures. Macroscopically, fracture surfaces are irregular and jagged, while microscopically, features such as fiber debonding, pull-out, and fracture are observed.

b.

Brittle Fracture

Abnormal fracture under mechanical stress and chemical exposure during normal operation, typically occurring at 10–20% of the rated load. The macroscopic fracture surface is partially smooth and flat, while the microscopic morphology exhibits step-like features. Stress corrosion cracking (SCC) is the most widely accepted cause. The use of acid-resistant core rods and optimized end-sealing techniques can effectively reduce failure probability.

c.

Decay-like Fracture

Caused by the multi-factor coupling of moisture, heat, electrical stress (discharge and current), and mechanical load, and characterized by the pulverization of core rods, the degradation of epoxy resin matrix, fiber breakage, and fiber–matrix interface failure. The degradation process involves moisture penetration, partial discharge, and thermal aging, necessitating special attention to sealing performance at the rod–sheath interface.

(2)

Electrical Failures

a.

Flashunder

Caused by poor interfacial bonding and moisture ingress, forming carbonized conductive paths and eventual breakdown. Enhancing sheath materials and optimizing grading ring designs can mitigate such failures.

b.

Abnormal Heating in Composite Insulators

Attributed to sheath aging, internal defects, and surface contamination. Hypothesized mechanisms include partial discharge and insulation resistance degradation, with water ingress and material deterioration serving as common features to both decay-like fracture and abnormal heating failures.

(3)

Preventive Measures

a.

Material Modification

The application of superhydrophobic coatings to prevent moisture ingress, the development of self-diagnostic materials for defect monitoring, and the utilization of self-healing epoxy resin to repair damage.

b.

Structural Design

Monitoring porosity in core rods, optimizing grading rings to manage electric field distributions, and adopting novel CEP insulators to enhance interfacial sealing.

c.

Operational Maintenance

Techniques such as IR detection, UV imaging, and electric field measurement are employed to identify abnormal heating and partial discharges, although environmental interference and accuracy limitations persist. Ultrasonic, X-ray computed tomography (XCT), microwave, terahertz, and other methods offer the potential for internal defect detection, but challenges remain for detecting minute flaws.

5.2. Future Outlook

Building upon the current research, future directions are proposed to further enhance the performance of composite insulators and ensure the safe and stable operation of transmission lines:

(1)

Deeper investigation of mechanism of decay-like fracture

Future studies should elucidate the coupled effects of moisture, thermal, electrical, and mechanical stresses at both the macro- and micro-scale; quantify their contributions; and leverage advanced experimental techniques to explain the degradation processes of resin matrices, fibers, and their interfaces.

(2)

Optimization of materials and processes

Improving silane coupling agents, sheath sealing properties, and core rod hydrolysis resistance is critical for enhancing their long-term reliability. The development of advanced materials should be prioritized to improve environmental adaptability. Simultaneously, manufacturing processes must be optimized to strictly control core rod quality and enhance product consistency and stability.

(3)

Innovation in detection technologies and algorithms

Future efforts should focus on developing integrated inspection techniques that combine multiple sensing modalities to improve sensitivity and accuracy. Additionally, artificial intelligence and digital twin technologies should be employed to deeply analyze inspection data, enabling the early diagnosis and predictive maintenance of degradation states, especially core rod detection technologies.

(4)

Technical and economic comparison of different insulation options

Due to advantages such as their light weight and strong pollution resistance, composite insulators have developed rapidly in recent years. However, their aging characteristics as organic materials must not be ignored. Therefore, when selecting external insulation, it is essential to balance short-term investment and long-term maintenance costs, and to consider investment payback periods and profitability.