Study on Live Temperature Rise and Electrical Characteristics of Composite Insulators with Internal Conductive Defects

Abstract

Internal conductive defects in composite insulators severely degrade their insulation performance and are considered concealed defects, posing a significant threat to the safe and stable operation of the power grid. Focusing on this issue, this study develops an electro-thermal multi-physical field simulation model and uses finite element analysis to investigate the electric field distribution and temperature rise characteristics. Composite insulator specimens with varying defect lengths were fabricated using the electrical erosion test. Charged tests were then conducted on these defective specimens, as well as on field-decommissioned specimens. The impact of internal conductive defects on the infrared, ultraviolet, and electric field distribution characteristics of composite insulators during operation was analyzed. The results indicate that the surface electric field of composite insulators with internal conductive defects becomes highly concentrated along the defect path, with a significant increase in electric field strength at the defect’s end. The maximum field strength migrates toward the grounded end as the defect length increases. Conductive defects lead to partial discharge and abnormal temperature rise at the defect’s end and the bending points of the composite insulator. The temperature rise predominantly manifests as “bar-form temperature rise,” with temperature rise regions correlating well with discharge areas. Conductive defects accelerate the decay-like degradation process of composite insulators through a positive feedback loop formed by the coupling of electric field distortion, Joule heating, material degradation, and discharge activity. This study identifies the key characteristics of electrical and temperature rise changes in insulators with conductive defects, reveals the deterioration evolution process and degradation mechanisms of insulators, and provides effective criteria for on-site diagnosis of conductive defects.

1. Introduction

With the rapid advancement of ultra-high-voltage (UHV) projects in China, composite insulators have been widely adopted in power systems due to their strong pollution flashover resistance, high mechanical strength, lightweight nature, and ease of installation [1,2,3]. According to statistics, the number of composite insulators in operation at voltages of 110 kV and above has exceeded 20 million, with over 1.3 million units having been in service for more than 15 years. The large number of insulators in use has raised higher demand for defect detection in composite insulators [4,5]. The study of composite insulators has spanned several decades, with foundational research dating back to the early 21st century or even earlier [6,7,8]. In recent years, as their application scope has expanded and service duration has increased, research has gradually shifted its focus toward aging mechanisms and failure modes [9].

Composite insulators are subject to long-term environmental influences during operation, which lead to material degradation and deterioration, thereby causing various defects that can result in issues such as line outages and grounding faults [10,11]. The interface between the silicone rubber sheath and the glass fiber-reinforced plastic (GFRP) core rod serves as a mechanical and electrical weak point in the composite insulator. Under the combined effects of moisture, heat, electrical, and mechanical stresses, micro defects at the interface can emerge, leading to local electric field distortion and, consequently, partial discharge. Environmental factors such as ultraviolet radiation and temperature–humidity cycles cause material degradation through photo-oxidation and thermal stress. Water penetration causes hydrolysis of the core rod, significantly reducing its volume resistivity and partial discharge threshold. Over time, this leads to the formation of internal carbonization channels within the insulator, resulting in conductive defects [12,13,14,15]. These defects severely reduce the insulation performance of the composite insulator and are concealed, posing a risk to the safe operation of transmission lines and increasing maintenance difficulties. Therefore, studying the impact of internal conductive defects on composite insulators is of significant importance for improving their reliability and the accuracy of fault detection.

Conductive defects can cause severe distortion in the electric field distribution, leading to abnormal temperature rise in composite insulators. Based on finite element simulations, Qin et al. [16,17] studied the electric field distribution characteristics of composite insulators with conductive defects in 500 kV transmission lines. The results showed that conductive defects cause significant distortion of the surface electric field of the composite insulator and enhance the electric field at the high-voltage end, allowing the defect’s location and length to be determined based on the degree of distortion. Zhao et al. [18] demonstrated through finite element simulations that the electric field at the high-voltage end of the insulator is significantly amplified due to the presence of conductive defects, and the electric field increases with the length of the defect. Wu et al. [19] established an electromagnetic–thermal coupling finite element model for the FXBW-500 composite insulator, demonstrating that conductive defects result in an abrupt temperature rise in localized areas at the defect’s location.

Conductive defects can cause severe distortion in the electric field distribution and an abnormal increase in operating temperature, which accelerate the aging process of composite insulators and reduce their long-term operational reliability [6,20]. The electrical erosion test can effectively recreate the key features of conductive defects in decay-like composite insulators. Lei et al. [21] simulated conductive defects in composite insulators by deeply embedding a copper wire in the underside of the shed, demonstrating that conductive defects at the high-voltage end reduce the discharge threshold of the composite insulator. Gao et al. [22] reproduced degradation and deterioration of the epoxy resin in the GFRP core rod of decay-like composite insulators using the electrical erosion test. Pang et al. [23] fabricated carbonization channels on the exposed surface of the GFRP core rod using an electrical erosion test in an acid mist environment. They found that the carbonization channels caused the core rod to heat up, and the temperature rise phenomenon exhibited regional characteristics.

Abnormal temperature rise in composite insulators can reduce their insulation performance, posing a threat to the safe and stable operation of power systems [24,25]. Therefore, it is crucial to detect and diagnose abnormal temperature rise in composite insulators in a timely manner. Such temperature anomalies may be caused by various factors, including environmental conditions, material aging, internal defects, and contamination, each potentially leading to different forms of temperature rise [26,27,28]. Yuan et al. [29,30,31] classified the abnormal temperature rise into “point-form temperature rise (PFTR)” and “bar-form temperature rise (BFTR),” proving that BFTR in composite insulators is caused by the degraded GFRP core rod. The temperature rise phenomenon of the GFRP core rod under low-humidity conditions results from the combined effects of polarization loss and conduction loss. Infrared thermography is a non-contact temperature measurement technique that can rapidly detect abnormal temperature regions in composite insulators. By analyzing the temperature distribution patterns in infrared images, it enables the diagnosis of defect types in composite insulators [32,33].

In summary, current research on internal conductive defects in composite insulators primarily relies on numerical simulation methods, with limited studies focusing on fabricating defective specimens to investigate their actual operational characteristics. Additionally, most studies concentrate on the impact of conductive defects on a single physical field, such as the electric field or temperature field, and lack in-depth exploration of the effects of internal conductive defects on the coupling interactions in multiple physical fields within composite insulators.

In light of this, this study investigates the impact of internal conductive defects on the operational characteristics of composite insulators. Based on the carbonization channels on the surface of core rods from decay-like specimens, composite insulator specimens with internal conductive defects of varying lengths were fabricated using the electrical erosion test. An electro-thermal multi-physical field simulation model for composite insulators was established, and finite element analysis was conducted to study their electric field distribution and temperature rise characteristics. Furthermore, charged tests were carried out, the energized temperature rise behavior, electrical characteristics, scanning electron microscopy (SEM) morphology, and energy dispersive spectroscopy (EDS) results of the defect-containing specimens fabricated in this study were compared with those of field-retired specimens. The similarities and differences in the operational characteristics of the fabricated specimens and decay-like specimens were discussed, and the impact of internal conductive defects on the operational characteristics of composite insulators was analyzed, considering multi-physical field coupling. The findings of this study offer a valuable reference for on-site defect detection in composite insulators and provide a theoretical basis for simulating the degradation characteristics of decay-like composite insulators.

2. Materials and Methods

2.1. Fabrication of Defect-Containing Specimens

Composite insulators mainly consist of three parts: a GFRP core rod, a silicone rubber housing, and metal end fittings. The first two components adopt typical composite material structures. The core rod is generally made of GFRP, with continuous unidirectional E-glass fibers as the reinforcement phase and epoxy resin as the matrix. The resin’s toughness and weather resistance are enhanced by the addition of curing agents and modifiers. The housing is made of high-temperature vulcanized (HTV) liquid silicone rubber, based on polydimethylsiloxane (PDMS), with fillers such as aluminum hydroxide (Al(OH)₃) or fumed silica (SiO₂) added to improve its flame retardancy and mechanical properties.

A batch of field-decommissioned composite insulator decay-like specimens was obtained in this study. During operation, these specimens were found to exhibit various degrees of abnormal temperature rise and other fault conditions. Through a comparative analysis of the appearance characteristics of these specimens, it was observed that multiple specimens exhibited several distinct, continuous carbonization channels on the surface of the core rods, as shown in Figure 1. The approximate path of the carbonization channels is marked with red dashed lines in the figure.

To study the staged impact patterns of conductive defects in composite insulators, this study intended to fabricate composite insulator specimens with internal conductive defects, referencing the appearance of carbonization channels on the core rods of the aforementioned decay-like specimens. According to the literature [22], the electrical erosion test can quickly produce GFRP core rod specimens with conductive defects under laboratory conditions, simulating the degradation state of the core rod under actual operating conditions. Based on this method, a GFRP core rod electrical erosion test platform was established in this study, as shown in Figure 2.

Within this platform, the GFRP core rod specimens with attached fittings are supported at both ends by two insulating tripods. The high-voltage end of the specimen is connected to a power-frequency high-voltage power supply, and a grounded electrode is connected at a specified distance from the end of the high-voltage fitting. The surface of the core rod between the two electrodes is wrapped with a uniformly thick, wet gauze that has been pre-soaked in a sodium chloride solution with a conductivity of 2500 μS/cm.

After the setup was completed, a continuous voltage was applied to the sample until discharge occurred at the high-voltage end. Initially, intense yellow discharges appeared on the surface of the core rod. As the experiment progressed, the discharge zone gradually moved toward the grounded end. Eventually, arc discharges spanned the two electrodes, and flashover occurred in the specimen.

Based on the platform described above, three GFRP core rod specimens with conductive defects of different lengths (12 cm, 24 cm, and 40 cm, respectively) were fabricated using the electrical erosion test. Under the action of electrical erosion, distinct continuous carbonized channels formed on the core rod surface, all originating from the high-voltage end. Their morphology was similar to the carbonization channels on the surface of the core rods in decay-like composite insulators under actual operating conditions, as shown in Figure 3. Using the injection molding process, composite insulator specimens with varying lengths of internal conductive defects were fabricated from the GFRP core rod specimens created in the preceding steps. At the same time, a defect-free composite insulator was also made using the same process for comparison. Each specimen was numbered, as shown in

Table 1. Specimen IDs for different defect conditions.

Specimen ID	A	B	C	D
Defect condition	defect-free	12 cm	24 cm	40 cm

.

2.2. Finite Element Simulation

The failure of composite insulators due to decay-like fractures is primarily concentrated in high-voltage transmission lines at 220 kV and above. Therefore, the three-dimensional simulation model in this study was designed using the specifications of a 220 kV composite insulator sample.

In the simulation, the electric field equations that must be satisfied for the current field calculation of the insulator model surface and the surrounding air are as follows:

For electrostatic fields, the electric potential φ satisfies Poisson’s equation:

$$\nabla \cdot \left(\epsilon \nabla \mathit{\phi }\right)=-\rho$$

(1)

In the equation, ε is the permittivity of the medium, and ρ is the charge density.

In the absence of free charges, Poisson’s equation simplifies to Laplace’s equation:

$$\nabla \cdot \left(\epsilon \nabla \mathit{\phi }\right)=0$$

(2)

The relationship between the electric field intensity E and the electric potential φ is given by:

$$E=-\nabla \mathit{\phi }$$

(3)

The relationship between the current density J and the electric field intensity E is given by Ohm’s law:

$$J=\sigma E$$

(4)

In the equation, σ represents electrical conductivity.

The electrical parameters for the materials of each component in the composite insulator are listed in

Table 2. Dielectric parameters of the composite insulator.

Material	Relative Permittivity (Real Part)	Dielectric Loss Tangent
Silicone rubber	3.4	0.0035
Core rod	4.2	0.005
Air	1	0.00008
End fitting	10,000	0.0005
Conductive defect	10,000	1

. To highlight the impact of conductive defects on the electric field distribution of the composite insulator, the relative permittivity of the defect area was set to be the same as that of the metal.

Under service conditions, composite insulators often experience overheating driven by two primary mechanisms: dielectric losses resulting from the polarization effect of the dielectric under the influence of the power-frequency alternating electric field, and losses caused by leakage current. As illustrated in Figure 4, the dielectric equivalent circuit includes C_g (lossless polarization capacitance), R_p and C_p (resistance and capacitance reflecting lossy polarization, respectively), and R_lk (leakage resistance), which encompasses both bulk and surface paths. I_lk denotes the leakage current through the dielectric [34].

Furthermore, the loss power per unit volume of the dielectric material can be expressed as follows:

$$p=E{J}_{\mathrm{r}}=E^2\omega \epsilon \tan \delta$$

(5)

In the equation, E is the electric field intensity distribution of the dielectric, J_r is the total active current density in the dielectric, ω is the angular frequency of the power-frequency sinusoidal electric field, ε is the permittivity of the dielectric, and tanδ is the loss tangent of the dielectric.

In thermodynamics, the three basic forms of heat transfer are conduction, convection, and radiation. In the case of load balance, the effect of radiation generated by the object is generally not considered. Therefore, this study only discusses the first two forms of heat transfer.

Assuming a static medium and no mechanical energy exchange with the surroundings, the differential equation for heat conduction can be derived based on energy conservation principles:

$$\left\{\begin{array}{l}\mathit{q}=-k\nabla \mathit{T}\\ \rho c{\displaystyle \frac{\mathsf{\partial }\mathit{T}}{\mathsf{\partial }t}}=-\nabla \cdot \mathit{q}+q_{\mathrm{v}}\end{array}\right.$$

(6)

In the equation, T(x,y,z,t) represents the spatiotemporal distribution of the temperature field; q is the heat flux density, with units of W/m²; k is the thermal conductivity, with units of W/(m∙K); ρ is the density of the medium, with units of kg/m³; c is the specific heat capacity of the medium, with units of J/(kg∙K); and q_v is the heat source intensity, with units of W/m³.

At the boundary interfaces, specifically between the insulator sheath, hardware, and surrounding air, convective heat exchange is described as follows:

$$-\mathit{n}\cdot \mathit{q}=h(T_1-T)$$

(7)

In the equation, h represents the convective heat transfer coefficient, measured in W/(m²∙K), and T₁ is the ambient temperature.

With Equations (6) and (7) established for simulating the temperature distribution in composite insulators, the corresponding Lagrangian functional integral equation is constructed via the variational approach:

$$J={\displaystyle {\int }_{\mathrm{\Omega }}(\frac{k}{2}}{‖\nabla T‖}^2-q_{\mathrm{v}}T)d\mathrm{\Omega }+{\displaystyle {\int }_{\mathsf{\partial }\mathrm{\Omega }}\frac{h}{2}}{\left(T_1-T\right)}^{2}d\mathrm{\Gamma }$$

(8)

After discretizing the above functional integral equation to seek the minimum value, the finite element equations can be obtained. The relevant thermal parameters of the insulator are presented in

Table 3. Thermodynamic parameters of composite insulator.

Material	Thermal Conductivity (W/(m∙K))	Specific Heat Capacity (J/(kg∙K))	Density (kg/m3)
Silicone rubber	0.25	1000	1200
Core rod	0.3	500	1800
Air	0.0011	1.01	25
End fitting	450	500	8000
Conductive defect	5	1000	1600

, and a detailed description of these parameters is not provided in this paper.

Based on the appearance of the carbonization channels on the surface of the core rod specimens described above, a modeling simulation of composite insulator specimens with conductive defects was carried out using 3D plotting software (SOLIDWORKS 2021). The electro-thermal coupled finite element simulation was conducted using COMSOL 6.2. The actual specimen and the simulation model are shown in Figure 5.

The carbonization channels in composite insulators typically originate at the high-voltage end, where the electric field intensity is more concentrated. Therefore, in the simulation model, the conductive defect was placed at the end of the junction between the insulator’s core rod and the high-voltage end fitting, with a length of 12 cm.

To ensure the reliability of the simulation results, a mesh independence study was conducted. The model was solved with three levels of mesh density (coarse, medium, and fine), and the resulting electric field strength and temperature rise values at critical defect locations were compared. The difference between medium and fine meshes was found to be less than 2%, indicating mesh convergence. Therefore, the medium mesh was adopted for the simulations in this study to balance accuracy and computational efficiency.

Additionally, possible error sources, such as simplifications in boundary conditions and uncertainty in material parameters, were considered and discussed. The influence of these factors was minimized by referring to literature-reported data and performing sensitivity checks where appropriate.

2.3. Charged Testing and Diagnostics

To verify the reproducibility of the impact of internal conductive defects on composite insulators, as revealed by the simulation analysis, this study conducted charged testing and operational characteristic inspections on the defective specimens under actual operating conditions.

To simulate the charged state of composite insulators under field operating conditions, this study applied a power-frequency voltage to the insulator using a power-frequency high-voltage power supply in a laboratory environment. The rated voltage of this power-frequency high-voltage source was 1500 kV, with a rated capacity of 2250 kVA. During the test, the sample was suspended, with the high-voltage end fitting connected to the power supply and the low-voltage end fitting connected to the ground. The test voltage was set to 127.02 kV (the theoretical phase voltage value for the 220 kV voltage level). The experimental setup is shown in Figure 6.

Although the decay-like fracture failures of composite insulators are mainly concentrated in transmission lines of 220 kV and above, defects in composite insulators, as well as abnormal operating characteristics, typically occur at the high-voltage end. Therefore, for the convenience of specimen preparation and testing, and with mechanical stress neglected, this study used two 110 kV composite insulators connected in series to replace the 220 kV insulators in the experiment. The high-voltage end of the defective specimen was connected to the power-frequency high-voltage power supply, while the other end was connected to an intact insulator.

For the experimental control, this study obtained several field-decommissioned decay-like specimens and aged sheath composite insulator specimens to observe their operational characteristics during the charged tests.

To observe the infrared temperature rise characteristics, ultraviolet discharge characteristics, and electric field distribution of composite insulator specimens with internal defects under energized conditions, an infrared thermal imager, ultraviolet imaging device, and integrated electro-optic crystal electric field sensor were used to monitor the specimens during the energized test. To ensure the reliability of the experimental data, repeated measurements were performed under identical conditions to minimize random errors. For ultraviolet detection, the observed ultraviolet intensity was qualitatively analyzed using a calibrated ultraviolet imaging system. The positioning and exposure conditions were kept consistent across all measurements to minimize environmental interference. These measures collectively ensured the reliability of the experimental data.

An FLIR T1040 handheld infrared thermal imager (Teledyne FLIR LLC, Wilsonville, OR, USA) was employed to detect the temperature rise in the composite insulator during energized testing. The imager offered a resolution of 1024 × 768 pixels, supported a temperature measurement range of −40 °C to 2000 °C, and provided a measurement accuracy of ±1%. The test distance was set to 6 m. After applying voltage for 30 min and waiting for the temperature of the insulator to stabilize, the infrared imager was used to record the heating characteristics of the insulator. Thermal images were analyzed to extract peak surface temperature values and identify the distribution of abnormal heating regions.

An OFIL Uvolle-VX handheld ultraviolet imager (OFIL Systems, Ness Ziona, Israel) was used to detect partial discharge in the composite insulator during energized testing. The imager was capable of detecting discharges as low as 1 pC at 12 m and covered a spectral range of 240–280 nm.

An LM-DE301 optical electric field measurement instrument (Beijing Panwoo Integrated Optoelectronic Inc., Beijing, China) and PicoScope 2406B USB-based PC oscilloscope (Pico Technology Ltd., St. Neots, Cambridgeshire, UK) were employed to measure the electric field distribution of the energized specimen. The sensor supported a measurement range of 30–3000 kV/m and a frequency range of 5 Hz to 40 MHz. It featured primary electrical insulation and did not interfere with the measured field. During testing, the sensor probe was positioned parallel to the surface of the insulator sheath at a fixed distance of 50 mm.

Moreover, to observe the characteristics of the specimens at the microscopic level, SEM and EDS tests were conducted on both the defect areas of the specimens and the defect-free specimens.

3. Results

3.1. Simulation Results

The surface electric field distributions of the intact composite insulator and the composite insulator with defects, obtained through simulation calculations, are shown in Figure 7. The unit is kV/m.

The surface electric field distribution of the defect-free composite insulator was uniform, with the electric field strength gradually decreasing from the high-voltage end to the grounded end, without any electric field distortion. By contrast, for the composite insulator with defects, the surface electric field near the high-voltage end exhibited severe distortion and became highly concentrated. The electric field at the defect’s end and its branching points significantly increased, with the maximum field strength located at the defect’s end, reaching 1933.2 kV/m. By contrast, the field strength at this location in the defect-free model was only 138.39 kV/m. Due to the conductive defect raising the potential along its path, the remaining insulating section at the high-voltage end experienced a reduced voltage drop, resulting in a lower electric field intensity in that region compared to the defect-free case.

The temperature rise is shown in Figure 8, with the unit in Kelvin (K). The surface of the defect-free composite insulator showed no significant temperature rise, with an initial temperature of 293.15 K. In the model with defects, a noticeable temperature rise appeared on the surface at the defect’s bending point, with the highest temperature reaching 315.44 K. The initial temperature was 293.15 K, and the maximum temperature rise was 22.29 K.

Analysis of the surface electric field distribution and temperature rise of the composite insulator, obtained through simulation, revealed that the presence of internal conductive defects in the composite insulator significantly increased the relative permittivity of the dielectric in this area. The electric field at the high-voltage end underwent severe distortion, and the surface electric field became highly concentrated along the defect path. Additionally, the electric field at the defect’s end and the ends of its branches increased significantly, with the highest surface electric field occurring at the end of the main branch. The high-voltage end defect area of the composite insulator exhibited abnormal temperature rise, characterized by BFTR. Under the combined influence of the electric field and temperature, the components in the vicinity of the defect underwent long-term electrothermal aging, which accelerated the degradation process of the composite insulator.

To verify the rationality and accuracy of the simulation results, this study conducted charged tests on composite insulators with internal conductive defects and performed various operational characteristic tests, including infrared and ultraviolet inspections.

3.2. Infrared Inspection Results

In this study, the maximum difference between the insulator surface temperature at steady state and the ambient temperature is defined as the temperature rise (ΔT) of the insulator. The measured ambient temperature during the test process for specimens A–D and decay-like specimens was 6.0 °C. By contrast, the ambient temperature during the test process for the aged sheath specimens was 17.0 °C.

The temperature rise characteristics of specimens A–D are presented in Figure 9. Specimen A exhibited no significant temperature rise, with a slight temperature rise observed in the sheath area near the high-voltage end, showing a maximum temperature rise of 3.9 °C, which did not differ significantly from the ambient temperature. Specimen B showed two distinct areas of abnormal temperature rising, located at the connection between the high-voltage end sheath and the fitting, and near the 1st shed, with a maximum temperature rise of 14.2 °C. There was a noticeable difference in the infrared characteristics compared to the defect-free specimen. Specimen C also exhibited two distinct areas of abnormal temperature rise, located at the connection between the high-voltage end sheath and the fitting, and near the 6th to 8th shed, with a maximum temperature rise of 23.5 °C. The two temperature rising areas were more widely spaced, and the abnormal temperature rise was more severe. Specimen D displayed two prominent regions in abnormal temperature rise, located near the 6th to 9th shed and the 13th to 14th shed, with a maximum temperature rise of 60.2 °C. The abnormal temperature rise situation further intensified, and a slight temperature rise phenomenon was also observed in the high-voltage end sheath area.

As shown in Figure 10, the temperature rise pattern of the aged sheath composite insulator specimens was primarily PFTR. Since the maximum surface electric field typically occurred at the high-voltage end, the aging phenomenon of the sheath usually began at this location. As a result, the temperature rise spots for this type of defect were concentrated at the junction between the high-voltage end sheath and the fitting, with a maximum temperature rise of 16.2 °C.

As shown in Figure 11, the decay-like composite insulator specimens exhibited two areas of abnormal temperature rise, with the temperature rise pattern primarily characterized by BFTR. The temperature rise was concentrated between the high-voltage end and the 6th large shed, as well as between the 13th and 15th large sheds. This highly coincided with the areas of severe deterioration in the specimen, with the maximum temperature rise reaching 76.7 °C.

According to the infrared detection results in this study, the composite insulator specimens fabricated using the conductive defect creation method presented in this paper all exhibited multiple distinct areas of abnormal temperature rise, primarily in the form of BFTR. The temperature rise regions were mainly concentrated near the bending points and the ends of the internal defects, as illustrated in Figure 12, and showed a good correspondence with the simulation results. As the defect length increased, the maximum temperature rise of the specimens also increased. Compared to the aged sheath specimens, the infrared characteristics of the composite insulator specimens with conductive defects fabricated in this study were more similar to those of the decay-like specimens. By comparing the maximum temperature rise and temperature rise patterns, it could be concluded that conductive defects are the dominant cause of abnormal temperature rise in composite insulators.

3.3. Ultraviolet Inspection Results

To facilitate observation of the measurement results, the gain of the ultraviolet imager was set to G = 100, and the ultraviolet discharge detection results at the moments of maximum discharge severity for each specimen were compared. The detection results for specimens A–D are shown in Figure 13.

Specimen A exhibited slight discharge phenomena, primarily concentrated near the high-voltage end fitting, possibly caused by electric field distortion at the high-voltage end. The specific electric field distribution is described in Section 3.1 and Section 3.4. Specimen B showed a discharge phenomenon slightly more severe than the intact specimen, with the discharge area expanded somewhat. Concentrated discharge phenomena were observed from the high-voltage end fitting to near the 1st shed. Specimen C exhibited two distinct concentrated discharge areas. The first area was located near the high-voltage end fitting, exhibiting weak discharge phenomena, while the second area was situated near the 3rd to 9th sheds, characterized by more severe discharge phenomena. Specimen D also showed two regions of concentrated discharge. The first area was located near the high-voltage end fitting, exhibiting weak discharge phenomena, while the second area was situated near the 9th to 16th sheds, where the discharge phenomena were further intensified.

As illustrated in Figure 14, the aged sheath specimen also exhibited slight discharge phenomena only at the high-voltage end, with the discharge severity slightly greater than that of the intact composite insulator specimen.

As shown in Figure 15, the decay-like specimens exhibited two concentrated discharge areas, located at the high-voltage end and the end of the severely deteriorated region.

From the above results, it could be concluded that internal conductive defects, resulting from the presence of the tip electrode at the defect’s end and the concentration of the electric field, trigger partial discharges in the composite insulator. As the defect length increases, the discharge phenomenon under charged testing gradually intensifies, with the areas of severe discharge closely coinciding with the defect’s end position. The discharge characteristics more closely resemble the decay-like composite insulator specimens.

3.4. Electric Field Measurement Results

Due to the complexity of the spatial electric field distribution and the difficulty in accurately fitting it, coupled with the challenge of ensuring sensor positioning stability during the testing process, this study adopted a fixed-point measurement approach. The specific selection rule for the test points is as follows: starting from the high-voltage end sheath, the midpoint between each pair of large sheds is chosen as a fixed measurement point, continuing until the defect’s end region. The measurement results were normalized based on the electric field magnitude between the 2nd and 3rd large sheds for the specimen with a carbonization channel length of 40 cm, and the results are shown in Figure 16.

The electric field intensity peak for the intact composite insulator specimen was located just before the 1st large shed at the high-voltage end, showing a trend of gradually decreasing from the high-voltage end to the grounded end. For the specimens with carbonization channel lengths of 12 cm, 24 cm, and 40 cm, the electric field intensity peak appeared before the 1st large shed, between the 2 and 3 large shed, and between the 2–3 and 4–5 large sheds, respectively. The internal defects caused significant distortion in the surface electric field distribution of the specimens, with an abnormal increase in electric field strength in multiple regions. The electric field distortion region highly correlated with the bending points and end points of the defects. Internal conductive defects resulted in a significant increase in the surface electric field intensity near the defect, with the maximum value occurring at the endpoint or bending point of the defect, demonstrating good correspondence with the simulation results.

4. Discussion

4.1. Equivalence Analysis

To analyze the similarities and differences in the operational characteristics between the specimens in this study and the actual decay-like specimens, specimen D, which exhibited the most distinct features, was selected for comparison. Its infrared temperature rise characteristics and ultraviolet discharge characteristics were compared with those of the decay-like specimens.

By comparing the infrared measurement results in Figure 17, it can be observed that both specimens exhibit a temperature rise pattern primarily in the form of BFTR, with two distinct temperature rise areas and similar maximum temperature rise values. This suggests that the temperature rise in the BFTR pattern is primarily due to the internal carbonization channels. However, the first temperature rise area of the decay-like specimen is longer, starting from the high-voltage end and extending to near the 6th large shed, while the first temperature rise area of specimen D is primarily concentrated around the defect’s bending point. The reason for this is that the decay-like specimen has multiple temperature rising factors at the high-voltage end, such as sheath aging and cracking, core rod exposure, and carbonization channels, which lead to more severe temperature rising than in specimen D.

As shown in Figure 18, both specimens exhibit two concentrated discharge areas under charged test conditions. The discharge areas of the decay-like specimen are concentrated at the high-voltage end and the end of the severely deteriorated region, while the discharge areas of specimen D are focused at the high-voltage end and the defect’s tail, showing good correspondence with their respective abnormal temperature rise regions and accurately reflecting the locations of severe deterioration or defects. The discharge in the decay-like specimen is caused by defects, such as corrosion in the core rod, sheath, and detachment of the protective layer, that expose the core rod to air. Air fills the defect to form a gap [35]. By contrast, the discharge in the specimens from this study is due to the low-resistance channel formed by the defect, which causes electric field distortion and local concentration, leading to electron avalanche and, consequently, partial discharge.

To compare the similarities and differences at the microscopic level, SEM and EDS tests were conducted on the defect areas of the two specimens and the defect-free specimen, as shown in Figure 19 and Figure 20, respectively.

Figure 19 presents the SEM image comparison of three types of specimens. In the defect region of the specimens fabricated in this study, the glass fiber surface appears smooth and clean, with the resin matrix almost completely ablated. By contrast, the actual decay-like specimen shows more pronounced degradation features, including significant hydrolysis and erosion of the resin and partial exposure of the glass fibers. The remaining resin is sparsely and unevenly distributed, with numerous micron-scale pores and electrical erosion holes clearly observable. The defect-free specimen, on the other hand, exhibits a dense and well-organized structure. The interface between the glass fibers and the resin matrix is intact and firmly bonded, with no signs of hydrolysis, discharge marks, or electrical erosion.

Compared to the intact core rod material, the decay-like core rod shows an increase in the contents of Si and O elements and a decrease in the content of C element [11]. This study employed area scanning in the EDS analysis, and from the EDS results in this study, it is evident that the elemental composition of the specimens fabricated in this study is very similar to that of the actual decay-like specimens, with similar proportions of Si, O, C, N, Al, and other elements. Compared to the intact specimens, both exhibit a significant increase in the contents of Si and O elements, as well as a substantial decrease in the content of C element, which is consistent with the characteristics described above. Therefore, the elemental composition and variation trends of the specimens fabricated in this study are similar to those of the decay-like specimens.

After long-duration charged testing, the sheath at the end region of the defect in specimen D underwent carbonization, forming electrical erosion holes. The morphological features of these holes are highly similar to those observed on the decay-like specimens, as shown in Figure 21. This confirms that internal conductive defects are a significant factor in the formation of electrical erosion holes on the insulator sheath surface. The presence of these electrical erosion holes will significantly exacerbate moisture ingress and discharge activity in the composite insulator, accelerating its decay-like degradation process [36,37].

By comparing the charged infrared characteristics, ultraviolet discharge characteristics, SEM, and EDS test results of the specimens in this study with those of the decay-like specimens, it was found that the characteristics of both specimens are highly similar. This suggests that the specimens fabricated using the defect creation method in this study can, to some extent, simulate the operational characteristics of decay-like composite insulators. It also validates the rationality and scientific basis of the defect fabrication method.

4.2. Influence of Conductive Defects

The key features observed in defect-free, defective, and decay-like specimens are summarized in

Table 4. Summary of test results for composite insulator specimens.

Specimen Type	Defective Specimen	Decay-Like Specimen	Defect-Free Specimen
Infrared characteristics	Two BFTR regions at defect tip and bending point	Two or more BFTR regions at high-voltage end and degraded section	No abnormal heating observed
Electric field distribution	Significant enhancement at defect tip; shift of peak field to grounded end as defect length increases	Not yet measured	Uniform field distribution; no concentration zones
Ultraviolet characteristics	Continuous ultraviolet discharge at the high-voltage end and the defect terminal	Intense ultraviolet discharge at the high-voltage end and the severely deteriorated region	No ultraviolet discharge detected
SEM observations	Smooth glass fiber surface; resin almost completely ablated; electrical erosion holes observed	Severe resin hydrolysis; exposed fibers; porous surface with micro-eroded pits	Dense resin matrix; good bonding to glass fibers; no signs of erosion
EDS results	Low C content, high Si and O contents	Low C content, high Si and O contents	Stable elemental composition

. This comparative analysis highlights the differences in thermal, electrical, and morphological responses among the three specimen types, thereby providing a more complete understanding of the defect evolution and its impact on insulator performance.

The electric field simulation results in this study showed a significant correlation with the electric field measurement data from the charged tests, validating the accuracy of the simulation model. The research indicated that when a continuous carbonization channel exists within an insulator, a low-resistance conductive path forms at the interface between the GFRP core rod and the silicone rubber sheath [23,38]. The relative permittivity in this region increases significantly compared to the surrounding materials, leading to the formation of a high electric field strength area at the defect. The maximum field strength shifts toward the grounded end as the defect length increases. The electric field near the defect becomes highly non-uniform, causing the electric field endured by the sheath to far exceed the conventional design value, significantly accelerating the material degradation process.

The ultraviolet detection results indicated that the discharge initiation location closely aligns with the high electric field strength area predicted by the simulation, exhibiting a high degree of spatial overlap. Due to the electric field distortion, the electric field in the defect area increases significantly. When the local electric field strength exceeds the breakdown threshold of the material, ionization occurs in this region, leading to an electron avalanche. The energy released during this process triggers partial discharge. During discharge, high-energy electrons and ions are injected into the surface or interior of the insulating material, forming a space charge layer that further distorts the electric field in this area. The Joule heat and chemical reactions produced by the discharge carbonize the insulating material, either expanding existing defects or creating new ones.

Due to the presence of conductive defects, a low-resistance path forms inside the insulator, and under the influence of a high electric field strength, the current density increases locally. The Joule heat generated by polarization loss and conduction loss, combined with the Joule heat from discharge, causes abnormal temperature rise at the defect location. This is primarily manifested as BFTR in the defect’s bending and end areas. The measured maximum temperature rise reached 60.2 °C. Long-term thermal exposure leads to a further increase in the conductivity of the core rod and sheath materials, forming a positive feedback loop of “temperature rise–conductivity increase–current increase,” accelerating the degradation process of the composite insulator.

It can be seen that the impact of conductive defects on the performance of composite insulators is not simply the result of the superposition of a single physical process but rather the outcome of the coupling of multiple factors, including electric field distortion, Joule heating effects, material degradation, and discharge activity, as shown in Figure 22. Electric field distortion triggers discharge and concentrates the current, leading to a temperature rise. The temperature rise alters the material’s dielectric properties, further intensifying the electric field distortion. Discharge activity expands the existing defects and reconstructs the multi-physical field distribution.

5. Conclusions

This study developed an electro–thermal coupling simulation model for composite insulators with defects, analyzing the impact of internal conductive defects on the surface electric field distribution and temperature rise of composite insulators. A GFRP core rod electrical erosion test platform was established, and composite insulator specimens with varying lengths of conductive defects were fabricated. Charged tests were conducted on the fabricated specimens, as well as on field-decommissioned decay-like specimens and aged sheath specimens. Infrared, ultraviolet, and electric field tests were performed to analyze their operational characteristics. A comparative analysis was conducted on the abnormal temperature rise, ultraviolet discharge characteristics, SEM, and EDS test results of the conductive defect specimens and decay-like specimens. The study examined the impact of internal conductive defects on the operational characteristics of composite insulators. The specific conclusions are as follows:

• The simulation results in this study showed good correspondence with the charged test results. The findings indicate that internal conductive defects in composite insulators cause significant non-uniformity in the electric field near the defects, with the surface electric field becoming highly concentrated along the path of the defect. The electric field at the defect’s end and bending points increases substantially. These defects trigger partial discharge and abnormal temperature rise at the defect’s end and bending points, with the temperature rise predominantly manifesting as BFTR. The temperature rise regions correlated well with the discharge areas. As the defect length increases, the maximum field strength increases and shifts toward the grounded end, and the maximum temperature rise and discharge intensity also increase accordingly.
• The main characteristics of composite insulators with conductive defects under charged test conditions are as follows: Single or multiple regions of abnormally elevated electric field strength appear along the axial direction. In addition to the high-voltage end, the defect section exhibits continuous ultraviolet discharge, and two or more BFTR regions form on the sheath surface. These characteristics are primarily concentrated on the high-voltage side of the insulator. They can serve as effective criteria for on-site diagnosis of conductive defects, significantly improving the accuracy of defect identification.
• The SEM and EDS test results showed that partial discharge causes ablation of the resin matrix on the surface of the core rod, leading to the exposure of glass fibers and the formation of microscopic electrical erosion holes in the defect area. This confirms that partial discharge is the primary cause of the formation of conductive defects. Continuous partial discharge leads to carbonization of the sheath, forming electrical erosion holes and triggering a temperature rise. This indicates that internal conductive defects are the dominant factor in abnormal temperature rise and the formation of electrical erosion holes in composite insulators. The electrical erosion test in this study effectively reproduced the key defect characteristics of decay-like composite insulators, showing a high degree of consistency with field-aged specimens in infrared, ultraviolet, SEM, and EDS results, thereby validating the scientific soundness of the specimen preparation method.
• This study introduces a defect diagnosis method based on infrared and ultraviolet imaging features, which enables the identification of defect morphology and severity. This approach holds practical value for engineering diagnostics and enriches the composite insulator defect identification framework.
• The mechanism by which conductive defects affect the performance of composite insulators is clarified as a positive feedback process driven by multi-physical field coupling. This study reveals the evolutionary sequence comprising electric field distortion, discharge, temperature rise, material degradation, and further electric field distortion, thereby deepening the scientific understanding of the decay-like deterioration mechanism and providing a theoretical foundation for the development of prevention strategies.