Ester insulating liquids feature a fire point above 300℃ and are biodegradability. It is often used as a substitute for mineral oil in transformer retrofilling engineering. Nevertheless, the old oil is difficult to be completely removed, and the residual mineral oil will deteriorate the physicochemical and dielectric properties of mixed system. Detecting the mineral oil content in the mixture is an important method for performance evaluation, but the existing detection methods are difficult to simultaneously balance accuracy and efficiency. In this paper, the applicability, accuracy, and economy of six detection methods including thermal conductivity method, kinematic viscosity method, fire point method, Fourier transform infrared spectroscopy (FTIR) method, dielectric properties method, and iodine value method in the detection of mineral oil content in mixed insulating liquids of natural esters (FR3, RAPO) and synthetic esters (KI50EX, TFO 100) systems were systematically compared. By constructing linear regression models between detection parameters and mineral oil mass fractions, the prediction accuracy of each method was evaluated based on the coefficient of determination (R²). The results show that for natural ester-based mixed systems, the 60℃ kinematic viscosity prediction method has the highest priority, followed by the 40℃ thermal conductivity method, iodine value method, and FTIR method. For synthetic ester-based mixed systems, the 60℃ kinematic viscosity prediction method is the most cost-effective, with the thermal conductivity and FTIR method following closely behind. The iodine value method is inapplicable to synthetic ester systems due to the saturated chemical structure of synthetic esters. Fire point tests cannot provide a clear indication of the residual mineral oil content but can help quickly determine whether mixed insulating liquids meet the K-level flame-retardant requirements. Dielectric parameters are significantly affected by environmental factors such as moisture and are not suitable as quantitative detection indicators for mineral oil residues.

Dry-type transformer windings are directly exposed to the air, and are highly susceptible to interlayer short-circuit faults due to environmental factors such as temperature and humidity. Moreover, the early insulation degradation is difficult to detect. Therefore, this paper aims to investigate the electromagnetic characteristics of winding interlayer insulation faults, providing a theoretical basis for optimal insulation configuration and condition diagnosis. First, an interlayer short-circuit current testing method using dry-type transformer taps was proposed, and a 3D "field-circuit" coupled model consistent with actual transformer dimensions was established for experimental validation. Subsequently, based on the temperature-humidity-resistance correlation characteristics of insulation materials, a parametric model was established for winding interlayer insulation resistance. The evolution laws of fault-layer current and spatial magnetic flux density under different fault locations were simulated and analyzed during the insulation resistance decay process from 10⁶ Ω to 0 Ω. The results show that the fault layer current and magnetic flux density exhibit a non-linear and highly sensitive response to the changes in interlayer insulation resistance. When the insulation resistance drops to 10³ Ω and 10 Ω, respectively, the electromagnetic parameters undergo significant abrupt changes, and the change rates of fault layer current and magnetic flux density increase to 12.55 A/Ω, 0.01 mT/Ω, and 66.69 A/Ω, 14.65 mT/Ω, respectively. Furthermore, after the interlayer insulation resistance decreases to 10 Ω, as the fault location moves from the outer to the inner layer, the fault layer current and spatial magnetic field first increase and then decrease, and presenting a spatial distribution pattern of "maximum in the center, minimum at the edges". On the basis of these abrupt change characteristics, 10³ Ω and 10 Ω are determined as the critical resistance criteria for interlayer insulation degradation and breakdown of winding layers in dry-type transformers, respectively. According to their spatial distribution characteristics of current and magnetic field under interlayer insulation fault, it is concluded that the middle layer of the winding is the area with weak insulation, while the edge layer is the area where fault detection is unfavorable.

Dielectric loss factor measurement is an effective means for detecting defects in capacitive electrical equipment. However, due to the limitation of the instrument power supply capacity, there are many difficulties in the application of high-capacitance equipment. The damped AC voltage method requires low power supply capacity, and the oscillation frequency can be adjusted to be close to the power frequency. However, the current dielectric loss factor measurement using this method is based on the waveform attenuation parameters of damped AC voltage, which is far less accurate than digital bridges and can only reflect particularly severe defects. Therefore, a method to measure the dielectric loss factor of high-capacitance equipment by using damped AC voltage combined with digital bridge was proposed in this paper. Firstly, a dielectric loss factor measurement system based on damped AC voltage was established. The dielectric loss factor was extracted by full-phase Fourier calculation. The dielectric loss factor measurement results of sinusoidal waves and damped AC voltage with different frequencies were compared, the difference between the two method were analyzed, and the correction method was proposed. Then, comparative tests were conducted on the cable samples aged in the laboratory and those exposed to water and moisture, as well as on-site 220 kV CVT, and the feasibility of dielectric loss factor measurement based on the damped AC voltage combined with digital bridges was verified. The results show that the absolute deviation between The absolute deviation between the measurement results of the measurement system established in this article under 50 Hz damping oscillation voltage and the measurement results of the standard dielectric loss measuring instrument is less than 0.1%, which can meet the requirements of engineering applications.

With the acceleration of urbanization, 10 kV distribution cables, as core components of urban power grids, face severe challenges in safe operation. Cable joints are prone to explosion accidents due to electromagnetic-thermal-mechanical multi-field coupling effects, and traditional explosion-proof methods exhibit deficiencies in heat dissipation and stress balance. In this paper, the cable with model YJV22 8.7/15 3×35 mm² as the research object, and a two-way coupling model of electromagnetic field, temperature field, and solid mechanics field were established to study the temperature and stress distribution patterns of 10 kV cable joints under overload and short-circuit conditions systematically and explore the influences of dimension parameters (radius ratios K1, K2) of the explosion-proof box and sealing materials on explosion-proof performance. The results show that although installing the explosion-proof box impedes heat dissipation, leading to the peak temperature of cable joint rise by about 4℃, while by optimizing the radius ratios (K1) of explosion-proof box to 1.5-1.6 and K2 to 1.6-1.7, the heat dissipation efficiency of cable joint improves significantly and the stress concentration decreases. When the thermal conductivity of the sealing material exceeds 0.5 W/(m·K), the heat dissipation disadvantages of the explosion-proof box can be compensated and the current-carrying capacity increases. Additionally, polyurethane sealant can reduce the stress peak valueof the explosion-proof box by approximately 47 times compared to epoxy resin.

To investigate the partial discharge behaviors of typical insulation defects under normal operating temperatures, DC cross-linked polyethylene cables were taken as the objects, three kinds of defects such as bubbles, protrusions, and scratches were introduced to the cables. High-frequency current signals were collected at 70℃ to analyze their discharge pulse characteristics, and the change laws of partial discharge inception voltage (PDIV) and extinction voltage (PDEV) were tested within the temperature range of 40-80℃. The results show that the PD behavior of different defects at the same temperature are significantly different, the protrusion defects exhibit the highest discharge pulse repetition rate, whereas the bubble and scratch defects show larger pulse amplitudes. Spectral analysis reveals that the frequency band of discharge signal from protrusion defect is broader and the energy distribution is wider. Temperature elevation significantly reduces the PDIV and PDEV for the three kinds of defects, among which bubble and protrusion defects are more sensitive to temperature, while scratch defects show a smaller change with temperature. Additionally, the difference between PDIV and PDEV of bubble defects is the largest, and the discharge sustainability is the strongest.

Malfunctions in the insulation system of high-voltage cable terminations can directly trigger breakdown faults, which severely undermines the normal operation of high-voltage transmission lines. In this paper, after analyzing a 110 kV cable terminal breakdown accident, it is found that the lead seal at the terminal tail pipe had partially fallen off, meaning that the metal sheath of the cable had failed to ground at this location. Using the PSCAD/EMTDC electromagnetic transient simulation software for calculation, it was revealed that when only the lead seal detachment (metal sheath grounding resistance) was considered, the voltage rise inside the terminal was relatively weak, and it would be difficult to cause insulation failure of the terminal in a short period of time. Based on this, this paper considered the situation where the cable was invaded by lightning overvoltage. In this case, due to the complex internal structure of the cable terminal, it was more prone to electric field distortion, which eventually led to terminal insulation deterioration, failure or even breakdown. For this scenario, the finite element method was employed to simulate and calculate the electric field distortion inside the terminal when lightning over-voltage intrusion occurred under different degrees of lead seal detachment. The simulation results show that the degree of lead seal detachment has a significant influence on the induced voltage of sheath. With the increase of grounding resistance, the induced voltage on the cable metal sheath first rises and then tends to stabilize, and the equivalent resistance reaches approximately 10⁵ Ω at the stabilization stage. Under the same conditions, the internal electric field distortion of the the cross-interconnected grounding mode is more serious than that of the head-end grounding mode. Additionally, after fitting the results with the least squares method, it is found that the field strength distortion of the terminal main insulation is linearly affected by the cable core voltage and the sheath induced voltage, while the field strength distortion of the sheath surface is only linearly related to the sheath induced voltage.

This study aims to enhance the dielectric energy storage performance of poly(vinylidene fluoride) (PVDF) composite dielectrics. A new type of all-organic composite dielectric film (P-M) was prepared by filling different mass fraction of poly(methyl methacrylate) (PMMA) into PVDF by simple physical blending. The electric strength, energy storage density, and energy efficiency of composite dielectric films were systematically evaluated. Combining theoretical calculations and experimental characterization, the mechanism of electrostatic interactions between PVDF and PMMA molecular chains on energy storage performance of composite dielectric films was analyzed. The results show that the electric strength of the PVDF-based composite dielectric films with a PMMA mass fraction of 40% (P-M40) is 856.2 kV/mm, which is 59.0% higher compared to that of pure PVDF. The maximum energy storage density (U_e) of the P-M40 composite dielectric film is as high as 17.2 J/cm³, which is 40.9% higher than that of pure PVDF, and the charging and discharging efficiency (η) reaches 72.5%. Theoretical calculation reveals that there is an electrostatic interaction between the PVDF and PMMA molecular chains, which can regulate the arrangement and crystallization behavior of the PVDF molecular chains. Fourier transform infrared spectroscopy and X-ray diffraction analysis reveal that the introduction of PMMA promots the formation of the low-loss α phase, inhibits the generation of the β phase, and reduces the grain size of PVDF. Scanning electron microscopy shows that the microstructure of the composite dielectric film is denser, and the physical defects such as internal pores significantly decrease. These structural changes increase the carrier transport barrier, suppress the leakage current and conductivity loss, and improve the breakdown and energy storage performance of the composite dielectric film.

Epoxy resin is extensively utilized in high-voltage power equipment and electronic encapsulation, but it is prone to microcracks under prolonged multi-field stresses, leading to insulation degradation and even equipment failure. Introducing self-healing microcapsules can effectively solve this problem, however conventional microcapsules have limitations such as strict triggering conditions, requiring external intervention, and the potential to damage the matrix. Therefore, this study designed a non-contact light-triggered microcapsule with urea-formaldehyde resin as the wall material and polyurethane acrylate oligomer as the core material. By embedding TiO₂ and SiO₂ nanoparticles with an average particle size of 200 nm into the wall material, an ultraviolet shielding layer was constructed, then the microcapsule was uniformly dispersed in the epoxy resin to prepare a composite material to achieve self-healing under natural light. The results show that the microcapsule structure remains intact at 210℃ and its thermal stability is excellent. When the mass fraction of microcapsules is 4%, the electric strength and dielectric performances of the composite materials are basically the same as those of the pure base material, with only a slight increase in tensile strength. After the damaged material is repaired, the recovery rate of tensile strength, electric strength at room temperature and high temperature, as well as volume resistivity all exceed 85%, the repair performance is excellent. This study provides a novel technical pathway to prolong the service life of epoxy resin insulating materials.

Polyimide (PI) film has become a key material for turn-to-turn insulation in electric drive motors due to its excellent high-temperature stability, superior mechanical properties, and outstanding electrical insulation performance. In this paper, the aggregation structure of PI were regulated by change the types of catalysts during the chemical imidization process. The effects of different catalysts on the aggregation structure of PI were discussed, and the effects of catalyst types on electro-thermal-mechanical properties and corona resistance of PI were studied systematically. The results show that the aromatic catalyst isoquinoline can promote the orderly arrangement of molecular chains through π-π interactions, and significantly enhancing the dielectric and corona resistance properties of PI. The PI film prepared using isoquinoline catalyst exhibits excellent dielectric performances. At a frequency of 1 kHz, its dielectric constant can reach 3.47, while the dielectric loss factor reduces to 0.001 1, and the electric strength at room-temperature reaches 561.39 kV/mm. Furthermore, its corona resistance performance is greatly improved. Under 20 kHz, the corona resistance lifetime reaches 5.8 min, which is 1.68 times that of the thermally imidized film.

With the increases of voltage level and main insulation thickness of high-voltage cable, the thermodynamic operating conditions of cross-linked polyethylene (XLPE) main insulation have become more complex. High-temperature thermal elongation performance is the key to evaluating the thermodynamic properties of XLPE. However, the mechanism research on the differences in thermal elongation performance of different XLPE samples remains insufficient. This paper aimed to comprehensively elucidate the underlying mechanism of thermal elongation performance differences of XLPE high voltage cables under different atmospheres. The thermal elongation performance of three kinds of domestic and international XLPE samples were measured under air, nitrogen, and vacuum atmospheres, and their differences in cross-linking density were analyzed. The cross-linking network structure of XLPE was characterized through gel content and molecular chain structure, and the thermal-oxidative ageing characteristics of XLPE were discussed combined with gel content, carbonyl content, and oxidation induction time. The results show that the high-temperature thermal elongation performance of XLPE is determined by its cross-linking network structure and thermal-oxidative ageing resistance. The XLPE with higher cross-linking density and better thermal-oxidative ageing resistance exhibits lower thermal elongation and permanent elongation. The thermal elongation test under air atmosphere will induce the thermal-oxidative ageing reaction of XLPE, resulting in the reduction of XLPE gel content and the increase of carbonyl content, which will destroy the cross-linked network structure, making the thermal elongation and permanent elongation of XLPE slightly higher. Conversely, the thermal elongation tests under vacuum or nitrogen atmospheres can prevent the damage of thermal-oxidative ageing to cross-linking network, and reflect the true crosslinking degree and thermodynamic properties of XLPE. This work can provide a more effective theoretical basis and testing method for assessing the thermodynamic performance of XLPE.