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.

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.

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.

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.

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.

Oil-immersed power transformers are the core power transmission and transformation equipment in power systems. The internal oil-paper insulating materials of these transformers undergo deterioration such as moisture absorption and ageing during long-term operation, which seriously threatens the stable operation of the transformers. The AC conductivity behavior of oil-paper insulation composite system can effectively characterize the deterioration degree of the insulation. However, traditional AC conductivity calculation models of oil-paper insulation do not consider the effect of multiple relaxation processes in medium, resulting in low availability of these conventional models. In this paper, the frequency domain dielectric response measurements were performed on aged and damped oil-paper insulation samples at different test temperatures. A theoretical calculation model of AC conductivity variation characteristics in multiple relaxation process of oil-paper insulation system was constructed by means of relaxation and conductivity two shifting. The results show that after ageing, both the conductivity activation energy and relaxation activation energy of the oil-paper insulation decrease, while the effects of moisture on these two activation energy are opposite. The AC conductivities of oil-paper insulations with different ageing degrees at high frequency are basically consistent, and there is a significant transition area in the medium and high frequency ranges, while there is no smooth transition in the AC conductivity curve of the damped sample across the entire frequency range. Meanwhile, using the modified calculation model proposed in this paper can improve the accuracy of low-frequency AC conductivity calculation.

To address the stringent service requirements of AC contactor contact supports, linear phenolic resin was used for the blending modification of amino molding compounds. The effects of phenolic resin contents on mechanical properties, heat resistance, wear resistance, and electrical insulation properties of samples were investigated systematically. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were employed to characterize the microstructure of samples, and the interaction between molding temperature and formulation was further explored. Meanwhile, the mechanical life tests of finished supports were carried out. The results show that within the range of phenolic resin mass fraction from 0% to 25%, condensation reaction occurs between phenolic resin and amino resin to form a semi-interpenetrating polymer network. When the mass fraction of phenolic resin is 15%, the comprehensive performance of the sample is the best. Its unnotched impact strength, flexural strength, deflection temperature under load, and power frequency electric strength increase by 47.9%, 41.1%, 69.1%, and 32.8%, respectively, compared with those of the pristine amino molding compound. Further increasing the content of phenolic resin, the mechanical properties of samples only slighty decrease. When the mass fraction of phenolic resin is 25%, the wear resistance of the sample is the best, and the average wear loss is 87.2% lower than that of the pure amino sample. Within the molding temperature range of 160-180℃, phenolic resin with a mass fraction of 15% and high-temperature curing has a synergistic effect on the molding compound. The modified materials prepared by using phenolic resins with a mass fraction of 15% or above are used to form the contactor's contact switch, and the mechanical life of the contactor can reach 12 million cycles, which can meet the requirements for long-term service.

Acoustic signal is the key characteristic parameters for identifying partial discharges (PD) in power transformers. To address the limitations of conventional PD monitoring techniques, such as weak anti-electromagnetic interference and inability to achieve internal deployment, a fiber Bragg grating (FBG) with a frequency range of 5-60 kHz covering audible sound and low-frequency ultrasound was designed in this paper. The sensor was employed to monitor acoustic signal during the discharge process between column-column electrodes in transformer oil, and the frequency domain gravity center and audible acoustic energy proportion of acoustic signal were extracted. Further, the prepared sensor was embedded within a full-scale transformer to verify its monitoring effectiveness for the internal discharge sound signals of transformer. The results show that the dominant frequency bands of the column-column electrode discharge acoustic signals in transformer oil are within the range 5-10  kHz and 46-60  kHz, and the frequency domain gravity center and the audible acoustic energy proportion of the acoustic signal show a trend of increasing first and then decreasing with discharge severity. The proposed FBG sensor can effectively monitor the discharge development in the ±400 kV converter transformer.

Aiming at the problem that organic diluents in conventional superhydrophobic coatings are prone to cause environmental pollution and safety hazards, 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS-17) was used to modify the surface of micro/nano silica (SiO₂) particles in this paper. A diluent free superhydrophobic anti-icing composite coating (RTV@Fn-SiO₂(11)#Fm-SiO₂) with macroscopic structure was prepared using room temperature vulcanized silicone rubber (RTV) as the substrate, and a macro-micro-nano multi-scale rough structure was constructed by knurling embossing technology, and its wettability, anti-icing performance, and environmental tolerance were systematically tested. The results show that the water contact angle of the prepared coating reaches (162.4±1.8)°, and the sliding angle is only (2.1±0.2)°, showing excellent superhydrophobic performance. In both dynamic and static icing tests, the coating demonstrates excellent anti-icing performance. The static ice formation time is extended to 1 292 s, which is 14 times that of the bare aluminum substrate, and the icing adhesion strength reduces significantly. After acid and alkali corrosion, ultraviolet ageing, tape stripping, and sandpaper wear resistance tests, the coating still maintains stable superhydrophobic performance. The diluent free superhydrophobic anti-icing composite coating with macrostructure has excellent anti-icing performance and envi-ronmental stability, which has potential application value for the protection of insulating equipment in power transmission systems.