To improve the accuracy of cable insulation status assessment, this paper proposed an assesement model of insulation condition based on Bayesian optimization (BO) algorithm and light gradient boosting machine (LightGBM) algorithm. First, all the features in the dataset were combined to form different feature subsets. By traversing all the feature subsets, the optimal feature combination corresponding to the highest accuracy from five-fold cross-validation was identified to complete the input feature selection. Then, the BO algorithm was used to optimize seven hyperparameters in LightGBM. Finally, the proposed BO-LightGBM algorithm was used to assess the cable insulation status. The results show that the feature subset method proposed in this paper can better improve model performance compared with principal component analysis (PCA) and mutual information-based feature selection methods. After optimization by the BO algorithm, the accuracy of the LightGBM model is further enhanced. Compared with particle swarm optimization (PSO) algorithm and genetic optimization (GA) algorithm, the computational efficiency of BO algorithm increases by approximately 80% and 86.9% at the same accuracy level, respectively. Furthermore, compared with other commonly used machine learning algorithms, the performance metrics of the proposed model are optimal.

To study the ablation mechanism of water-blocking buffer layer in high-voltage cable and reveal its critical conditions, the dielectric performance parameters of water-blocking buffer layer material with different water contents were measured. A three-dimensional non-axisymmetric corrugated aluminum sheath structure cable geometric model was constructed based on gravity effects to conduct electro-thermal-flow multi-physics field coupled finite element simulation. Experimental verification was conducted using a constant current simulated ablation test system, and the voltage and temperature variations during the ablation process were monitored in real-time. The results show that poor contact and moisture are necessary conditions for ablation. When the aluminum sheath is poorly contacted with the buffer layer, electric field concentration occurs near the gaps at the well-contacted positions, and partial discharge can be triggered when the moisture content reaches 5%. In well-contacted areas of buffer layer, capacitive current concentration causes localized temperature rise, and the local temperature would reach the ablation initiation threshold at the water content of 25%. According to the experimental results, ablation only occurs when the aluminum electrode is poorly contacted with the buffer layer, while only electrochemical corrosion is observed under good contact conditions. Electrochemical corrosion initially produces high-resistance white powder, followed by partial discharge, which makes the voltage and temperature rise steadily. With the occurrence of partial discharge, the voltage can reach 147 V, and the temperature can reach 187.8℃, causing partial melting of polyester fibers in the buffer layer, further leading to current concentration, and ultimately causing thermal breakdown of the water-blocking buffer layer material.

To investigate the critical conditions of high voltage cable buffer layer ablation, the types of insulation shielding damage caused by actual cable ablation were summarized in this paper. A three-dimensional asymmetric high-voltage cable model was established by finite element simulation. The ablation development process of high-voltage cables under both dry and wet conditions, as well as the mechanisms of their damage to insulation shielding, were studied separately. The difficulty of ablation under the two conditions was compared and analyzed. The results show that damage under dry condition is attributed to current-induced thermal effects caused by prolonged poor contact. When poor contact extends to 7.5 m, the surface current density on the buffer layer reaches as high as 442 A/m², and the temperature reaches high as 200℃ within the buffer layer. Damage under wet condition arises from current-induced thermal effects caused by the high resistance products from electrochemical corrosion, and these products can penetrate areas where current and temperature are concentrated within the buffer layer. When the coverage rate of white powder reaches 97.9%, the surface current density on the buffer layer is up to 416 A/m². Simulation results align closely with actual dissolution observed in faulty cables affected by ablation, thus validating our analysis findings presented in this paper.

To investigate the effects of voltage stabilizers on the insulation properties of crosslinked polyethylene (XLPE) under DC voltage, anthraquinone (EK) was selected as the voltage stabilizer, and XLPE composites with different mass fractions of EK were prepared through physical blending. The impact of different content of EK on the DC insulation properties of XLPE was analyzed by differential scanning calorimetry (DSC) as well as surface potential decay, space charge, and conduction current measurements. The results show that as the mass fraction of EK increases, both space charge accumulation, space charge injection deepth, maximun distortion field strength, and conductivity of XLPE composites decrease at first and then increase. This phenomenon is attributed to the change of the trap distribution characteristics in XLPE by EK. When the mass fraction of EK does not exceed 1%, the deep traps are mainly introduced, which contribute to field homogenization, reduce space charge accumulation, and inhibit carrier migration, thereby decreasing DC conductivity. When the mass fraction of EK is 1%, the improvement effect of XLPE insulation performance is the best.

To improve the thermal ageing resistance of crosslinked polyethylene (XLPE), different contents of anti-ager 2-mercaptothiazole (MB) were blended with PE, and then MB/XLPE sheet samples were prepared through electron irradiation process. The effect of MB content on the crosslinking degree, thermal elongation performance, insulation properties, oxidation induction period, and carbonyl index of XLPE were studied. The results show that with the increase of MB content, the gel content and thermal elongation performance of MB/XLPE composites reduce, while the volume resistivity maintains stable. The addition of MB significantly extends the oxidation induction period and suppresses the carbonyl index growth of XLPE during thermal ageing. When the weight part of MB is 1, the oxidation induction period of MB/XLPE significantly increases from 0.9 min (for pure XLPE) to 80 min; after thermal ageing for 168 h at 165℃, and the carbonyl index of MB/XLPE only increases from 0.04 to 0.06, indicating marked improvement in thermal ageing resistance. Based on the Arrhenius equation, it is concluded that this MB/XLPE exhibits a service life of 76.2 years at 90℃ which is 2.5 times longer than that of conventional XLPE.

To study the internal relationship between space charge transport and polarization/depolarization current of cross-linked polyethylene (XLPE) cable insulation in direct current (DC) field, a space charge-current combined measurement platform was established based on PEA method. The combined measurements were conducted on XLPE cable insulation samples with different ageing time. Parameters such as total charge, carrier mobility, and trap distribution in XLPE samples were evaluated using the combined variation data of space charge and current. The results show that at room temperature and 20 kV/mm, homopolar charges accumulation occurs obviously within the sample, and with the increase of test temperature; the charge polarity changes to the opposite. With the increase of ageing time, the number of accumulated heteropolar charges in the aged samples increases at first and then decreases, and the range of positive charge accumulation expands from near the anode to near the cathode; the decay rates of polarization and depolarization currents in the aged samples decrease, the steady value of polarization current increases, and the depolarization current increases at first and then decreases. With the increase of ageing time, the total amount of charge within the samples increases at first and then decreases, while the conductivity and carrier mobility increase. Notably, the carrier mobility significantly increases when the samples are ageing for more than 10 days, causing the charge accumulation range to gradually extend further from the anode, resulting in a significant reduction in the total amount of accumulated charges within the samples. The trap depth of XLPE samples increases gradually with the increase of ageing time, which is the reason for the reduced decay rate of depolarization current in the aged samples.

The extrusion process technology of semi-conductive shielding material (SCSM) for high-voltage cables is crucial for the uniform dispersion and distribution of conductive carbon black (CB), as well as the interfacial interaction between CB and matrix resin. To investigate the influence of processing methods and parameters on the structure and properties of SCSM, this study employed twin-screw extrusion granulation and single-screw extrusion sheet processes. The influence of extrusion temperature, screw speed on the microstructure, electrical properties, mechanical properties, and surface smoothness of the SCSM were investigated. The results show that the optimal parameters for the twin-screw extrusion granulation process are 160℃ of extrusion temperature and 120 r/min of screw speed. Under these conditions, the volume resistivity of the shielding material at 23℃ and 90℃ is 9.6 Ω·cm and 265.0 Ω·cm, respectively. In the single-screw extrusion sheet process, when the extrusion temperature and screw speed are 105℃ and 50 r/min, respectively, the CB achieves sufficient shear force and dispersion time, effectively preventing pre-crosslinking of the SCSM. Consequently, there is no protrusions larger than 50 μm in diameter observed on the surface of the shielding material.

The researches on the effect of liquid additives on the improvement of processing characteristics and physical properties of crosslinked polyethylene (XLPE) insulation materials for high-voltage cables are insufficient. In this paper, liquid crosslinking agents and antioxidants were taken as research objects. The effects and mechanisms of liquid additives on the scorch resistance, gel content, crosslinking by-products, thermodynamic and dielectric properties of XLPE were discussed. The results show that the liquid cross-linking agent 2,5-dimethyl-2,5-bis(tert-butyl peroxide)hexane with a high decomposition temperature and long-chain alkyl groups involved in crosslinking reaction can simultaneously enhance the scorch resistance and gel content of XLPE insulation materials, and reduce the content of crosslinking by-products. Meanwhile, the liquid antioxidants α-tocopherol and sorbitol exhibit excellent compatibility, they can form temperature-dependent proton transfer interactions, which can improve the anti-scorch performance of XLPE without compromising its gel content. Additionally, these liquid crosslinking agent and antioxidants can improve the breakdown performance of XLPE insulation while maintaining its thermodynamic properties. This study proves the feasibility of applying liquid additives to XLPE insulation for high-voltage cable.

The aggregate structure, microcrystalline structure, and surface chemical properties of domestically developed conductive carbon black (CB) were compared with that of international commercial products, and the comprehensive performance of semi-conductive shielding materials prepared by filling CB were studied. The results show that the domestic CB exhibits comparable aggregate structures, microcrystalline dimensions, and carbon content with the imported CB, while the domestic CB contains 0.51% of sulfur elements and demonstrates lower particle size uniformity, alongside a significant variance in the content of various surface functional groups. The semi-conductive shielding material prepared with domestic CB achieves equivalent electrical and mechanical performance to that prepared with imported CB, but its surface smoothness is inferior, which does not meet the industry requirements. It is concluded that the uniformity of particle size, contents of surface functional groups, and the sulfur content are important research direction for structural optimization and performance improvement of domestic conductive carbon black, and surface smoothness is a significant research focus for the development of domestic high-voltage semi-conductive shielding materials.

To investigate the influence of water on the breakdown position of oil-filled cable terminal, this study simulated and analyzed the failure reason caused by water in the 220 kV oil-filled cable terminal. A cable test platform was established in the laboratory, and the influence of the grounding state and electric field strength on the breakdown position was simulated. The partial discharge (PD) quantity and water film height of the cable in different grounding states and at different electric field strength were recorded by using high frequency current partial discharge detector and camera. The results show that the water content in oil, grounding state, and electric field strength are the important factors affecting the fault location of oil filled cable terminal. When the water is immersed in the cable terminal and reaches a certain amount, and the cable terminal is directly grounded, the higher the electric field strength, the more water molecules escape from the oil-water interface, the higher the water film height, the longer the distance between the breakdown point and the upper surface of stress cone, and the larger the partial discharge. The intensity of PD is positively correlated with water film height, and the silicone oil degradation defects within the cable terminal can be evaluated by partial discharge, which provides a reference for evaluating the insulation status of oil-filled cable terminal.