This paper summarized the global patent layout of corona resistant PI films, the influencing factors of corona resistance for inorganic nano hybrid films, and introduced the application status of corona resistant PI films at liquid nitrogen temperature, as well as the future development of corona resistant PI films.

The ageing problem of insulating oil under long-term electrical-thermal combined stress seriously threatens the safe and stable operation of oil-immersed transformers. In view of the transformer faults caused by the ageing of insulating oil, it is urgent to research the ageing characteristics, mechanism, and diagnostic methods of insulating oil. Therefore, the development status and key technologies in the electrical thermal ageing characteristics and diagnostic methods of mineral insulating oil and vegetable insulating oil were reviewed. This paper first summarized the physical and chemical properties of mineral oil and vegetable oil for transformers, and their high temperature performance, biodegradability, acid value, and electric strength were compared. Then the two basic forms of electrical and thermal ageing and their products for insulating oil were reviewed. On this basis, the influence of metal materials such as copper on the ageing of oil was emphasized. Finally, the commonly used ageing diagnostic methods of insulating oil were described. The multi-dimensional fusion characterization of feature parameters from different detection methods is expected to improve the diagnostic accuracy, which has a good engineering application prospect.

Compared with mineral oil transformer, vegetable oil transformer has the characteristics of strong overload capacity, high fire safety, and good environmental performance. At the same time, its noise generated during operation is smaller than that of dry-type transformer, and the loss is lower, the size is smaller, the price is lower. So more and more power grid and industrial users began to use vegetable oil transformer. The current research progress of vegetable oil transformer from the aspects of design, material application and research, production and operation maintenance was summarized in this paper, and the research direction and prospect of vegetable oil transformer were prospected.

A barium titanate/polyvinylidene fluoride (BT/PVDF) composites was prepared by solution blending method, and the effect of size distribution and content of BT particles on the dielectric properties, electric strength, energy storage properties, and thermal stability of the composites were investigated. The results show that compared with single particle size BT, the synergistic effect of double particle size BT filler makes the composites have more excellent comprehensive properties, and the synergistic effect is the most remarkable when the mass ratio of BT with double size is equal to 1. With the increase of filler content, the dielectric constant and thermal stability of the composites increase, and the dielectric loss factor of the composites maintains at the relative low level. When the mass fraction of BT is 60%, and the mass ratio of BT with double size is 5∶5, the dielectric constant of BT/PVDF composites reaches 46.5 at 100 Hz, which is 5 times bigger than that of pure PVDF; the energy storage density and polarization are 0.18 J/cm³ and 0.011 9 C/m², respectively, which are 176% and 310% higher than those of pure PVDF; the decomposition temperature corresponding to 5% of weight loss reaches 478.4℃, which is 1.3℃ and 30.3℃ higher than that of PVDF composites filled with S-BT and L-BT, respectively.

A high toughness composite epoxy resin system was prepared with bisphenol A epoxy resin as matrix, G2019 epoxy resin as the modified resin and hexanediol diglycidyl ether as diluent. The high-performance epoxy resin-based glass fiber reinforced composites for wind turbine blades were fabricated via the pultrusion process, and their mechanical and electrical properties were studied. The results show that the obtained composite epoxy resin system can be used as a high toughness epoxy resin matrix. When the content of G2019 epoxy resin is 30 phr, the impact strength of the epoxy resin composite system casting reaches 25.1 kJ/m², which is 151.5% higher than that of the pure bisphenol A epoxy resin, and the AC and DC electric strength reaches 48 kV/cm and 65 kV/cm, respectively. The pultruded epoxy resin-based glass fiber reinforced composites show outstanding comprehensive mechanical and electrical performances, which can meet the application requirements of wind power fields.

A thermosetting polyester wire enamel with high crosslinking density was prepared by using water-soluble polyester resin as matrix. The adhesion of paint film was regulated by the ratio of hard and soft monomer in polyester resin, the electric strength of paint film was improved by adjusting the branching degree of polyester resin, and the technology of paint film was improved by adding thickening agent. The results show that when the mole fraction of water-soluble functional monomer is 6%, the mole fraction of branched functional monomer is 14%, and the mole ratio of soft and hard chain segments in the remaining monomer is 2∶1, the wire enamel with the water-soluble thermosensitive polyester as matrix has the best comprehensive performance. The optimum mass fraction of curing agent is 3.0% of resin mass. When the mass fraction of thickening agent is 0.4% of the total resin, the elongation at break of the enameled wire is 40%, and the electric strength is 40 kV/mm. The final coating thickness of the enameled wire is 100 μm, its breakdown voltage is 4 kV, and its heat resistance is good, which has potential application value in the new sensor technology and equipment body integrated transformer.

Two kinds of nano-fillers nano-tiania (TiO₂) and muti-wall carbon nanotubes (MWCNTS) were selected. Sixteen different epoxy nanocomposite dielectrics were prepared by adding two kinds of nanofiller alone or simultaneously to the epoxy resin. The influence of nanoparticles on the surface flashover characteristics of epoxy nanocomposite dielectrics was explored through the volume resistivity test, surface potential decay and vacuum DC surface flashover experiment. The results show that the surface flashover voltage of epoxy nano-composite is related to the mass fraction of filler, and an appropriate amount of filler can enhance the flashover voltage effectively. Comparing with adding TiO₂ (improving 14.49%) and MWCNTS alone (improving 23.11%), adding TiO₂ and MWCNT simultaneously can further improve the surface flashover voltage up to 44.99 kV (improving 36.06%). The surface trap characteristics of the material are calculated by the surface potential decay curve. By analyzing the relationship between deep trap and surface flashover voltage, it is found that the flashover voltage is linearly correlated with the deep trap energy level, and the deeper the trap energy, the higher the flashover voltage. The depth of surface deep trap energy is improved by adding two kinds of nanoparticle into epoxy composites, thus the electron emission and charge transport processes are suppressed, and the surface flashover voltage increases.

In order to study the influence of crosslinking behavior on the mechanical properties and water tree characteristics of nanocomposite dielectrics, we prepared two nanocomposites of polyethylene/montmorillonite (PE/OMMT) and crosslinked polyethylene/montmorillonite (XLPE/OMMT) by melt blending, and their tensile properties, dynamic mechanical properties, and water tree ageing characteristics were tested. The results show that the OMMT in XLPE plays a role of physical crosslinked point and stress dispersion, which increases the tensile strength and toughness of nanocomposites. The homogeneously dispersed OMMT of lamellar heat resistance and physical support functions improves the storage modulus of nanocomposite at high temperature. The water tree initiation time of the nanocomposites is short, and the OMMT layer is beneficial to buffer the impact of expansion and deformation of water molecules on the molecular chain, so that the growth length of water tree is smaller. The three-dimensional network structure after crosslinking enhances the ability to withstand deformation of nanocomposites, thus the nanocomposites have better resistance to water tree.

The main negative effect of transformer insulation ageing is the decline of mechanical properties. Once the winding coil is impacted by external short circuit, it will probably produce deformation, which will lead to the loss of insulating ability and eventually cause transformer fault. A mechanical analysis model of insulating paper materials was established according to their structural characteristics on the basis of material mechanics. Combined with the ageing mechanism and influencing factors of insulating paper materials, a test platform for the mechanical properties of insulating paper materials was built. The mechanical properties of insulating paper materials under different ageing degrees were studied to obtain the ageing mechanical properties of insulating paper materials. The results show that the plasticity of the insulating paper material basically disappears under repeated cyclic loading, and the insulating paper under different ageing times can be approximately simplified as an elastic material.

XLPE cable insulation is prone to age under the long-term influence of electrical stress. In order to study the electrical ageing characteristics of XLPE cable, we conducted accelerated electrical ageing experiments on 10 kV XLPE cable, and the physic-chemical properties and electrical properties of the XLPE cable before and after ageing were investigated. The results show that the crystallinity of XLPE decreases significantly after electrical ageing, and the melting peak characteristic temperature of XLPE decreases slightly. The methylene content of XLPE increases, and carbon-carbon double bonds start to appear inside the material. PDC test results show that the polarization and depolarization current of the samples increase after electrical ageing, and the conductivity and low frequency dielectric loss of XLPE increase significantly. This is because the chemical bond break in XLPE molecular chain is broke by high-energy electron bombardment, resulting in the increase of short-chain molecules number and degradation of crystalline region, which ultimately decrease the physic-chemical and electrical properties of XLPE insulation material.

Partial discharge inception voltage (PDIV) under repetitive pulse voltage is an important parameter to evaluate the insulation performance of low-voltage random-wound inverter-fed motors. Different from the traditional sinusoidal voltage, the rise time and polarity of the impulse are critical parameters to be considered when PDIV test is carried out on the turn to turn insulation of the winding for inverter-fed motor at repetitive impulsive voltages. In this paper, the influence of repetitive impulsive voltage rise time on the PDIV and energy distribution features of turn to turn insulation for inverter-fed motors with three specification were investigated at repetitive impulse voltages with different impulse rise times (50 ns, 75 ns, 100 ns, and 200 ns), and the mechanism of variation of PDIV with different rise times was discussed. The results show that with the decrease of rise time, the PDIV of three motors decreases, and the maximum decline rate reaches 28.1%. The influence of repetitive pulse polarity on the PDIV test results of low-voltage random-wound inverter-fed motor is limited, indicating that both positive and negative can objectively reflect the PDIV performance of motor. With the increase of impulse rise time, the discharge energy in frequency domain moves from high frequency to low frequency. The research results are expected to provide reference for PDIV test of low voltage inverter-fed motor insulation and the revision of related standards.

It is increasingly urgent to the demand of ultra-high voltage direct current (UHVDC) cable equipment because of the implementation of UHVD transmission project. In order to study the structural design of UHVDC cable joints, we start from the influence of nonlinear conductivity characteristics of polymer insulation materials on the insulation electric field distribution of DC cable joints to analyze the effect of modified silicone rubber on the optimal design of the insulation structure of UHVDC cable joints. The results show that the electric field in the reinforced insulation of DC cable joints will reverse polarity with the increases of the cable load current. The nonlinear conductivity of the silicone rubber is greatly enhanced by doping copper calcium titanate nanofibers inorganic filler, the activation energy and electric field strength coefficient of the silicone rubber are improved, and the threshold electric field strength of conductivity entering the nonlinear region is reduced. The distortion of tangential electric field of double-layer dielectric interface and surface electric field of stress cone and high-voltage shielding tube in the UHVDC cable joint are obviously suppressed by the modified silicone rubber reinforced insulation material. The modified silicone rubber, used as the reinforced insulating material for UHVDC cable joints, can make up for the insufficient that the field distortion in the joint cannot be suppressed effectively by adjusting the structural size of cable joints. The research results can effectively solve the electric field control problem in the design of UHVDC cable joints.

In humid and dirty environment, it is easy for high voltage switchgear to trigger discharge or insulation breakdown fault. To improve the design and operation level of high voltage switchgear, we simulated the electric filed distribution of high voltage switchgear under micro-environment and calculated the minimum discharge distance. Firstly, a multi-physical field finite element simulation model of bus chamber for 40.5 kV high voltage switchgear was established by COMSOL simulation software, and the temperature and humidity distribution in the bus chamber were obtained. The electric field distribution characteristics in the presence or absence of micro-environment were studied by numerical analysis method. Based on the air streamer discharge initiation criterion, the minimum discharge distance in micro-environment was calculated by COMSOL and MATLAB software. Finally, the influence of different insulation partition layout parameters (thickness, location) on the electric field distribution and minimum discharge distance in micro-environment was studied. The results show that the maximum temperature in the bus bar chamber can reach 347.15 K under high temperature and high humidity environment, and the overall temperature presents a gradient distribution of upper and lower. The insulating partition, which is located in the area of low temperature and high humidity, is easy to accumulate wet dirt. In the micro-environment, the electric field at the bus tip and the minimum discharge distance decrease, but the surface electric field increases, and the flashover probability increases.

Mechanical impact is an important factor that damages the stator bar insulation material. At present, systematic researches on the damage of stator bar insulation materials by mechanical impact are still lack. In this paper, the effect of mechanical impact on the stator bar insulation material was studied by combining the finite element simulation and experiment. The results show that when the insulating material is subject to mechanical impact, the average equivalent stress-strain and electric strength firstly increase or decrease, and then become stable within a certain impact strength range under the same punch structure. Under the same impact strength, both the average equivalent stress-strain and electric strength increase with the increase of the tip area of punch head. The correlation between the average equivalent stress-strain and the electric strength can be used to evaluate the insulation damage of stator bar caused by mechanical impact.

In order to study the ablation mechanism of buffer layer of corrugation aluminum (Al) sheath high voltage cross-linked polyethylene (XLPE), firstly, we analyzed a ablation failure of 110 kV XLPE insulated cable, and proposed that the concentrated radial current was the cause of ablation failure. Secondly, a simulation model of the faulty cable was established. The current density and its distribution of the contact part of buffer layer and corrugated Al sheath were simulated, and the simulation results were proved by model experiment and calculation. It was found that the size and distribution of radial current were affected by the embedded depth of Al sheath in the buffer layer and the volume resistivity of the buffer layer. Finally, simulation experiments were designed to prove that the radial current concentrated was one of the causes of buffer layer ablation, and the experimental conditions of the simulated ablation experiment were controlled in the thermostat. The surface morphology of the experimental sample and the ablated Al sheath were compared by optical microscope. The results show that the ablation process in the ablation experiment is the same as that in the actual faulty cable, and with the increase of the current density in buffer layer, the ablation start time decreases. This paper reveals the physical mechanism of radial current concentration in the ablation fault of corrugated Al sheath, which provides guideline for relevant fault diagnosis and protection.

In order to study the ablation mechanism of buffer layer in cable, we analyzed the change of contact state between buffer layer and aluminum sheath according to the defect phenomenon, and then a circuit model was established to analyze the defect. According to the model, the induced voltage of insulation shielding layer and the main factors affecting the induced voltage were analyzed. A 15 m defective cable sample was intercepted from the breakdown accident line, and a partial discharge detection experimental platform was built with the defective cable sample. The results show that the induced voltage of insulation shielding layer is directly proportional to the number of defects, the resistivity of composite layer, and the thickness of white powder at the defect, and is inversely proportional to the contact area between buffer layer and aluminum sheath, and the discharge signals inside the defect cable have obvious characteristics of poor contact discharge.

In order to verify the applicability of DGA fault diagnosis methods in oil-filled submarine cable, we chose domestic dodecylbenzene (DDB) insulating oil as research object. First, the decomposition temperature of DDB insulating oil was determined by thermogravimetric test under different heating rates. Subsequently, a test system was built to carry out thermal fault simulation tests of DDB pure oil, oil-paper, and 25^# mineral oil at 150, 250, and 350℃. After test, the percentage of five characteristic gases (H₂, CH₄, C₂H₆, C₂H₄, C₂H₂) dissolved in the fault oils was measured to study the gas production law of DDB insulating oil under thermal fault. Finally, existing DGA diagnostic methods (classic Duval triangle, quadrilateral graphic method) were selected to analyze the gas data. The results show that the proportion of C₂H₄ dissolved in DDB insulating oil under thermal fault increase significantly with the increase of fault temperature, and the proportion of C₂H₄ in mineral oil is at a low level in the whole temperature range. In the thermal fault diagnosis, the classic Duval triangle and quadrilateral graphical method can detect thermal faults by the dissolved gas in DDB insulating oil, but the temperature range of thermal fault cannot be determined accurately, and the fault boundary need to be further modified.