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.

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.

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.

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.

The thermal ageing behavior of insulating paper and the evolution of refractive index of cellulose fiber during thermal ageing process were explored. Accelerated thermal ageing experiments were carried out on the oil-paper samples at 120℃ and 130℃, respectively. The degree of polymerization, furfural content, and carbon and oxygen gas content were measured at different ageing periods, and their changing laws were explored. According to the change trend of the crystallinity of insulating paper in the test, a reasonable inference was made about the change of its refractive index, and further analysis was carried out by the Abbe refractometer. On the basis of dispersion staining color method, the evolution rule of refractive index of the oil-paper cellulose fiber during thermal ageing was explored. The results show that as the ageing progresses, the degree of polymerization decreases, while the furfural content and the carbon oxygen gas content increase, the refractive index and crystallinity of the insulating paper increases, and the refractive index of the oil-paper cellulose fiber increases also. The refractive index of cellulose can be considered as a new type of ageing characteristic of insulating paper.

In order to apply the frequency domain spectroscopy test technology to nondestructively test of ageing state for large electric machine stator bar insulation, we conducted accelerated ageing tests on the epoxy-mica insulation of stator bar under the combined action of multiple factors such as heat, electricity, and machinery, and tested the frequency domain spectroscopy of samples with different ageing degrees. The Davidson-Cole model parameters of frequency domain dielectric response were extracted as characteristic parameters by least square method, and the ageing degree of epoxy-mica insulation was analyzed according to the parameters. Finally, the relationship between characteristic parameters and ageing time was established by fitting. The results show that the dielectric loss factor (tanδ) of epoxy-mica insulation increases with the increase of ageing degree, especially at the frequency of 10^-3–10^-1 Hz; the characteristic parameters β and τ extracted by the Davidson-Cole model can be used to characterize the ageing degree of insulation. With the increase of ageing time, β decreases linearly, τ decreases exponentially, while Δε increases obviously. A formula for quantitatively characterizing the degree of insulation ageing is obtained through the fitting relationship between τ and ageing time, which can be considered as an empirical formula for evaluating the ageing degree.

In areas with large temperature difference between day and night, negative pressure is easy to form in the expander of oil-immersed inverted current transformers, which will affect the distribution of water in the oil-paper insulation, and then affect the insulating properties of the oil-paper insulation system. In this paper, the saturation solubility of water in insulating oil under different pressure and temperature was measured by indirect method. On the basis of the test results, the relationship between the saturation solubility of water in insulating oil and temperature and voltage in Arrhenius equation was fitted by recursive least square method, and the effect law was studied. The results show that with the increase of negative pressure, the water saturation solubility in insulating oil decreases constantly, and this trend becomes more obvious with the increase of temperature. Under the effect of negative pressure, water in the oil separates out and turns into suspended water in insulating oil or transfers to insulating paper. The translation factor n related to pressure was introduced in this paper, and a mathematical expression involving pressure, temperature, and water saturation solubility in oil was established. The acid value, temperature, and pressure were taken as the comprehensive factors to evaluate the water saturation solubility in insulating oil, and the multi factor influence law was established, in which the influence of acid value and pressure can cancel with each other to some extent. The water distribution and insulation state of the oil-paper insulation system for oil-immersed inverted current transformers can be evaluate more accurately by the above research results.

A high temperature resistant insulating varnish for traction motors was prepared by modifying polyester-imide insulating varnish with meta-aramid resin. The effects of meta-aramid resin on the adhesion, electrical properties, flame retardancy, and heat resistance of the insulating coating were studied. The results show that when the content of meta-aramid resin is 15%, the electrical strength, the adhesion, the oxygen index of the insulating coating reaches 65 kV/mm, 12.9 MPa, 28.7%, respectively. After high temperature test at 220℃ for 500 h, the paint film shows slight discoloration without cracking and peeling off, which can meet the requirements of the traction motors.

The influence of sampling period, voltage rising speed, and overcurrent breaking time on the breakdown voltage of insulating materials in the process of breakdown voltage test was analyzed. Proper improving technology and method was select to eliminate or reduce the error. The results show that the dispersion of test results is reduced by digital filtering, weighted average, and other methods, and the accuracy of dynamic measurement is improved at the same time.

High voltage power cables demand stable and reliable connections with the power system through cable accessories, and silicone rubber (SIR) and ethylene-propylene-diene monomer (EPDM) are the main reinforced insulation materials for cable accessory due to their excellent dielectric properties and mechanical properties. However, with the continuous improvement of high voltage transmission level, the electrical properties of cable accessories also need further development. In this paper, the research status inside and out of China was summarized, then the factors affecting the electrical properties of SIR and EPDM for high voltage cable accessories were discussed. The research progress in improving method of electrical properties were analyzed, and the research progress in the interfacial electrical properties of double-layer dielectric was introduced. At last, the research difficulties of electrical properties modification of rubber reinforced insulation for high voltage cable accessories were summarized, and the research trends of the rubber insulating materials for high voltage cable accessories were prospected.