Thermoplastic polypropylene material has excellent electrical and thermal properties. It does not require cross-linking and degassing in the production process of power cable insulation, has low energy consumption and good environmental friendliness, and can be recycled compared with cross-linked polyethylene. In this paper, medium-voltage polypropylene cable and cross-linked polyethylene cable insulation were used as the research object. The directly synthesized polypropylene and cross-linked polyethylene were analyzed by differential scanning calorimetry, Fourier infrared spectroscopy, and X-ray diffraction analysis, and their mechanical tensile properties, dielectric constant and loss, breakdown strength, and other macroscopic properties were tested. The results show that the structure of the directly synthesized polypropylene is interspersed with ethylene segments, which leads to the melting temperature of polypropylene material decrease, but its melting temperature is still much higher than that of the cross-linked polyethylene. The ethylene segment will participate in the crystallization process of polypropylene, but will not crystallize alone. The elongation at break of polypropylene is 712%, which is higher than that of the cross-linked polyethylene (566%), which is more conducive to the transportation and installation of cable. The breakdown strength of polypropylene at 90℃ is 91.5 kV/mm, which is 123% higher than that of the cross-linked polyethylene at the same temperature, and the breakdown strength of polypropylene has better stability with respect to temperature. In terms of comprehensive mechanical properties and electrical properties, the performance of directly synthesized polypropylene is not inferior to cross-linked polyethylene, which can be used as an insulating material for medium voltage cables.

Gas chromatography is a common way to determine the dissolved gas content in insulating oils, but there is many types of natural insulating oil, and the Ostwald coefficient is not clear. In order to be able to accurately calculate the dissolved gas content in natural insulating oils, the Ostwald coefficient of seven typical dissolved gases in four natural ester insulating oils was measured using double equilibrium method and compared with the Ostwald coefficient in mineral oil. The results show that there is little difference on Ostwald coefficient of the seven dissolved gases in the three natural esters, while the Ostwald coefficient of dissolved gas in the modified esters is slightly greater than that in the natural esters. The Ostwald coefficient of dissolved gas in FR3 has little relative error compared with the available data, which proves that the data obtained by the method has good credibility.

In the extrusion process of XLPE cable materials, when the local extrusion temperature is higher than the decomposition temperature of crosslinking agent, the crosslinking agent will be thermally decomposed in advance, resulting in a low degree of pre-crosslinking in materials. To study the effect of pre-crosslinking on the space charge characteristics of XLPE DC cable materials, the low-degree pre-crosslinking treatment at different temperatures (140℃, 150℃, and 160℃) was carried out before high-temperature crosslinking of XLPE DC cable materials. The aggregated structure was characterized by X-ray diffraction, differential scanning calorimetry, and crosslinking test, and the space charge characteristics at different temperatures were studied. The results show that pre-crosslinking will hinder the normal crosslinking process of XLPE at high temperature, which leads to the decrease of crosslinking degree and crystallinity. Incomplete crosslinking structure makes the pre-crosslinked XLPE samples accumulate a lot of the same polarity of charges injected by electrodes. At the same time, more impurity molecules remaining in the sample will dissociate with the increase of temperature to produce large number of the different polarity of charges, which leads to serious electric field distortion. The decrease of crystallinity also leads to the increase of trap density and depth at the interface between the crystal region and the amorphous region, which limits the charge migration and detrapping process.

The effect of oil contaminated environment in oil-cooled inverter-fed motor on the partial discharge inception voltage (PDIV) of aramid papers with different thickness was studied at different temperatures using a ball-plate electrode configuration. The results show that the PDIV of aramid papers increases with the increase of thickness and is greatly affected by temperature. With the continuous increase of temperature, the PDIV decreases to a great extent. Cooling oil can significantly increase the PDIV at room temperature, but at high temperature, the PDIV will reduce, which may affect the insulation properties of aramid papers. Therefore, in order to improve the electrical properties of insulation system when aramid paper is used as the main insulation in inverter-fed motor, the above characteristics need to be considered.

The extrusion process of high-voltage cable insulating material is very important to the quality of high-voltage cable. The viscosity properties and temperature stability of cable insulation low density polyethylene (LDPE) base material determine the extrusion processing characteristics of cable insulating material. In this paper, three kinds of LDPE base material were extracted from three different brands of cable insulating materials. The molecular chain structure of LDPE base material was tested by gel permeation chromatography (GPC), and the rheological properties of LDPE base material were obtained by rotational rheometer at different temperatures (120-150℃). The results show that the shear thinning behavior of LDPE base material with different molecular chain structures is more obvious with the increase of temperature. The complex viscosity and zero-shear viscosity are affected by the weight-average molecular weight and its distribution. The viscous flow activation energy decreases with the increase of shear rate, which is about 20-30 kJ/mol in low frequency region and 9-16 kJ/mol in the high frequency region, it is closely related to the synergistic effect of branching degree, weight-average molecular weight and its distribution.

Aiming at the problem that the poor heat dissipation effect of the cable laying in piping leads to the cable line appear ampacity bottleneck, we use the 2×3 piping laying of 110 kV power cable with two loops as research object. The changes of cable ampacity before and after backfilling low thermal resistance materials in the pipes was studied through thermal circuit models and large current tests, and the relationship between the ampacity of cable and the thermal conductivity of backfill material was analyzed based on finite element simulation model. The results show that when two circuits of 110 kV 630 mm² cables are laid with 2×3 rows of pipes, compared with the case without backfilling in the pipes, the ampacity of cables after the pipes are backfilled with low thermal resistance materials increases by 25.22%. With the continuous increase of thermal conductivity of the backfill material, the ampacity of cable also continues to increase, but the increase rate is slower and slower, and the ampacity of cable tend to be stable eventually. When the thermal conductivity of backfill material increases from 1 W/(m·K) to 7 W/(m·K), the increase rate of cable ampacity is 8.99%. When the thermal conductivity of backfill material increases from 7 W/(m·K) to 13 W/(m·K), the increase rate of cable ampacity is only 2.00%.

In this paper, the thermal oxidation process of soybean insulating oil was studied by molecular dynamics simulation. The oxidative decomposition path of soybean insulating oil in oxygen atmosphere and temperature rise was simulated. The decomposition products of vegetable insulating oil were analyzed, and the oxidation activation energy of vegetable insulating oil was calculated. The results show that the oxidation activation energy E_a of palmitic acid, oleic acid, and linoleic acid required for the oxidation reaction decreases in turn, indicating that there are differences about the thermal oxidation stability of palmitic acid, oleic acid, and linoleic acid. According to the decomposition and polymerization of substances after reaction, the high temperature oxidation reaction can be divided into two types: oxidative decomposition reaction and oxidative polymerization reaction. The products of high temperature oxidation reaction mainly include oxidized triglyceride, oxidized triglyceride polymer, and short carbon chain triglyceride and volatile compounds. The formation modes of these three types of oxidation products are different, but the types of compounds used are similar. Oxygen is easy to diffuse in soybean oil, and the effect of fatty acids from vegetable insulating oil on the antioxidant properties of soybean oil from high to low is palmitic acid, oleic acid, and linoleic acid. The more unsaturated the fatty acids, the easier they are oxidized.

Typical boron nitride (BN)/epoxy resin (EP) nanocomposite and barium titanate (BT)/polyvinylidene fluoride (PVDF) nanocomposite were selected as research system, and the influence of nano-particle volume fraction, particle size, and size distribution on the thermal conductivity and dielectric properties of the nanocomposites was studied by finite element method. The equivalent thermal conductivity and dielectric constant of the nanocomposite was calculated by mean field theory. The results show that with the increase of nano-particle volume fraction, the thermal conductivity and dielectric constant of BN/EP and BT/PVDF increase. Under the same volume fraction and size distribution, the thermal conductivity and dielectric constant of BN/EP and BT/PVDF increase with the increase of particle size. Under the same volume fraction and particle size, and the thermal conductivity and dielectric constant of BN/EP and BT/PVDF increase with the increase of particle size standard deviation. Consequently, it is an effective way to improve the properties of material by changing the filler content, particle size, and size standard deviation appropriately according to the specific needs.

The insulation defects are likely to occur during operation of on-board transformers due to the harsh operating environment, resulting in the appearance of partial discharges and the degradation of oil-paper insulation, which might affect the safe and stable operation of trains. Therefore, according to the inter-turn insulation structure of on-board transformer, a three-layer oil-paper partial discharge model was designed by using aramid paper, and partial discharge simulation tests were carried out in lab. The partial discharge signal was recorded by partial discharge detector, and the dynamic change rate of discharge statistical parameters was proposed to characterize the development process of partial discharge. The results show that the discharge phase spectrograph of oil-immersed aramid insulating paper is symmetrically distributed, and the discharge magnitude rapidly increases to nC level after initial discharge. The discharge process of oil-immersed aramid insulating paper can be divided into four stages, including starting stages, development stages, dangerous stages, and breakdown stages according to the discharge phase spectrograph and dynamic change rate of discharge statistical parameters. Besides, the dynamic change rate of discharge repetition rate has an inflection point during discharge process, which can be used to evaluate the partial discharge development process of oil-immersed aramid insulating paper.

Both power frequency and 0.1 Hz cosine square wave voltage can be used for withstand voltage assessment of cable, but the equivalence of assessments under two voltages is not clear. For this reason, on the basis of needle plate electrode model, breakdown tests under power frequency and 0.1 Hz cosine square wave voltage were carried out on the samples with different insulation residual thickness defects. The Weibull distribution of breakdown time under different voltages was statistically analyzed, the morphological characteristics of breakdown channel were observed and compared, and the reasons for the difference in the strength of assessment under power frequency and 0.1 Hz cosine square wave voltage withstand voltage were analyzed. The results show that under the condition of needle plate electrode model, the breakdown voltage and breakdown time of the defects under 0.1 Hz cosine square wave voltage are both greater than that under power frequency voltage, which indicates that the strength of cable withstand voltage assessment under 0.1 Hz cosine square wave voltage is significantly lower than that under power frequency voltage. Before the sample is broken down under 0.1 Hz cosine square wave voltage, there is no obvious traces of burning and carbonization, and the cumulative effect is not significant. The reduction of composite electric field intensity and insignificant cumulative effect caused by the charge injection of cosine square wave DC stage may be the reason that the 0.1 Hz cosine square wave voltage on the cable withstand voltage assessment is significantly lower than the power frequency voltage.