The distribution transformers serving in the distribution network of urban villages have been in heavy overload operation for a long time, and the number of transformers approaching or even exceeding their design life is increasing. Prolonging their service life by replacing natural ester insulating oil is a feasible method of life extension. In this paper, the thermal ageing characteristic of aged mineral insulating oil-impregnated paper in natural ester was tested to analyze the pyrolysis characteristics of cellulose. Then the residual life of oil-impregnated paper after replacing natural ester was predicted by the pyrolysis kinetic model of oil-impregnated paper cellulose. The results show that the influence of the residual mineral insulating oil in the oil-impregnated paper on the pyrolysis process of cellulose after replacing the natural ester can be ignored. The moisture content of insulating paper is an important factor affecting its pyrolysis reaction, and the replacement of natural ester can significantly decrease the moisture content of insulating paper and delay the pyrolysis rate of insulating paper cellulose.

Thermoplastic polypropylene materials have excellent electrical properties and thermal properties, and their production process does not require cross-linking and degassing, with low energy consumption and recyclability. Compared with cross-linked polyethylene, it is environmentally friendly and one of the best materials for the production of power cable. Taking polypropylene materials as the discussion object, the basic structure and physical properties of polypropylene and their relationship with electrical properties were introduced in this paper. The effects of copolymerization modification, blending modification, chemical modification, and nano particle modification on the structure, mechanical, thermal and electrical properties of polypropylene were reviewed. The research progress and application of polypropylene cables at home and abroad were introduced. Finally, the furure development of polypropylene materials were summaried and prospected from the production and preparation technology of polypropylene, relationship between structure and properties, and modification techniques.

The general properties, viscosity-temperature relationship, storage stability, and heat resistance of ELAN 3322-300 HV1 epoxy impregnating resin were systematically studied. Winding bar and coil samples were made to simulate the insulation structure of high voltage motor, and their electrical performance was tested. The class H insulation structure was conducted thermal ageing tests. The results show that the ELAN 3322-300 HV1 epoxy impregnating resin has low curing volatile and good storage stability, its temperature index is 211, and its temperature classification is class H. The winding bar and coil samples after impregnating have excellent electrical insulation performance and thermal ageing resistance, and the temperature index of class H insulation structure is 182. The ELAN 3322-300 HV1 epoxy impregnating resin is suitable for VPI insulation treatment of class F and H high voltage motor.

Epoxy composite is easy to accumulate space charge under high temperature and high electirc field condition, which would lead to local electirc field distort, and in severe case, partial discharge would produce and insulation would break down. Nano-MgO/EP composites with different doping rate were prepared by mixing nano-MgO particles into epoxy resin. Their glass transition temperature was measured by differential scanning calorimetry (DSC). Their trap characteristics were calculated by thermal stimulation depolarization current method (TSDC). Their space charge characteristics were measured by pulse electroacoustic (PEA) method. The results show that the addition of nano-MgO particles can increase the glass transition temperature of epoxy resin and inhibit the space charge accumulation in epoxy resin. With the increase of nano-MgO doping rate, the glass transition temperature of epoxy composite increases at first and then decreases, the deep trap energy level and density of nano-MgO/EP composite increase at first and then decrease, the space charge density of nano-MgO/EP composite decreases at first and then increases, and the electric field distortion trend is similar with that of space charge. When the doping rate of nano-MgO is 3%, the glass transition temperature of the nano-MgO/epoxy composite reaches the maximum value, and the ability to inhibit the space charge accumulation and electric field distortion is the best.

Enamelled rectangular magnet wires used in driving motor for electric vehicle need to test their partial discharge initial voltage (PDIV). However, the current IEC and national standards have no normative parameter on the PDIV test condition, and domestic motor manufacturers have different requirements on the PDIV performance. In order to obtain accurate PDIV data, the testing principle of instrument for measuring partial discharge was studied. The influence of test methods on the PDIV results of polyamide-imide and polyimide enamelled rectangular magnet wires under different voltage increase rate, determination threshold, and fitting length of wires was analyzed. The results indicate that if 10 V/s and 10 pC are taken as test condition and the fitting length of wires is more than 90 mm, the PDIV results are the closest to the theoretical value. If higher rising rate and higher threshold are used, the measured PDIV results will be higher, and the stability of test results will decrease.

Partial discharge optical measurement method has strong anti-interference ability and high sensitivity, becoming one of the most important and effective methods to evaluate the insulation condition of electrical equipment, whose signal detection capability is closely related to the layout way of fluorescent fiber sensor. An experimental platform for fluorescent fiber PD detection was built in this paper. The influence of the relative distance between sensor and insulation defect on the PD detection effect was studied in linear layout and Archimedean layout, and then compared with the commonly used UHF method. The results show that the sensitivity of the Archimedes layout is better than the linear layout. When the distance between fluorescent fiber and PD defect is less than 45 cm, the two layout methods of fluorescent fiber sensors have higher detection sensitivity than that of the UHF sensors. For the linear layout, increasing horizontal distance has a more detrimental effect on PD detection than increasing vertical distance. For the Archimedean layout, the lateral offset of fluorescent fiber has a greater effect on the detection PD than the horizontal distance. Finally, according to the characteristics of the two layouts and the location of switchgear where PD is likely to occur, recommendations for placing fluorescent fibers in the switchgear were given.

In this paper, the modification methods of polypropylene, the development and operation of polypropylene cables, and the existing regulations and specifications of polypropylene cables were reviewed. According to the research and development status of polypropylene cables, the problems that may be encountered in the future operation of polypropylene cables were summarized and research suggestions were put forward, which can provide reference for the subsequent research and commercial application of polypropylene cable.

On the basis of the crystal structure of piezoelectric effect of polyvinylidene fluoride (PVDF) polymer, the effects of different annealing temperatures (60-160℃) on the surface crystal morphology, crystal size, crystallinity, and key dielectric parameters including polarization response curve and piezoelectric coefficient of PVDF film samples were studied. The results show that the sample crystal size and crystallinity increase with the increase of annealing temperature in the range of 60-140℃. The polarization curves show higher residual polarization intensity and hysteresis, and the piezoelectric coefficient increases from 11 pC/N to 27 pC/N with the increase of annealing temperature. When the annealing temperature reaches 160℃, cracks appear on the surface of sample, and consequently the breakdown is easy to occur in the polarization process, and the dispersity of piezoelectric coefficient is large.

A base gel was prepared with bisphenol A epoxy resin E124 as the main resin, polymercaptan as the curing agent, and imidazole as the accelerator. A single component high thermal conductive structural adhesive was prepared by adding different proportions and sizes of spherical alumina and irregular alumina into the base gel. The effects of different filling content and different particle sizes of alumina on the thermal conductivity, bonding strength, viscosity, and thixotropy of the single component high thermal conductive structural adhesive were studied. The results show that when the mass ratio of base gel to the three different particle sizes of alumina is m(base gel)∶m(40 μm Al₂O₃)∶m(2 μm Al₂O₃)∶m(0.5 μm Al₂O₃) =15∶59.5∶25.5∶6, m(base gel)∶m(40 μm Al₂O₃)∶m(5 μm Al₂O₃)∶m(0.5 μm Al₂O₃) =15∶59.5∶25.5∶8, m(base gel)∶m(40 μm Al₂O₃)∶m(10 μm Al₂O₃)∶m(0.5 μm Al₂O₃)=15∶68∶17∶8, three kinds of high thermal conductive structural adhesives cured at 90℃ for 15 min can be obtained, their thermal conductivity is 2.73, 3.05, 2.85 W/(m·K), their bonding strength is 5.49, 5.55 and 5.31 MPa, and their viscosity is 145 000, 127 467 and 118467 mPa·s, respectively. The volume resistivity of the three kinds of high thermal conductive structural adhesives is above 10¹⁴ Ω·m, and the adhesive survival rate after high temperature and humidity, cold and thermal shock is more than 70% and 90%, respectively. The performance of the high thermal conductive structural adhesive meets the basic performance requirements of screen printing and heat dissipation materials for chip packaging.

As a kind of environment friendly liquid dielectric with high ignition point and degradability, natural ester insulating oil is widely considered as a good substitute of mineral oil in power transformers. In order to improve the electrical and thermal ageing properties of natural ester-paper insulation system, the surface of cellulose insulation paper was modified by silane coupling agent, and the oil-paper composite insulation combinations of natural ester, traditional mineral oil, ordinary insulation paper, and silane modified insulation paper were conducted accelerated thermal ageing tests. The ageing characteristic paremeters of oil-paper during the thermal ageing of four combinations were compared and analyzed. The results show that the insulating paper modified by silane can maintain a relatively complete three-dimensional network structure of cellulose during thermal ageing, and improve the ageing resistance and electrical performance of the natural ester-paper system.