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.

In view of the problem and difficulty of isothermal relaxation current method in evaluating the insulation state of distribution cables, a new sample of domestic unaged 10 kV distribution cable was used as the test object in this paper, and the isothermal relaxation current of the cable section and the high-field conductance current of the insulation slice during the 288 hours of accelerated thermal ageing process at 140℃ were detected. The variation trend and range of current relaxation component, ageing factor A, and threshold electric field E_t during thermal ageing were analyzed. The results show that there are three obvious polarization relaxation peaks in the isothermal relaxation current, and the time constants τ₁, τ₂, and τ₃ are in the range of 7-12 s, 31-39 s, and 210-536 s, respectively. During the thermal ageing process, the material traps corresponding to isothermal relaxation current Peak2 and Peak3 change greatly, and the change of Peak3 is the most obvious. The ageing factor A based on the isothermal relaxation current is in the range of 1.72-3.17. The A decreases at first and then increases with the increase of ageing time, and the corresponding conductance threshold electric field E_t shows the opposite trend of rising at first and then decreasing, which indicates that there is a clear correlation between the threshold electric field E_t and the ageing factor A, they are both the macroscopic appearances of internal molecular structure of the insulating material. The ageing factor A of the domestic cable before ageing is 1.91, which is close to the "aged" state of the German standard DIN VDE 0276, and the A drops to 1.72 after ageing for 48 h, which reaches the "better" state of the German standard. The high A of the domestic cable may be related to the residues of cross-linking by-products of cables, so the degassing process should be improved in the production of domestic cables.

Infrared temperature rise detection is an effective means to screen faulty composite insulators. However, the causes of abnormal heating of composite insulators are diverse, and not all causes will evolve into malignant faults. Therefore, it is particularly important to distinguish the abnormal heating caused by different causes for the safe, stable, and economic operation of power grid. In this paper, the physical and chemical properties of two 220 kV retreated composite insulators with different heating characteristics were studied by SEM and FTIR, and their dielectric properties were also studied. The results show that the causes of different abnormal temperature rise are different. For the composite insulator whose heating part is concentrated at the end of high-voltage end and temperature rise is lower than 5℃, the epoxy resin of the core rod at the heating part has no obvious degradation, and the dielectric constant and dielectric loss factor have no obvious increase, while the sheath has holes and shallow microcracks, and the dielectric constant and dielectric loss factor increase with the increase of humidity. For the composite insulator whose heating part extends to multiple umbrella skirts and temperature rise is up to 40 ℃, the epoxy resin of the core rod has obvious degradation, and the dielectric constant and dielectric loss factor are 3.67 times and 79.4 times bigger than those of the normal part, respectively.

The meltable polytetrafluoroethylene (PFA) was used as the matrix resin. Firstly, the PFA resin was modified by blending and filling with polymer and inorganic fillers to prepare suspension. Then, the suspension was coated and sintered in turn to prepare fluororesin-based film. Finally, the fluororesin-based film and copper foil were pressed together to prepare flexible copper clad laminate (FCCL). The results show that the prepared FCCL has 0.001 5 (10 GHz) of dielectric loss, 38 MPa of tensile strength, 0.05% of water absorption rate, greater than 1.0 kgf/cm of peel strength, and there is no delamination, oxidation, foaming in the soldering resistance test (300℃, 10 s, 3 times), which indicates that the FCCL has better overall performance.

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.

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.

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.

In order to solve the prediction problem of water tree density of XLPE cable, an analysis method combining the macro detection data with the micro water tree profile of aged cable was proposed. Two new parameters (carbon and oxygen index and water content index) were proposed to quantitatively describe the changes of EDS and FTIR results in the water tree region. Firstly, the polarization and depolarization current method (PDC) was used to detect the water-tree ageing cables under different experimental conditions. A series of macroscopic parameters such as cable ageing factor, DC conductivity, and 0.1 Hz dielectric loss factor were obtained. Then, the micro parameters of water tree were obtained by microscopic observation of cable slice. The water tree growth model was established to calculate the water tree density of cable, and the correlation between the macroscopic parameters of cable and the microscopic water tree density was analyzed. The results show that the longer the ageing time, the greater the carbon oxygen index and water content index, and the faster the rate of increase. Based on the correlation coefficient, a new mathematical model for predicting the growth density of water tree inside XLPE cable is established, which realize the combination of microstructure of water tree inside cable and macro test parameters.

In this paper, the physicochemical and electrical properties of oil-paper insulation in the process of surface discharge deterioration under static and flowing oil conditions were studied. The discharge product and moisture of oil-paper insulation were analyzed by infrared spectrometer and micro-water measuring instrument. The multilayer microstructure of oil-paper insulation was analyzed by viscosity tester, X-ray diffractometer, and scanning electron microscope. The conductivity, electric strength, and partial discharge of oil-paper insulation were analyzed by electrical measurement. The multi-type discharge deterioration characteristics of oil-paper insulation in static and flowing oil were compared and analyzed. The results show that the flow of oil slows down the surface discharge deterioration of oil-paper insulation, retards the rise of electrical conductivity and the decline of electric strength, and inhibits the occurrence and development of surface discharge. The surface conductivity determines the difficulty of surface discharge, and its increase results in the statistical distribution of surface discharge times showing a repetitive characteristic of "wave" gradually. Discharge frequency determines the surface discharge deterioration and destruction degree of oil-paper insulation, which is an efficient characteristic quantity to evaluate the deterioration state of oil-paper surface discharge.