High voltage cable fusion joint with defects operating for a long time is easy to cause operation failure. In order to timely troubleshoot the joint defects, we taked high-voltage cable fusion joints as research object. The main insulation and conductor recovery process of high-voltage cable fusion joint was detected by digital radiography (DR) technology, and the bending limit of conductors was analyzed by finite element method. The results show that the DR detection technology can effectively detect bubbles, conductor bending, eccentricity and other defects in the XLPE main insulation of fusion joint. The accuracy of detection results is demonstrated by field dissection, which provides an effective means for the defect detection of high voltage cable fusion joint. The bubble defect of XLPE main insulation is caused by the decrease of temperature and nitrogen pressure in the crosslinked pipeline. Considering the safety margin of the joint, it is recommended that the bending gap should not exceed 11 mm and the insulation thickness at the bending position should not be less than 30 mm.

The dissociation of impurities such as crosslinking by-products can easily lead to the accumulation of heteropolar space charges, resulting in local field strength distortion. At the same time, after the cross-linked polyethylene (XLPE) DC cables is degassing, the crosslinking by-product impurity in the insulating layer will form a concentration gradient distribution of "high inside and low outside", and the effect of impurity concentration gradient on the space charge is still unclear. Therefore, based on the bipolar charge transport model, the effect of cross-linked by-products impurity with uniform distribution and gradient distribution on the XLPE space charge and electric field distribution were simulated and calculated, and various factors affecting the gradient effect of impurity were analyzed. The results show that under the impurity gradient distribution, the space charge and electric field distribution of XLPE show obvious impurity gradient effect, that is, there are more opposite polarity charges accumulated on the low concentration side, which leads to the enhancement of the nearby electric field. While the impurity dissociation barrier and the impurity distribution concentration are the main reasons of affecting the impurity gradient effect. When the impurity concentration gradient is constant, the lower the activation energy, the higher the temperature, the lower the dissociation barrier, and the lower the carrier mobility, which results in the impurity gradient effect more obvious. The larger the insulation layer thickness, the higher the concentration of impurity distribution, and the more obvious the impurity gradient effect.

In this paper, class H insulation system of high voltage motor was studied. The basic properties of electromagnetic wire, dry mica tape, and insulating impregnated resin used in class H insulation structure were tested. The coil sample was manufactured, and the electrical properties, electric ageing, and electrothermal ageing tests were conducted on the coil sample. The results show that the insulating coil sample has excellent electrical performance in breakdown voltage and dielectric loss factor, which can meet the technical requirements of class H 10 kV high voltage motor.

In order to study the effect of winding preload on thermal ageing characteristics of insulating pads, four groups of insulating pads were conducted accelerated thermal ageing tests at 135℃ after appling 0, 200, 500, and 1 000 N of preload, respectively, and the mechanical properties of insulating pads were tested. At the same time, in order to simulate the mechanical ageing of insulating pad under the influence of winding vibration in the actual working condition, a sinusoidal cyclic mechanical load with 5 N of amplitude and 2 Hz was applied on the accelerated thermal ageing of insulating pad with 200 N of preloading force. The results show that that thermal ageing causes the stress-strain curve of insulating pad gradually shift to the right under 0 N preload, and its elastic modulus decreases gradually. The elastic modulus of insulating pad decreases to 79.935% of the new insulating pad after accelerated ageing for 336 h at 135℃. Applying preloading force can slow down the thermal ageing of insulating pad, and the greater the preloading force, the lower the ageing rate and ageing degree of insulating pad. But applying cyclic mechanical load will accelerate the ageing of insulating pad. The research results have certain reference and practical application value for the research of insulation pad ageing and transformer vibration.

In order to study the effect of organic and inorganic fillers on the performance of insulating paper, nano titanium dioxide (TiO₂) treated with dopamine hydrochloride was coated with nano-fibrillated cellulose to get a multiple mixed filler (PDA-TiO₂@NFC). Then the multiple mixed filler was filled into the cellulose insulating paper, and a sandwich structure insulating paper modified by different multiple mixed fillers was prepared. The optimal concentration of multiple mixed fillers was explored, and the tensile strength, DC volume resistivity, breakdown characteristics, and space charge of the modified insulating paper with different multiple mixed fillers were analyzed. The results show that when the multiple mixed fillers concentration is 0.5%, compared with the unmodified insulating paper, the tensile strength of the modified insulating paper increases by 17.90%, the AC and DC breakdown strentgh of oil-paper increase by more than 17%, and the injection of space charge can be effectively inhibited. The effect of multiple mixed fillers on the DC volume conductivity is not obvious. The mechanical properties and electric strength improvement of the sandwich structure insulating paper prepared using this method are significant, which provides reference for jointly improving the performance of insulating paper.

HVDC cross-linked polyethylene (XLPE) cables will be damaged by operation impulse voltage caused by switching and other operations in the power system under working condition, which will affect the insulation. However, at present, there is a lack of relevant research on the cable insulation deterioration generated by space charge accumulation caused by multiple impulse cumulative effect. Based on the PEA space charge measurement system under DC superimposed impulse voltage in the laboratory, the space charge characteristics of XLPE cable insulation under DC voltage, impulse voltage, and DC superimposed impulse voltage were measured in this paper. The results show that XLPE is dominated by heteropolar charges under positive DC voltage, and negative charges of the same polarity appear on the SC electrode under negative DC voltage. Continuous high-amplitude impulse voltage can cause large number of homopolar charges injection on the two electrodes, and the amount of injected charge is larger under negative impluse voltage. Under DC superimposed impulse voltage, the same polarity superimposition method can promote the injection of homopolar charge on the two electrodes more than the opposite polarity superimposition method. Moreover, the longer the impulse voltage is applied, the more charge accumulates in the XLPE sample.

The equivalent extended Debye model of oil-paper insulation in the damp state no longer has linear characteristics, which makes the evaluation method of oil-paper insulation based on time-frequency conversion have errors. In this paper, the advantages of time domain and frequency domain dielectric spectroscopy were integrated, and the grey correlation algorithm was used to construct the evaluation model of oil-paper insulation damp condition on the basis of the time and frequency domain dielectric response parameters. Firstly, oil-paper insulation samples with different moisture content were prepared, and the influence of moisture and temperature on the polarization and depolarization current (PDC) and frequency domain spectroscopy (FDS) curves was analyzed. Furthermore, the characteristic parameters of PDC and FDS curves under different damp conditions were extracted. On this basis, the grey correlation algorithm was used to integrate the time and frequency domain dielectric characteristic parameters of oil-paper insulation to obtain its standardized reference vector. Finally, the accuracy of evaluation model was verified by laboratory sample data and field transformer data, which provides a new method for warning and diagnosis of transformer early fault.

Compared with the traditional cable joint, the cable fusion joint has the advantages of no stress cone and no movable interface, which has more promising in the new cable operation project and the old cable reconstruction and maintenance project. In order to evaluate the performance of fusion joint after long-term operation in the power grid, the fusion joint was conducted electro-thermal-mechanical coupling simulation under the operation of power grid. The stress distribution and temperature distribution of cable joint were studied through changing the effective value of operating current and considering the short-time overload and material parameter difference between new and old insulation layer. The simulation results show that the stress distribution at the interface between the old and new insulation layer is uneven when the material parameter difference between the new and old insulation layer increases. The increase of operating current will not only increase the overall temperature of fusion joint, but also increase the stress of insulating layer. The radial stress of short-time overload operation for 1 hour is about 4 times bigger than that of steady-state operation, the axial stress is about 3 times bigger than that of steady-state operation, and the core temperature is about 80 K higher.

A new type of insulating paper with aramid nanofiber as the basic structural unit was prepared, and its ageing characteristics in natural ester insulating oil and mineral insulating oil were systematically investigated and compared with the commercial Nomex T410 insulating paper. The results show that the aramid nanofiber-based insulating paper has better thermal stability and ageing resistance, and the mechanical and electrical properties of the aramid nanofiber-based insulating paper are better than those of Nomex T410 insulating paper after accelerated thermal ageing test under the same condition. In addition, the new oil-paper insulation system composed by natural ester insulating oil and aramid nanofiber-based insulating paper can effectively extend the life of the transformer, which provides a reference for the insulation design and development of green low-carbon electrical energy equipment.

Effect of swelling effect of silicone rubber (SiR) caused by silicone grease coated on the interface between high voltage cable accessories and cable body on the electrical tree characteristics of SiR under monopolar impulse voltage (250/2 500 μs) was studied. The isothermal surface potential decay (ISPD) and equilibrium solvent swelling were carried out to characterize the influence of swelling effect on the trap characteristics and crosslinking structures of SiR. The results show that with the increase of swelling time, both the initiation probability and fractal dimension of electrical tree in SiR increase. After 340 h of swelling, the tree initiation voltage of 50% probability under positive and negative impulse voltage decrease from about 37 kV and about -41 kV to 32 kV and about -39 kV, respectively. In addition, the polarity effect can be observed in the electrical tree process of SiR under impulse voltage. The initiation and growth of electrical tree is easier under positive impulse voltage than negative impulse voltage. With the increase of swelling time, the shallow trap density increases, and the deep trap and physical crosslinking density decrease. The destruction of physical crosslinking structure leads to the expansion of free volume, which intensifies the collision ionization processes. Meanwhile, the decrease of deep trap density enhances the charge transport processes. Multiple physical processes intensify the fracture of molecular chains, which furtherly weakens the resistance to electrical tree of SiR. The polarity effect of electrical tree in SiR under impulse voltage is mainly caused by the different distribution of hole-type and electron-type traps.