The oil-paper insulation systems, composed of insulating paper and monoester insulating oil or mineral insulating oil, were conducted thermal ageing tests, the ageing temperature was 130℃ and 140℃, and the ageing atmosphere was nitrogen and air, respectively. The thermal ageing characteristics of two insulation systems were compared. The results show that after the ageing of oil-paper insulation, the acid value and water content of monoester insulating oil are higher than that of the mineral insulating oil. Under the nitrogen atmosphere, the monoester insulating oil can prolong the ageing life of insulating paper significantly compared with mineral insulating oil, but at 130℃, the monoester insulating oil only prolong the ageing life of insulating paper slightly compared with mineral insulating oil. There is little difference between the thermal ageing life of insulating papers in the two insulating oils under air atmosphere.

Micro-water is one of the main factors leading to the deterioration of transformer oil-paper insulation, so it is important to discuss the affecting mechanism of micro-water in the pyrolysis process of oil-paper insulation. In this paper, a compound molecular model of oil-paper insulation was established, and on the basis of reactive molecular dynamics, the pyrolysis process of oil-paper insulation containing micro-water was simulated. The affecting mechanism of water molecules in different pyrolysis stages of oil-paper insulation was investigated, and the roles of hydrogen ions and hydrogen radicals in the pyrolysis process were analyzed. The results show that in the oil-paper insulation composite model containing micro-water, the initial formation time of small molecules such as water and hydrogen molecules is earlier, and their formation rate and quantity are higher than that of the oil-paper insulation composite model without micro-water. By analyzing the reaction path, it is found that at the initial stage, water molecules firstly destroy the hydrogen bond network of insulating paper, and with the increase of high temperature time, the water molecule act as a catalyst to promote the pyrolysis of insulating paper. As the reaction proceeds, water molecules as carriers carrying hydrogen carboxylate ions diffuse into the insulating oil, which promotes the pyrolysis of insulating oil. During the reaction process, the hydrogen carboxylate ion has the effect of accelerating the insulation pyrolysis, and which can accelerate the pyrolysis of oil-paper insulation synergistically with water molecules, while the hydrogen radical does not have the accelerating effect.

In order to explore the influence of distributed optical fiber on the ageing life of transformer insulation system, we carried out accelerated thermal ageing tests on the transformer oil sample containing distributed optical fiber at 110, 120, 130℃. The breakdown voltage of the samples was measured regularly during thermal ageing process. On the basis of the breakdown voltage cumulative loss rate, the relationship between the activation energy of the transformer oil containing optical fiber and pure oil was analyzed, a method for calculating activation energy of transformer oil containing optical fiber was proposed, and a thermal ageing life model of transformer oil containing optical fiber was constructed. The results show that the optical fiber in the transformer oil can affect the strength of the transformer insulation system, and the ethylene tetrafluoroethylene (ETFE) optical fiber has the least effect on the ageing of transformer oil. The calculated results of three life evaluation models of transformer oil containing ETFE, poly(tetrafluoroethylene) (PTFE), and thermoplastic polyester-ether elastomer (TPEE) fiber established based on calculated activation energy and breakdown voltage cumulative loss kinetic equation, are in good agreement with the evaluation results of 10℃ life criterion in engineering experience, which verifies the effectiveness of the model.

Mastering the theoretical calculation model of gas relative insulation strength can greatly accelerate the research of SF₆ substitute gas. On the basis of molecular topological index theory, a molecular topological index of insulating gas was proposed, and the electrical topological state indexes of 22 types of atomic for 47 insulating gas molecules were calculated. At the same time, the electrical topological state indexes of carbonyl and cyano groups were introduced. A multivariate linear prediction model of gas insulation strength was constructed by stepwise multiple regression method, and the groups which have significant indigenous influence on the insulation strength were obtained by screening principle. The results show that >CH-, =C=, -F, -C≡N, -Cl, and >C=O have significant effects on the gas insulation strength. Among them, -F, -C≡N, -Cl, and >C=O have higher electronegativity, and their electrical topological state index (ETSI) value is high and has a positive contribution to the insulation strength. These groups can be preferentially considered when designing and screening SF6 substitute gases.

A transformer oil circulating flow device was established, the flow transformer oil containing metal particles was conducted partial discharge (PD) experiment, and the PD characteristic parameters were extracted. By combining with the movement behavior of metal particles in flow transformer oil, the influence mechanism of electrode covering paper on the PD of metal particle in flow transformer oil was discussed. The results show that when the electrode is covered with paper, the partial discharge initial voltage (PDIV) increases, and the discharge amplitude and frequency decrease. When the double electrodes are bare or only the lower electrode is covered with paper, the discharge concentrates in two phase sections of 0°-90° and 300°-360°. When the two electrodes are covered with paper, the discharge phases concentrate near 45° and 160° mainly. The metal particles will be settled on the paper because of the existence of insulating paper and lead to the increase of horizontal distance between two adjacent collision points. In addition, after the electrode is covered with paper, the electric field in the oil will decrease, which makes the vertical velocity of the particles decrease and the collision frequency between particles and electrodes decrease, resulting in the decrease of PD strength and change of the discharge phase distribution.

In this paper, an evaluation method based on polarization and depolarization currents (PDC) multi feature weight for cable insulation ageing state was proposed. Firstly, on the basis of PDC method, several ageing characteristic quantities reflecting insulation state were extracted. Secondly, the ordered binary comparison quantization method and principal component analysis method were used to determine the weight of feature quantity from subjective and objective aspects, and on the basis of DS (Dempster/Shafer) evidence theory, the subjective weight and objective weight were fused effectively to get the fusion weight. Thirdly, on the basis of the fuzzy comprehensive evaluation method, the effective evaluation of cable insulation ageing state was realized by combining with the fusion weight. Finally, the effectiveness of the evaluation method was verified by an example. The results show that the proposed method can evaluate the cable insulation ageing status effectively and accurately, which can provide a new direction for the evaluation of cable insulation ageing status.

A two-dimensional fluid-solid coupling model of 110 kV power transformer was established, the winding temperature rise characteristics of natural ester insulating oil power transformer and mineral insulating oil power transformer were compared and analyzed by finite-volume method, and the correctness of the calculating method was verified through experiments. At the same time, the winding overload characteristics for the same transformer with two insulating oils were compared. The results show that the winding temperature rise of the natural ester insulating oil transformer is slightly higher than that of the mineral insulating oil transformer, and the difference of their hot spot temperature rise is 4.7℃, but the natural ester insulating oil can improve the transformer temperature rise limit by 10 K. Thus, the winding temperature rise characteristics of natural ester insulating oil transformer is better than that of mineral insulating oil transformer. The overload capacity of natural ester insulating oil transformer is better than that of the mineral insulating oil transformer, and the difference between their overload performances expands further as the overload multiplier increases.

In order to study the failure mechanism of transformer sealing ring in low temperature environment, we collected some failed rubber sealing rings of transformers in extremely cold area, and studied their microstructure and mechanical properties. The changes of micro morphology caused by low temperature and mechanical stress were characterized by scanning electron microscope, the changes of molecular chain structure and thermal motion of rubber samples were characterized by Fourier transform infrared spectroscopy and differential scanning calorimetry, and the changes of micro indentation hardness and reduced Young's modulus of rubber samples were characterized by non-destructive micron indentation test. The results show that due to the long-term joint action of low temperature, insulating oil, and mechanical stress, both the structure and arrangement of the molecular chain in nitrile butadiene rubber (NBR) have changed, resulting in the obvious decrease of micro indentation hardness and reduced Young's modulus of the rubber, and the sealing effect loses.

In this paper, we proposed an intelligent recognition method for composite insulator hydrophobicity on the basis of image processing. For the composite insulator images with different hydrophobicity levels, the images were performed histogram equalization and filtering treatments at first, then the water drop/water trace was separated from the background by the Otsu threshold segmentation method, so as to extract clear and complete water drop/water trace contour. The water drop/water trace coverage rate, the maximum water drop/water trace area ratio, and the water drop/water trace average size were used to quantify the water drop/water trace, and the support vector machine was used to establish the feature classification model, so as to realize the hydrophobicity intelligent recognition of composite insulators. The results show that the hydrophobicity intelligent identification method of composite insulator based on image processing can effectively identify seven kinds of hydrophobicity levels, and the average identification accuracy rate is above 80%.

In this paper, a multi-physics finite element model of actual 10 kV T-type cable terminal joint was established. The electric field and temperature field distribution characteristics of the T-type cable terminal joint with air gap defect at insulation interface were studied, and the change laws of electric field and temperature field amplitudes were analyzed, then the characteristic quantities reflecting the air gap defect of T-type cable terminal joints effectively and the monitored position were proposed. The results show that different degrees of air gap defects in T-type cable terminal joints have a certain impact on the electric field distribution characteristics at the interface, but the electric field and temperature field amplitudes at other typical positions do not change significantly. The air gap defect position would near breakdown under air gap damping condition, at this time, the increase of electric field amplitude on flange base and upper surface of box can reach 20%, and the temperature rise can reach 15℃. In actual engineering, the potential critical hazard of partial breakdown discharge in T-type cable terminal joint can be reflected by monitoring the increase of electric field amplitude and temperature rise of the flange base and the upper surface of box.