Charge trapping and transport over chemical defects in polyethylene have significant impacts on its electrical and dielectric properties. However, the dynamics of this phenomenon and its underlying mechanisms remain unclear. To understand this fundamental aspect, we conducted a time-domain ab initio nonadiabatic molecular dynamics study of phonon-assisted holes dynamics in polyethylene over C═O and C-OH defect states. Our results suggest that the hole transfer and energy fluctuations substantially depend on temperature and local morphology. When the temperature decreases from 300 to 100 K, the hole transfer efficiency and the energy fluctuations are severely suppressed due to the weakened interactions between holes and phonons. Furthermore, amorphous polyethylene exhibits a severe suppression of the hole transfer process compared to crystalline polyethylene. An explanation for the influence of morphology on the hole transfer process can be found in the differences in the hole-phonon coupling and the electronic coupling between two chemical defect states in crystalline and amorphous polyethylene. Advancing the fundamental understanding of the dynamics of hole transfer over chemical effects in polymers is a key to improving their insulating properties for the next-generation high-voltage cables.

CaTiO3 perovskite has drawn extensive attention in recent years for its photocatalytic applications related to environmentally friendly fields, such as photodegradation of organic pollutants, carbon dioxide reduction, etc. However, most research was focused on the experimental modification strategies for increasing its photo-catalytic efficiency, little is known about the mechanisms behind it, especially at the molecular level. Our focus here, is the electronic and optical properties of La/Ce-doped CaTiO3 perovskite with different mass fractions. A 2 × 2 × 2 supercell of La/Ce-doped CaTiO3 with concentrations of 2.5 wt

In order to accurately predict dissolved gas concentration in transformer oil and to anticipate transformer faults, a prediction model based on historical dissolved gas data in transformer oil is proposed by combining Sparrow Search Algorithm (SSA) and Long Short-Term Memory network (LSTM). First, the concentrations of dissolved gases (H 2 , CH 4 , C 2 H 6 , C 2 H 4 and H 2 ) in oil were monitored and collected continuously for one year from the same transformer as the training and testing dataset of SSA-LSTM model. Then, SSA algorithm was employed to optimize the parameters of LSTM, including the number of hidden units, the maximum training cycle, the initial learning rate and so forth. By comparing and analyzing different ratios of the training and testing sets, 80% of the entire dataset was chosen as the best training set due to its good balance between prediction accuracy and convergence time. Finally, SSA-LSTM was used to predict the gas concentration in the oil over the next 7 days. The results reveal that, in comparison with traditional prediction methods (BP and LSTM), our proposed SSA-LSTM model has a better prediction performance as measured by prediction accuracy (Acc), mean absolute error (MAE), root mean square error (RMSE) and determination coefficient (R 2 ). In conclusion, our proposed model can more accurately describe the variation rule of the dissolved gas concentration in the oil and provide a strong guarantee for the safe and stable operation of the power transformer.

电缆接头复杂的结构与界面情况,使其成为电缆系统中的薄弱点,需要重点研究.但是由于空间电荷测量方法的局限性,只能获得简单结构(例如薄膜或同轴电缆试样)的空间电荷分布,而复杂绝缘结构中的空间电荷分布无法直接测量.利用COMSOL仿真软件,基于双极性载流子输运模型,建立三维电缆接头仿真模型,对其空间电荷与电场分布进行了仿真计算与分析研究.仿真结果显示,电缆接头中,空间电荷主要积聚在应力锥的根部和高压屏蔽的顶端,并导致这两处电场发生畸变,成为电缆接头中的薄弱点,而温度梯度的存在将加剧电场的畸变.与考虑电导率的传统方法相比,空间电荷仿真结果反映了由载流子注入、抽出和迁移所引起的电缆接头中的电场畸变,为电缆接头的设计与优化提供了参考.

Based on the detection and analysis of the abnormal phenomenon in UHV transformer partial discharge test, this paper eliminates the influence of external disturbance and the test system, analyzes the structure characteristics of transformer magnetic shielding and grounding, and carries out the discharge location. It first discovers the fracture of magnetic shielding ground line in UHV transformer, which causes the magnetic shielding floating potential discharge and thereby produce a large amount of acetylene gas. The defect is eliminated successfully, and then the transformer operates normally by replacing the magnetic shielding ground line. The analysis shows that the integrated use of partial discharge test, ultrasonic positioning method and oil chromatographic analysis can accurately diagnose defect types and position them.

As a main insulation material in high voltage DC power equipment, such as convertor transformer, oil-paper is influenced heavily by space charge under DC voltage.At the same time, the oil-paper insulation always undertakes temperature differences from inside to outside.However, it is still unknown that how the temperature differences affect space charge property of oil-paper insulation.On the basis of former experimental study, we developed a transport model based on bipolar charge transportation and trapping level of materials to simulate space charge property in oil-paper insulation.The temperature gradient effect, namely, the heterocharges accumulating near lower temperature side and further distort local electric field, was gotten by simulation in both single and double layers oil impregnated papers.By comparing calculation results and experimental results, we concluded that the simulation model was reliable.Using this model, the effects of mobility of carriers, thickness of oil impregnated paper, and the temperature of lower temperature electrode were further studied.The results show that the increase of the mobility of oil impregnated paper and the injection rate of electrodes with the increase of temperature is mainly responsible for temperature gradient effect.And the temperature gradient effect turns to be more severe with the increase of mobility of carriers and the decrease of thickness of oil impregnated paper and the temperature of lower temperature electrode.

As the temperature gradient always exists in the insulation of power equipment, the influence of temperature gradient on oil-impregnated paper insulation is studied by experiments and simulation. It's found that in the both single-layer and double-layer structures, the heterocharges always appear near the lower temperature electrode, and increase with the enhancement of temperature gradient and field stress, resulting in more considerable field distortion. However, there are more heterocharges in the single-layer samples while the field distortion is more severe in double-layer samples. Numerical simulations were carried out to investigate the mechanism of temperature gradient effect. It is indicated that the space charge accumulation is mainly due to the temperature dependence of the charge injection and conduction. The behavior of interface charges can be well described by assuming an energy barrier for charge transportation at the interface.

Polyethylene (PE) and oil impregnated paper are widely used as insulation materials in high voltage power equipment however, at high voltages the space charge inside the insulating material can distort the electric field distribution and thus greatly influence aging and degradation. Moreover, there is usually a temperature gradient across the insulation when the equipment is in service, and this can also influence the space charge accumulation. In order to study the effect of temperature gradient systematically a PEA system suitable for measurement under a temperature gradient has been developed and appropriate corrections for the measurement error introduced by the temperature gradient implemented. Separate determinations of the space charge distributions of PE and oil-impregnated paper at different temperatures and voltages have been made. The results for both materials showed some similar characteristics. Obvious heterocharge accumulates at the low temperature side and the higher the temperature difference between the two electrodes or the higher the applied voltage, the more the heterocharge accumulates. As a result the electric field is greatly distorted and the maximum field always appears at the low temperature side. The effects of temperature gradient on the space charge distribution are discussed in detail, and the differences between the two materials are identified. The conductivity of both materials has also been investigated, and the space charge induced by the conductivity gradient produced by a temperature gradient in both materials has also been evaluated. The results show that in oil-impregnated paper the space charge induced by a conductivity gradient plays a great part in space charge formation, while in LDPE the conductivity gradient contributes much less to the space charge. These differences are discussed.

Space charge behavior is important in the oil-immersed-paper insulation in converter transformers for HVDC transmission. This paper reports an investigation of the space charge property in samples with two layers of oil-immersed-paper and oil. Based on pulsed electroacoustic (PEA) technique, a waveform recovery method was proposed to get rid of the effects of interface and anisotropy on PEA result, and the space charge distributions in the two-layer samples under different voltage and oil thicknesses were measured. It's found that the charges with the same polarity as oil side electrode were accumulated at the interface and gradually increased with the increase of applied voltage. However, the positive charge charges always appeared in the oil under positive and negative voltages. Moreover, the charges in samples increased with the increase of applied voltage, while the amount of interface charges decreased with the increase of oil thickness. Due to the existence of interfacial charges, the electrical field in oil-immersed-paper was distorted with its maximum close to the interface. It is indicated that due to the different mobility properties of charges in paper and oil and the interface trap state, the interface between paper and oil plays an important role in charge formation and decay.

According to our former investigation on space charge properties of two layers of oil-impregnated-paper and oil, the interface charge between oil and paper showed different properties with Maxwell-Wagner theory in which macroscopic conductivity and permittivity were only taken into consideration. It is assumed that the charge barrier existing at interface was responsible for the interfacial charge formation. For verifying the charge barrier, in this paper, we built a numerical model to study the charge barrier effect under different voltages and oil depths. The result showed that due to the different mobilities of electrons and positive charges, the amounts of interfacial charges were different under positive and negative voltages, and the increasing of oil depth would reduce the interfacial charges. Furthermore, we used this model in multi-layer of oil-immersed-paper and oil to investigate the effect of interface charge on electrical field distortion.

As the temperature difference exists between the inner and outer conductors of high voltage DC cables in practice, it is important to study space charge behavior under temperature gradient. However, due to the effect of temperature on the acoustic propagation, it is necessary to study the waveform recovery method in pulsed electroacoustic (PEA) measurement. In our previous study, only the effects of temperature on acoustic velocity and attenuation were considered in the waveform recovery. As the density of material will not be continuously uniform under a temperature gradient, the dispersion of acoustic waves due to the change of acoustic resistance should be also taken into consideration. In this report, the characteristics of acoustic propagation under temperature gradient are discussed, and a new algorithm for waveform recovery is put forward, and the distortion of the waveform can be successfully suppressed.

Temperature gradient existing in the oil impregnated paper insulation such as converting transformer or DC bushing affects the space charge distribution. The space charge profiles and field distributions in double layers of oil impregnated paper within 20 minutes were measured by the pulse electro-acoustic method under different temperature differences(Δθ=0、20、40°C), and DC voltage of - 3. 4, - 10. 2, - 20. 4 kV (DC electric field intensity of 10, 30, 60 MV/m), respectively. The test results show that homocharge exists in both electrodes when the electric field intensity was low, and the heterocharge appears in the low temperature side due to the temperature difference. Moreover, the more remarkable the temperature difference and electric filed intensity were, the more the heterocharge at the low temperature side is, resulting in a greater field distortion. The homocharge of the upelectrode starts to accumulate in the space between two layers of oil impregnated paper. The higher the temperature and electric field intensity were, the more the space charge were while the density of the charge is saturated. The result suggests that the oil paper interface plays the role in preventing the charge from passing by, and the temperature is the main factor which affects the space charge distribution and the electricity conduction of the sample.

在直流高电压工程中油纸绝缘材料的空间电荷积聚问题是影响绝缘放电的重要因素之一，对于油纸绝缘的结构设计有着重要意义。在研究过程中，通常采用计算机仿真的方法来检测假设理论的可行性。本文首先提出了一种基于载流子和陷阱势垒理论的油纸绝缘介质空间电荷特性的理论模型，并依此建立了不同温度梯度下单层油纸和双层油纸结构的数学模型。通过数值计算结果与实际中实验结果的比对，验证了这种数值仿真过程的可行性。研究表明：电极注入是电荷的主要来源；双层油纸界面可以对正负电荷产生阻挡作用；温度梯度对于单双层油纸绝缘介质的电荷分布有着重要的影响，温度梯度越大，电场畸变越严重，且双层结构畸变程度远大于单层油纸结构。