Molecular Dynamic Simulation of the Effect of Initial Surface Temperature on Arc Erosion Due to Ion Bombardment

This study focuses on the effects of initial surface temperature on arc erosion caused by ion bombardment. The simulation results show that higher surface temperature leads to a greater number of lost Cu atoms and an increased size of the erosion crater. This is due to the ability of the incident ions to have a greater sputtering yield at higher temperatures. Moreover, the Cu atoms tend to agglomerate and form clusters after ion bombardment while leaving the surface

Measurements of DC Resistivity of Insulating Oils

Insulating oils are widely used in power transformers. The oil insulation degrades under multiple stresses during the transformer operation, which affects the insulation integrity and consequently could cause transformer failures. DC resistivity is one of the dielectric parameters of insulating oil for assessing transformer insulation conditions. This paper presents results of DC resistivity measurements of oil samples including a mineral oil and a synthetic ester in fresh and thermally-aged conditions as well as two in-service aged oil samples in a cylindrical test cell under temperature range of 20oC to 100oC.Two standards of DC resistivity measurement namely IEC 60247 and ASTM D 1169 were applied. For mineral oil, the results of ASTM D 1169 are higher than those of IEC 60247 which is caused by polarity dependence of DC resistivity. For the other samples, the results are the same for both standards. Further investigation on polarity dependence shows that it might be caused by different ion injection strength of the inner and outer electrodes

Accurate Estimating Algorithm of Transfer Function for Transformer FRA Diagnosis

Frequency response analysis (FRA) is an effective tool to detect winding deformation. If variation occurs to the FRA trace, a fault may be indicated. A new method to estimate the transfer function from FRA measurement results on largebandwidth is proposed. A MATLAB program is developed to auto-process the measured data and to obtain the transfer function expression. Measurement results from large frequency ranges are divided into several ranges, and key information is abstracted from each range to form a new transfer function. Afterwards, correction is still needed to eliminate the difference between estimated transfer function and measured data. This method is verified as valid for estimation of transfer functions of 48 groups of FRA data of a 400/275 kV auto-transformer

Inception and Breakdown Voltages of Insulating Liquids under DC Stress

This paper investigated the streamer inception and breakdown properties of a mineral oil and a synthetic ester liquid under DC stress up to 100 kV. To observe the effect of tip radius on streamer inception, the point-plane electrodes configuration with a fixed gap distance of 10 mm and point electrode with theradii of 5 μm, 10 μm, 20 μm and 50 μm were used. To observe the effect of gap distance on breakdown voltage, a point-to-plane electrode configuration with a fixed tip radius of 10 μm and variable gap distances of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were used. Streamer current and emitted light signals were measured by using a 10 Ω current shunt and photomultiplier tube (PMT) respectively. The results indicated that initiation voltages of the synthetic ester liquid are slightly lower than those of the mineral oil for all the tip radii under both positive and negative polarities. Moreover, the synthetic ester liquid has lower breakdown voltages than the mineral oil for all the gap distance under both positive and negative polaritie

Electrical performance of ester liquids under impulse voltage for application in power transformers

Ester liquids including both natural ester and synthetic ester are being considered as potential alternatives to mineral oil, due to their better environmental performance and for some liquids their higher fire point. Although these liquids have been widely used in distribution and traction transformers, it is still a significant step to adopt ester liquids in high-voltage power transformers because the high cost and severe consequence of a factory test failure and the high level of safety and reliability required in service for these units, tend to lead to a cautious approach to any step change in technology. Lightning impulse strength as basic insulation level is of importance for insulation design of power transformers and lightning impulse test is commonly required in the factory routine tests for high-voltage power transformers, so this thesis is aimed to investigate the electrical performances including pre-breakdown and breakdown of natural ester and synthetic ester under impulse voltage. Two types of field geometry were considered in the study, one is sphere-sphere configuration which represents the quasi-uniform fields inside a transformer and another is strongly non-uniform point-plane configuration which represents the situation of a defect or a source of discharge. In quasi-uniform field study, standard breakdown tests were carried out under negative lightning and switching impulse voltages. Influence of various testing methods on the measured lightning breakdown voltage was studied and the 1% lightning withstand voltage was obtained based on Weibull distribution fitting on the cumulative probability plot built up using the approximately 1000 impulse shots. As for strongly non-uniform field study, streamer propagation and breakdown event in ester liquids either with or without pressboard interface were investigated at various gap distances under both positive and negative lightning impulse voltages. A relationship between the results under lightning impulse and previously published results under step voltage was built up to predict the lightning breakdown voltage of ester liquids at very large gaps. The results indicated that impulse strengths of ester liquids for both breakdown and withstand in a quasi-uniform field, are comparable to those of mineral oil. In a strongly non-uniform field, streamers in ester liquids propagate faster and further, than in mineral oil at the same voltage level. Thus breakdown voltages of ester liquids are generally lower than those of mineral oil, which could be as low as 40% at a large gap distance of approximately 1000 mm. Introduction of parallel pressboard interface has no influence on the streamer propagation and thus does not weaken the breakdown voltage, but it tends to reduce the acceleration voltage particularly for mineral oil under positive polarity. Last but not least, a unique phenomenon of secondary reverse streamer (SRS) was observed in ester liquids, which occurs subsequently and well after the extinction of the primary streamer (PS) propagation within a single shot of impulse voltage and has the reverse polarity to the PS. The formation mechanism of SRS is explained mainly due to the reverse electric field induced by the residual space charges left by the PS.EThOS - Electronic Theses Online ServiceI MaterialsNational GridScottish PowerTJH2b Analytical ServicesUK Power NetworksElectricity North WestGBUnited Kingdo

Overloading of power transformers

With the increase of the integration of Distributed Energy Resources, network operators must address the challenges posed by the intermittent nature of renewable energy. This article highlights the crucial role of Dynamic Transformer Thermal Modeling in assessing transformer ageing and overload capabilities. The article provides insights into optimizing transformer performance and ensuring grid reliability by utilizing advanced monitoring technologies like Direct Temperature Monitorin

Studies on the Property and Application of Starch Sugar Ester Dodecenylsuccinic

In this study, we have prepared starch and Brown algae sugar ester dodecenylsuccinic, and by using infrared rays, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC), we studied the structures and properties of the starch and Brown algae sugar ester dodecenylsuccinic. In addition, we studied the possibility of using this modified starch and Brown algae as emulsifier that can be used in ice cream

Investigation of the Total Flow Rates in Oil Natural Transformer Retrofilling Scenarios

Retrofilling transformers with a new liquid has been considered as an option for life extension and/or uprating purposes. The determination of the new total liquid flow rate is of vital importance in assessing the effect of retrofilling on flow and temperature distributions in the winding, especially for ON transformers. This paper presents the determination of flow rate ratios for four liquids—a mineral oil, a GTL oil, a synthetic ester and a natural ester—in ON transformer retrofilling scenarios. Flow rate ratios among different liquids are provided for a simplified condition of linear radiator oil temperature variation. For a more realistic exponential radiator oil temperature variation, the detailed transformer geometric information is needed to estimate the total flow rate. In addition, a hydraulic winding network model is established to prove that “minor pressure losses” due to change of flow directions is indeed minor for ON transformer liquid flow conditions, justifying neglecting the minor losses in the deduction of the total liquid flow rate

Influence of sympathetic inrush on voltage dips caused by transformer energisation

Energising a transformer with other adjacent transformers in service may induce sympathetic inrush that could lead to long duration voltage dips. This study addresses this issue through analysing voltage dip events observed on a 400/132 kV grid when energisations of several generator step-up transformers were performed. With the help of a number of field measurements of voltage dips involving sympathetic inrush, a detailed network model in alternative transients program/electromagnetic transients program has been developed and validated. Based on the network model, comprehensive simulation studies have been performed. In addition, because of the nature of multiple controlling parameters, sensitivity assessment has been carried out to identify the key influential parameters. It is found that the sympathetic inrush, although does not affect voltage dip magnitude, can prolong voltage dip duration by 143% under the worst energisation condition. The sympathetic inrush occurring in substation transformers gives rise to a network-wide sympathetic interaction, resulting in further prolonged voltage dips on the 132 kV grid. Finally, it is proven that application of static var compensator is able to speed up the voltage dip recovery. © The Institution of Engineering and Technology 2013