Copper Release Kinetics and Ageing of Insulation Paper in Oil-Immersed Transformers

The paper provides a general overview of chemical processes leading to the degradation of oil-paper insulation in oil-immersed electrical current transformers. Previous knowledge available in literature is complemented by new results placing a specific emphasis on the physicochemical factors which affect the copper release in the insulation oil and the oil oxidation kinetics. It is demonstrated that various ageing processes interact with each other, with one or another process dominating under specific conditions. Comprehensive but disjoint studies focusing on separate sub-processes may produce rather misleading results, and occasionally, lie behind rather irrelevant quality demands imposed on the insulating liquids.

Low Frequency Dispersion Mechanism of Dielectric Response for Oil-paper Insulation Diagnosis

Both the real part and imaginary part of complex permittivity approximately have a log-linear frequency dependency at low fre- quencies, especially at ultra-low frequencies under conditions of different moisture concentrations and temperatures, which is recognized as the low frequency dispersion (LFD). In order to explain this dispersion, a new mechanism of dielectric response of LFD of oil-paper insula- tion is proposed. A simplified one-dimensional mathematical model of concentration polarization carrier caused by slow migration is developed and solved, which indicates that ion mobility is closely related to the size of gap and the adsorption capacity of cellulose molecu- lar chains to ions. A stochastic statistical model of the carrier mobility induced LFD is also developed. Moreover, actual tests under 50 °C and 2% moisture content were put forward, as well as simulations with according current waveforms. The simulation results agreed well with the experimental data in that concentration polarization of carriers caused by slow migration is the probable cause of low frequency dispersion of dielectric response for oil-paper insulation diagnosis.

Surface Partial Discharge Characteristics of Oil-paper Insulation Under Combined AC-DC Voltage

The valve side windings of converter transformers bear AC, DC, impulse, and reversal-polarity voltages during operation, which could result in serious insulation problems of the equipment. By performing experiments with surface discharge model of oil-paper insula- tion at 80 ℃ under combined AC-DC voltage for 200 h, we studied the spectrums and statistical parameters of partial discharges at different discharge stages. Furthermore, some fingerprint parameters were calculated in order to estimate the development situation of par- tial discharge, while the characteristic gases dissolved in the transformer oil were measured by gas chromatography. The surface discharges in the experiments were observed using a high speed camera, and a full discharge process could be marked off into four stages as follows. ①The elementary stage. When a partial discharge occurs near electrodes, electrical charges are injected into the region near electrodes and causing bubble generation. ②Due to their high resistivity and low dielectric constant, the bubbles would bare the major part of the voltage applied to samples. Therefore, discharge happens inside the small bubbles, and it emits a lot of light. ③Micromolecules of gas are produced in discharge, and further ionization in the transformer oil takes place simultaneously when high-energy electrons collide with oil molecules. ④The carrier charge moves forward to electrodes driven by the applied electric field, till they neutralize with the charge from electrodes, and hence discharge channels are formed subsequently.

Improvement on Condition Assessment Model for Oil-paper Insulation

Identification of the aging condition and the failure probability of oil-paper insulation in transformer is important for improving the reliability of electric power transmission system and applying life cycle cost (LCC) management to electrical equipment. Based on data obtained in a series of multi-accelerated-aging experiments, two approaches for calculating failure probability of oil-paper insulation were compared in aspects of degree of polymerization (DP) and condition ranking. In the experiments, mineral oil and cellulose paper are sub- jected to electrical and thermal stresses, and several parameters, including dissolved gases’ volume fraction, furfural content, moisture content, and degree of polymerization, are measured after the aging process. Results show that weight of carbon oxide, which has a close relationship with cellulose paper degradation, is much higher in DP model than in condition ranking model. Moreover, it is concluded that DP model is more practically accurate than condition ranking model, because aging of cellulose paper rather than mineral oil is the key and critical factor of oil-paper insulation aging.

Space Charge Characteristics of Oil-paper Insulation with New Paper and Aged Oil

Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performances. The insulation gradually degrades under combined thermal, electrical and chemical stresses during routine operations, mainly because of space charges inside. This work investigated the space charge characteristics in oil-paper insulation under oil aging circumstance. New trans- former oil samples are thermally aged to obtain different aging states, and their physical and chemical properties are analyzed. New Kraft papers are dried in vacuum and fully immersed in these different aged oil samples, and three kinds of oil-paper samples are obtained. We use the pulsed electro-acoustic (PEA) method to measure space charge under both DC voltage-on and voltage-off conditions at room temperature. The effect of oil aging state on characteristics of space charge injection, accumulation, and decay is analyzed and discussed. The results show that comparing with the DC voltage-off condition, more charges are injected into samples at the interface of electrode and dielectric when DC voltage is on. When the oil-aged state gets worse, more charges are induced at both cathode and anode, more space charges are accumulated in the bulk, the area of negative charges is larger, and local electric field is distorted more seriously. Moreover, for the voltage-off condition, aged oil is good for space charge decay, and trapped positive space charges decay faster than trapped negative charges.

Aging Effect on Characteristics of Oil Impregnated Insulation Paper for Power Transformers

This paper deals with experimental investigations of the electrical and physical properties of oil impregnated insulation paper for power transformers at different temperatures. The ac breakdown voltage, tensile strength and water content of insulation papers impregnated in mineral oil for different time periods were investigated. The effect of insulation paper thickness on the electrical and mechanical properties has also been studied. The results showed that the breakdown voltage and the tensile strength decreased with increasing the time of immersion of insulation paper in oil at room temperature, at 5℃ and at -12℃. Also, the thermal aging effect on the characteristic of insulation paper has been studied. It was found that high temperatures affect the breakdown voltage and the tensile strength to a great extent.

Influence of Moisture Content on the Degradation of Natural Ester Impregnated Paper Insulation

Natural ester is a kind of fire-resistant and environmentally friendly dielectric liquid, which has already been widely used in power transformers. The influence of moisture content on the degradation of natural ester (camellia oil and FR3 oil) impregnated paper in- sulation was investigated and compared with that of mineral oil impregnated paper in this paper. The initial moisture contents of insulating paper samples were 0.3%, 2% and 4%, respectively. Thermal aging experiment was conducted at 130 °C in hermetical stainless vessels for 80 days. Aging parameters such as the degree of polymerization (DP) of insulating paper, furfural mass concentration in oil and acid number of oil were measured. Chain scissions number (CSN) of aged insulating paper was calculated. The results show that moisture could accele- rate the degradation of both natural ester impregnated paper insulation and mineral oil impregnated paper insulation, while the degradation rate of natural ester impregnated paper insulation is much lower than that of mineral oil impregnated paper insulation. High moisture satura- tion solubility and hydrolysis process of natural ester may be the reason.

Chaotic Characteristic of Time Series of Partial Discharge in Oil-Paper Insulation

The chaotic characteristics of time series of five partial discharge （PD） patterns in oil-paper insulation are studied. The results verify obvious chaotic characteristic of the time series of discharge signals and the fact that PD is a chaotic process. These time series have distinctive features, and the chaotic attractors obtained from time series differed greatly from each other by shapes in the phase space, so they could be used to qualitatively identify the PD patterns. The phase space parameters are selected, then the chaotic characteristic quantities can be extracted. These quantities could quantificationally characterize the PD patterns. The effects on pattern recognition of PRPD and CAPD are compared by using the neural network of radial basis function. The results show that both of the two recognition methods work well and have their respective advantages. Then, both the statistical operators under PRPD mode and the chaotic characteristic quantities under CAPD mode are selected comprehensively as the input vectors of neural network, and the PD pattern recognition accuracy is thereby greatly improved.

The Research of Connection between Degree of Polymerization and Frequency Domain Dielectric Spectroscopy Characteristics during Oil-Paper Insulation Aging

In order to study the linkage effects between degree of polymerization and frequency domain dielectric spectroscopy characteristics of oil-paper insulation, the frequency domain dielectric response test platform of oil-paper insulation is set up. Complex permittivity of oil-paper insulation respectively composed by new or aged oil and insulation paper with different DP are tested, and complex permittivity of oil-paper insulation respectively composed by insulation respectively composed by new oil and insulation paper with different DP and low or high moisture content are tested. The test results are analyzed, and the analysis results show that the degree of polymerization of insulation paper has an influence on complex permittivity of oil-paper insulation though influencing the distribution of moisture and acids between oil and paper.

Fractal Dimension Analysis Based Aging State Assessment of Insulating Paper with Surface Microscopic Images

The insulating paper of the transformer is affected by many factors during the operation,meanwhile,the surface texture of the paper is easy to change.To explore the relationship between the aging state and surface texture change of insulating paper,firstly,the thermal aging experiment of insulating paper is carried out,and the insulating paper samples with different aging times are obtained.After then,the images of the aged insulating paper samples are collected and pre-processed.The pre-processing effect is verified by constructing and calculating the gray surface of the sample.Secondly,the texture features of the insulating paper image are extracted by box dimension and multifractal spectrum.Based on that,the extreme learning machine(ELM)is taken as the classification tool with texture features and aging time as the input and output,to train the algorithm and construct the corresponding relationship between the texture feature and the aging time.After then,the insulating paper with unknown aging time is predicted with a trained ELMalgorithm.The numerical test results show that the texture features extracted from the fractal dimension of the micro image can effectively characterize the aging state of insulating paper,the average accuracy can reach 91.6%.It proves that the fractal dimension theory can be utilized for assessing the aging state of insulating paper for onsite applications.

Comparative Evaluation of the Thermal Aging of Solid Insulation in Mineral Oil and Methyl Ester of Palm Kernel Oil

The purpose of this work is to determine the impact of thermal aging on the dielectric and physicochemical properties of the oil/paper mixed insulation. We performed a comparative analysis of dielectric paper dipped in two cooling fluids: palm kernel oil methyl ester (MEPKO) and mineral oil (MO). Two types of dielectric paper were used: Thermally Upgraded Kraft paper (TUK) and Nomex-910 paper (NP-910). An accelerated aging test was realized at 110°C during a total of 96 hours. Samples of oil and paper were collected after 0, 48, 72 and 96 hours for analyses purposes. The analyses performed included the measurement of the Breakdown voltage (BDV) of the dielectric papers, the Total Acid Number (TAN) and the Decay Dissolved Products (DDP) of the liquid dielectrics. The BDV of NP-910 is greater than the BDV of TUK. Concerning the type of oil, the BDV of dielectric papers impregnated with MEPKO is greater than the BDV of similar papers impregnated with MO, indicating a better preservation of paper when dipped in methyl esters. The analyses of TAN and DDP revealed that Nomex-910 improves the oxidation stability of MO, but reduces the oxidation stability of MEPKO. These results prove that methyl esters can be used as a substitute to replace mineral oils in power transformers. Furthermore, they show that NP can be used mainly in areas of transformer where solid insulation is subjected to high thermal and electrical stress, and TUK other places where solid insulation is required. Such combination could assure money savings and a better preservation of the oil viscosity.

高热故障下变压器油纸绝缘系统的产气机理研究

针对高热故障下超/特高压变压器内部绝缘系统状态变化和产气机理不清晰而严重制约变压器状态分析诊断的问题,本文基于热焓理论与仿真模拟手段相结合的方式对超/特高压变压器高热故障下油纸绝缘系统特征气体的生成路径与油纸绝缘系统的裂解机理开展研究。依据热焓理论得到气体的生成机理,并利用仿真模拟验证本文所提出的产气机理与路径。结果表明:高热故障下油纸绝缘系统中链烷烃要比环烷烃和双环芳香烃更容易发生裂解,各组分裂解生成特征气体的速率从大到小依次为纤维素、链烷烃、环烷烃、双环芳香烃。根据气体生成能可知,特征气体CH_4和C_(2)H_6最容易生成,C_(2)H_(2)最难生成,可通过特征气体在油纸绝缘系统中的生成速率判断高热故障的严重程度。

融合频谱特性与聚类云-证据推理的油纸绝缘老化程度诊断

油纸绝缘系统老化程度与电力变压器的稳定运行密切相关。针对油纸绝缘频域介电谱(frequency-domain spectroscopy,FDS)所提相关特征量单一和未考虑频谱特性指标间冲突性与随机性而导致老化诊断结果误差大的问题,提出了融合频谱特性与聚类云-证据推理的油纸绝缘老化程度诊断方法。首先,根据不同老化试样的频谱特征提取老化诊断多特征指标,并利用非线性拟合构建老化特征数据库。其次,基于Kmeans聚类提出适配非线性变化指标的聚类云模型基本概率分配方法。最后,针对多特征量的冲突性及相关性差异,利用CRITIC-G1综合赋权法计算各指标对应的证据修正因子,并进行基本概率再分配,利用证据融合推理得出油纸绝缘系统真实老化程度。实验结果表明,所提方法可准确诊断复合油纸绝缘样本的老化状态,为电力检修策略的制定提供了理论指导。

天然酯绝缘油-纸界面的多分支流注放电仿真

油-纸界面的局部放电是油浸式变压器中油-纸绝缘失效的主要原因之一。文中在多分支流注放电数值模型的基础上进一步考虑油-纸界面的电荷输运特性,构建油-纸绝缘系统中的多分支流注放电模型,并利用有限元法分别对天然酯绝缘油和绝缘纸介电常数配比、针电极与纸板的间距影响下的沿面放电特性进行研究。结果表明,针电极与纸板的间距及绝缘油与绝缘纸的介电常数配比均会显著影响流注分支;针电极与纸板的间距越小,油-纸界面处的流注分支间的抑制越明显,油-纸界面的流注对z-轴流注的影响更显著;当绝缘油介电常数比纸板大时,流注在油中的发展和分支更为显著,反之则流注更易沿纸板表面发展及在油-纸界面聚集电荷。

植物绝缘油中水对甲醇扩散影响的分子动力学模拟

甲醇因其稳定性强、在绝缘油老化初期含量高等特点,是变压器油纸绝缘状态评估的重要特征物质,而水是油纸老化过程中必不可少的产物。为明确水对植物绝缘油中甲醇扩散的影响,分别构建含水量为1.0%、1.5%、3.0%、5.0%的植物绝缘油、水、甲醇共混模型,利用分子动力学方法计算模型中物质的扩散轨迹、扩散系数、相互作用能、氢键、自由体积,并从微观角度出发,解释水促进甲醇扩散的机理。结果表明:随着含水量的增加,甲醇与植物绝缘油的相互作用能减小,系统整体的氢键稳定性下降,自由体积增大,最终导致甲醇在植物绝缘油中的扩散作用增强。

基于界面电荷极化特性的变压器油纸绝缘寿命预测方法研究

准确预测变压器油纸绝缘寿命对于保障电力系统安全稳定运行具有重要意义。根据微观介电响应机制剖析界面极化与油纸绝缘老化的内在机理联系,并基于混合极化模型——一种在扩展德拜模型基础上引入界面极化支路的等效电路拓扑,探究界面电荷极化特性在油纸绝缘寿命预测中的应用。首先,分析温升过程中频谱特性变化背后蕴含的普适弛豫极化规律,基于混合极化模型重构回复电压极化谱。结合温度对主时间常数和模型参数的影响规律,提出一种单一温度测试下的油纸绝缘活化能计算方法,通过界面极化电容构建出频温平移因子。其次,剖析介质损耗因数随老化加深的变化规律,探究损耗峰偏移现象与界面极化弛豫时间的内在联系,凝练全新表征油纸绝缘聚合度含量的频域特征参量——界面极化支路极点Ph2。最后,联合界面极化支路极点Ph2、频温平移因子和损失累积动力学方程构建油纸绝缘寿命预测模型,并将不同运行年限变压器现场实测数据代入模型验证实效性,为油纸绝缘寿命预测提供了一种新思路。

基于二元相关退化的动车组车载变压器油纸绝缘剩余寿命预测

针对动车组车载变压器油纸绝缘剩余寿命预测中单性能退化指标难以全面反映油纸绝缘退化过程的问题,考虑车载变压器油纸绝缘退化的个体差异性及两性能指标间的相关关系,提出了基于Copula函数的两性能指标相关退化的油纸绝缘剩余寿命预测方法:采用具有随机效应的维纳过程建立油纸绝缘的两性能指标相关退化模型,基于赤池信息准则(Akaike Information Criterion, AIC)选择拟合效果更优的Copula函数来描述两性能指标间的相关关系,采用最大似然估计法估计初始时刻的模型参数,基于序列贝叶斯更新方法在线更新退化模型中的漂移系数,以实现油纸绝缘剩余寿命的在线预测。最后以加速热老化试验下油纸绝缘的聚合度和抗拉强度的退化数据进行实例验证。结果表明,两性能指标相关退化模型比单性能指标退化模型的剩余寿命预测值与实际值之间的平均绝对误差更小,预测的准确性更高,且随着模型参数不断更新,剩余寿命的预测值与实际值间的绝对误差在不断减小,预测结果的准确性在不断提升。

针板电极下酯类绝缘油放电特性及产气规律研究

酯类绝缘油在电力系统中的应用越来越广泛,其放电特性和产气规律与传统矿物油存在一定差异。为了研究酯类绝缘油变压器在放电故障发展过程中的放电特性和产气规律,该文以天然酯、改性酯以及合成酯为研究对象,搭建了油纸绝缘放电故障模拟试验平台,采用恒压法进行重复性试验,采集放电发展过程中的放电特征参量,并同时采集试验油样进行油中溶解气体分析。结果表明,酯类绝缘油放电脉冲幅值较高,脉冲重复率较低,其放电发展过程中所释放的能量较大;相同电应力作用下,酯类绝缘油易产生较多的气体产物,其中CO、C_(2)H_(6)和C_(2)H_(2)含量较多,尤其是天然酯FR3;根据放电统计特征参量和油中溶解气体产物可将整个放电过程划分为放电起始阶段、放电发展阶段以及预击穿阶段,其可为酯类绝缘油变压器故障诊断提供一定依据。