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.

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.

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.

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.

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。

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。

基于PSO-ELM的变压器油纸绝缘状态无损评估方法

油浸式电力变压器作为电网的重要组成部分,其可靠运行至关重要。针对变压器长期运行后无法定量评估其绝缘状态的问题,文中开展了油纸绝缘模型的加速老化及受潮试验,探究了油纸绝缘老化及受潮程度对其回复电压曲线的影响规律,并提出采用粒子群优化-极限学习机(particle swarm optimization-extreme learning machine,PSO-ELM)算法的参数预测方法,实现了基于回复电压曲线特征参量的油纸绝缘老化与受潮状态量化评估。由油纸绝缘模型理化性能分析的对比结果可知,基于PSO-ELM方法的预测值精度远高于传统ELM方法,油纸绝缘内含水率及纸板聚合度预测的绝对误差范围分别小于±0.4%、±30。

不同过热故障条件下呋喃分子在油-纸界面扩散行为的分子动力学研究

呋喃分子是油浸式变压器局部热故障时绝缘纸裂解的重要特征产物之一,变压器油中呋喃分子含量的检测是评估变压器运行状态的重要离线检测手段。为了明确绝缘纸中呋喃分子向植物油中的扩散过程及其与局部过热故障温度标签的对应关系,采用分子动力学的方法,研究了纳米尺度油-纸界面模型在室温(300 K)、低温过热(500 K)、中温过热(750 K)和高温过热(1000 K)条件下呋喃分子从绝缘纸向植物油中的扩散过程。结果表明:随着温度升高,油-纸绝缘中呋喃分子的扩散系数逐渐增大,而且从绝缘纸中扩散到植物油中的呋喃分子数量与温度密切相关。在300 K与500 K的条件下,鲜有呋喃分子扩散到植物油中;而在750 K和1000 K的条件下,部分呋喃分子扩散入植物油中,且温度越高,扩散到植物油中的呋喃分子数越多。上述仿真结果与油浸式变压器中、高温局部过热故障条件下可检测出呋喃族化合物的经验依据一致。

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.

不同含水率下电-热耦合应力对油纸绝缘界面小分子气体扩散特性的影响机制

油纸绝缘在实际运行过程中,受到电—热耦合应力作用,其内部产生的气体以气泡的形式从界面处析出,造成局部放电甚至绝缘击穿,危害变压器的运行安全。文中通过搭建油纸绝缘电—热耦合实验平台,分别对不同温度、不同电场强度以及不同含水率条件下的油纸绝缘模型进行实验,实验结果表明,随着电场强度以及含水率的提高,气泡析出的起始温度逐渐下降。由此进一步建立油纸绝缘界面分子动力学仿真模型,以绝缘纸老化裂解过程中产生较多的CO_(2)为例,对其在油纸绝缘界面处的聚集状态、相对浓度,扩散系数,氢键作用以及自由体积分数进行了研究。模拟结果表明,升温提高了自由体积分数,促进了CO_(2)分子的布朗运动,较多CO_(2)分子扩散至界面处与绝缘油中,扩散系数提高了187.96%。相同温度作用下,随着电场强度的增大,CO_(2)分子与其他分子间形成的氢键数量减少,导致CO_(2)受到的分子间作用力减小,290K时,电场从50kV/m增大到200kV/m时,扩散系数平均提高了90.41%;360K时,提高14.02%。含水率的提高使得CO_(2)分子的扩散系数平均提高了367.17%。结果表明,温度和含水率在油纸界面气体分子扩散过程中起主要作用。研究通过微观分子动力学仿真手段,揭示了电—热耦合应力对气体分子扩散的影响机制,为变压器安全运行提供了新的监测评估依据。

基于GWO-KELM模型的变压器油纸套管典型绝缘故障辨识方法

油浸式套管是变压器的重要组成部件,其安全运行与整个电网的稳定性息息相关。分别制作零屏受潮、金属杂质、末屏破损和末屏未接地共4类套管内部或安装过程中的典型故障,利用现有检测手段,实现套管典型缺陷的故障特性分析。建立了适用于4类典型套管故障辨识的GWOKELM算法模型,通过灰狼优化后,KELM诊断正确率从75%上升至95.8%,得到了明显的提升。依据正确率和训练时间这2个指标,对优化后算法与传统机器学习算法的进行对比,结果表明:优化后的分类结果和训练时间均表现优异,灰狼优化后KELM仍然保持了高泛化、低计算复杂度的优点,在所搭建的故障分类案例中表现优异。

基于灰色关联与时-频域介电响应的油纸绝缘受潮状态快速评估方法

受潮状态下油纸绝缘的等效扩展Debye模型不再具有线性特性,使得基于时频转换的油纸绝缘状态评估方法存在一定误差。本文综合时域与频域介电谱的优点,在获取时-频域介电响应参数的基础上使用灰色关联算法构建油纸绝缘受潮状态评估模型。首先制备不同含水量的油纸绝缘样品,分析了受潮和温度对极化去极化电流(polarization and depolarization current,PDC)以及频域介电谱(frequency domain spectroscopy,FDS)曲线的影响,进一步提取不同受潮状态下PDC和FDS曲线的特征参数。在此基础上,采用灰色关联算法整合油纸绝缘时频域介电特征量得到其规范化后的参考向量。最后,分别通过实验室样品数据和现场变压器数据验证了评估模型的准确性,为变压器早期故障预警与诊断提供了新方法。

Properties of Electrical Discharges Developing in Transformer Oil Under Lightning Impulse in the Setup of Insulated Electrodes

Looking at the problem of electrical discharge development in mineral oil, it is easy to see that most studies in this area focused on the setups of bare high voltage electrodes having a point-plane electrode arrangement. The setups with insulated electrodes are the mar- gin of these studies but it seems to be important to find the dependences between the paper insulation on high voltage electrode and parameters of the discharges initiated in the vicinity of this electrode. Hence, in this paper the results of researches intended to reveal the role of insulation wrapping on a HV electrode in the mechanism of electrical discharges in transformer oil under lightning impulse of both polar- ities are presented. This role is determined by analysis of the parameters characterizing the discharges (onset voltage, propagation velocity, time to initiation, rise-time of light impulses) and also by observation of their spatio-temporal development and oscillograms of the light emitted by their channels. The research was performed for two model electrode configurations: an electrode with paper insulation and a bare electrode which had the same outer dimensions as the insulated one. The most essential conclusion from performed experiment is related to times to initiation. These times, equal in the case of insulated electrodes and model bare electrode, indicate that the source of "weak points" of the paper-oil insulation system is the oil, not the surface of insulation wrapping or the metal.

基于IWOA-BP算法的变压器绝缘老化评估方法

为增强变压器运行的安全性以及稳定性,提出一种基于改进的鲸鱼优化(improved whale optimization algorithm,IWOA)—反向传播(back propagation,BP)算法的变压器绝缘老化状态评估模型。建立变压器油纸绝缘模型进行极化—去极化电流(polarization and depolarization current,PDC)实验,为避免含水率对评估结果造成影响,提取综合电导率作为老化特征量;以BP神经网络为主,PDC电流数据为输入,综合电导率为输出,建立算法模型对油纸绝缘老化状态进行评估;为提升算法模型的评估效果,以加入非线性收敛因子以及自适应权重为方法对鲸鱼优化算法(whale optimization algorithm,WOA)进行改进,并以IWOA对BP神经网络进行优化,获取更优的BP权重与阈值初值,提高BP神经网络的收敛速度以及精度。最终结果证明,IWOA-BP评估模型具有一定准确性以及实用性,且在油纸绝缘寿命评估方向上有一定发展空间。