Characterization of static electrification in power transformers

Static electrification due to oil now causes many field failures of large forced-oil cooled power transformers. Also, in practice under normal operating conditions, the oil volume in these power transformers is reduced due to the internal heat generated (from the energized windings) which accelerates the chemical processes occurring inside. To complete filling of the oil tanks of such transformers, either new oil of the same or different type is used. Laboratory analogs of these transformers; namely a closed and an open cycle, have been used for investigating this phenomenon. Such tests have been performed upon different types of fresh and aged oils and oil mixtures to measure the electrification current. Investigations of the effect of oil temperature, oil velocity, frequency and type of the applied voltage, type of the solid-phase material, and the length of the oil gap are introduced. Also, a 200 kVA, 3.3 kV/380 V distribution power transformer is used where the oil is forced and heated externally to examine and demonstrate this phenomenon for unenergized and energized cases and for both fresh and aged oils     

Flow electrification in high power transformers: BTA effect on pressboard degraded by electrical discharges

The flow electrification process occurring at the oil-pressboard interface in high power transformers seems to cause electrical discharge incidents and may cause failures. The goal of this experimental study is to measure and to compare the streaming current obtained from unused pressboard, pressboard artificially degraded by electrical discharges, and pressboard obtained from a damaged power transformer. The influence of additive 1,2,3 Benzotriazole (BTA) in oil is also analyzed. The results show that flow electrification is greatly increased by the degraded pressboards compared with the new one. On the other hand, adding of BTA significantly reduces flow electrification.     

浅析大型变压器油流带电现象

从油流带电产生的机理、影响油流带电的主要因素、油流带电的测试方法和广东省大型变压器油流带电的测试实例等方面探讨了油流带电现象,提出如何抑制油流带电现象的产生是当前急需解决的问题。     

Flow electrification in power transformers. Explanation of thewall-current measurements

The aim of our work is to understand better the mechanisms of flow electrification due to oil circulating inside a rectangular pressboard channel. This research has been done because of technical incidents in power transformers in which oil is flowing past pressboard with a velocity in the range of several meters per second. A possible explanation of the wall current observed along the channel is proposed     

Static electrification and partial discharges induced by oil flowin power transformers

This paper presents the study of the major parameters on static electrification, using full-scale shell type transformers (570 MVA, 405 kV and 100 MVA, 500 kV). The consequences of static electrification on the dielectric strength and the conditions of partial discharge occurrence also are studied with oil flow rate up to 5 times the nominal value. It shows that the leakage currents are strongly influenced by temperature, applied voltage and oil flow rate Q. The latter is the most influential parameter. The winding leakage current I varies as Qⁿ , where 1⩽n⩽3. The maximum value of I is observed at room temperature (20°C) when the transformer is energized     

Static electrification of pressboard/oil interface and transientphenomena

The static electrification phenomenon of insulating materials used in power transformers is investigated through two devices. The first is a cell with a rotating disk covered on both sides with a given pressboard and immersed in a metallic tank containing ~41 of oil; and second device enables us to measure the electrostatic charge tendency of oils. The electrostatic charge tendency (ECT) of insulating oils and the leakage current generated by the charge concentration gradient at the oil/pressboard interface are analyzed as function of temperature, water content and aging of oils, the nature of the pressboard, and the rotating speed of the disk. The influence of the surface roughness of the pressboard and antistatic additives on the aging and ECT also are considered. A correlation between ECT and the physico-chemical and electrical characteristics of the oil is established. It is shown that the temperature gradients and the water migration phenomena at the oil/pressboard interface play an important role in the charge separation. Transient currents are strongly affected by the presence of air in the pressboard and the aging of the oil     

Flow electrification in transformers: relevance of ECT and leakage current measurements

The accumulated charge Q on a well insulated pressboard part being probably the most relevant parameter for flow electrification evolution inside transformers but clearly not measurable in situ, we compare in this paper this parameter with others possible to measure. The two most commonly used are oil parameters: ECT (electrostatic charging tendency) and, to a smaller extent, the continuous aging test for tan /spl delta/. Another is not so often used because of measurements difficulties, but was considered to be one of the most relevant: the leakage current. Thus in this paper we compare on two facilities (specially made for that) the accumulated charge Q to the ECT and leakage current. We find that, even if sometimes a correct correlation exists between these parameters, several cases show a bad correlation. Thus monitoring transformers by analyzing only ECT or leakage current seems to be hazardous.     

Experimental study and modeling of static electrification in powertransformers

For the last 20 years, static electrification has been suspected to he responsible for failures in power transformers (electric “tree” paths, “worm holes,” etc.). The phenomenon consists in a preferential adsorption of negative ions from the oil (impurities) into the pressboard. This yields, on the one hand, a space charge in the oil which can relax in contact with grounded metallic walls and, on the other hand, a space charge in the pressboard which can accumulate depending on the leakage paths. As part of a research program by Electricite de France in the field of static electrification in transformers, an experiment has been carried out, at the Universite de Poitiers, Poitier, France, to study the most constrained parts in transformers with regard to this phenomenon: the insulated pressboards close to the oil inlet. For this, the experimental loop consists in impinging an immersed oil jet on a pressboard target in which annular insulated electrodes have been inserted to determine the charge in the pressboard. Considering the scale effect, the equipment involves taking oil from an operating transformer and making it possible to study the influence of the flow velocity and the length of the free jet. From a first modeling, compared to the authors' experimental results, it seems that the local wall current coming from the pressboard is proportional to the local wall shearing stress. Considering the magnitude of the process, it finally turned out that this new configuration increases greatly the charge generation process in comparison to a flow parallel to the pressboard     

Peak-switching currents of power transformers

The switching conditions of nonloaded transformers have been investigated to understand the effect of current inrushes and limitations by considering the connecting circuit of the windings, magnetization performances, contact parameters, type of switcher drive, and the value and sign of residual magnetization. The peak-switching currents have been calculated, simulated, and measured and the results have been compared. The obtained results make it possible to increase the power efficiency of the facility by decreasing the losses and maintainign the quality factor of electric power in transient modes.     

三相变四相电力变压器研究

对3种不同的三相变四相电力变压器接线原理及结构特点进行比较并总结了各自的优缺点;建立了3种不同变压器的仿真模型并进行了在不同负荷条件下的仿真实验,比较了3种不同变压器在三相与四相交流电转换过程中的性能特点。研究表明:阻抗匹配三相变四相平衡变压器是利用绕组间的阻抗约束关系实现绕组间电磁关系平衡,绕组间无曲折连接,材料利用率较高,在三相与四相不对称转换过程中具有明显的优越性。     

静态爆破技术在高压带电环境中的可行性分析

利用岩石的抗压强度高,抗拉强度低的特点,引进博亚牌液压分裂机应用于输变电工程中对岩石进行液压分裂,有效解决了高压带电环境中爆破安全维护难、成本高的难题。     

绝缘纸板老化对油纸绝缘沿面放电发展规律的影响

为研究绝缘纸板老化对油纸绝缘沿面放电发展规律的影响,采用加热老化方法制备了不同老化程度的绝缘纸板试样,设计了柱板放电模型,采用恒压法对各老化程度绝缘纸板试样进行长时间加压实验,测量并采集了局放信号,统计了不同老化程度绝缘纸板沿面放电各阶段总放电量、最大放电量、放电次数等变化规律,总结了绝缘纸板老化对其沿面放电发展规律的影响。将理论分析与仿真相结合,分析了产生该影响的主要原因。实验结果表明:放电前期,放电主要由处在高场强区的变压器油产生,此时绝缘纸板老化程度对沿面放电总放电量、放电次数、最大放电量等无明显影响;随着放电的发展,绝缘纸板内由老化产生的孔隙中填充的变压器油分解产生气体,加剧了放电程度,此时绝缘纸板老化程度对其沿面放电影响较大,老化程度越高,总放电量、放电次数、最大放电量等增加速度越快,数值也越大。     

双电源输入环境下的电力电子变压器运行研究

为达到现代电网供电可靠性高、连续性强的标准,设计了一种适用于双电源输入电力电子变压器(DPI-PET)拓扑。其中,输入级为并联模块化多电平变流器(MMC),实现一路电源因故障停电时能快速配合另一路电源工作,并保持网侧以及直流侧电压平稳;输出级为YN联结的三个单相逆变器,解决各种异常工况下电能质量的影响,各环节结合运行状态给出了相应的控制策略。在Matlab/Simulink中建立10 kV/380 V的配电网DPI-PET进行仿真研究,验证了所提拓扑的有效性与实用性。     

发电厂低压厂用变压器主保护配置分析

近年来大型发电厂低压厂用静电除尘变压器的容量一般为2 500kVA,在运行中的谐波分量较大,在带50%以上负荷时,经常出现差动保护误动跳闸的现象,给生产运行带来不便.通过对低压厂用变压器速断保护整定原则及灵敏度计算分析认为,在速断保护满足灵敏性要求的前提下,低压厂用变压器可以不装设差动保护,而用速断保护作为低压厂用变压器的主保护,以简化保护配置,减少误动机率.     

Increase of static electrification in an aged oil‐immersed transformer

Increase of static electrification activity and discharge generation was experienced during the service operation of an aged power transformer. The transformer oil had high sulfide and sulfoxide contents and was liable to show high ECT value after aging. However, there was no apparent increase in standard ECT (electrostatic charging tendency) measurement of the insulating oil and, therefore, deterioration of pressboard was suspected as the cause of the increase. The ECT tests with aged oil and pressboard sampled from the aged transformer were insufficient to demonstrate the increase in static electrification for the aged transformer. On the other hand, the results suggested a possibility that increase in static electrification can be caused by adsorption of some deterioration compound in the aged oil to the pressboard. In order to make more effective evaluation to the transformer insulation, potential measurement on the pressboard in the oil duct was carried out. The experimental results demonstrated strong increase of the accumulated charge by the aged pressboard, which suggested effectiveness of potential measurement for the assessment of static electrification in transformers. The device of potential measurement for practical use was designed by reducing the necessary amount of oil sampling. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(3): 10–19, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20789     

Sensitivity of UHF PD measurements in power transformers

The reliability of electrical energy networks depends on the quality and availability of electrical equipment like power transformers. Local failures inside their insulation may lead to catastrophic breakdowns and might cause high outage and penalty costs. To prevent these destructive events power transformers are e.g. tested for partial discharge (PD) activity before commissioning and currently also during service. The current work deals with the electromagnetic PD detection method, also known as UHF method. The disadvantage of the UHF method is still the missing possibility for a calibration or at least a verification of the sensitivity. The so-called Sensitivity Check might show in future a relation between unconventionally measured UHF quantities to the apparent charge level in pico Coulomb. An important aspect is the attenuation of UHF signals within power transformers which is investigated in this paper.     

直流输电对交流系统变压器的影响

列举了天生桥—广州直流输电系统、三峡—广东鹅城直流输电系统、贵州高坡—广东肇庆直流输电系统调试投运期间的大量现场实测数据,并对这些数据进行分析,结果表明直流输电系统单极大地回线方式或双极不对称方式运行时对交流系统中性点接地运行变压器的噪声、变压器中性线直流电流以及电网谐波电压均会产生较大影响,严重时可能损坏电力设备。该结论对直流输电系统的设计、建设、运行以及交流系统采取相应措施有极重要的参考价值。     

OLTC对含DG电力系统静态电压稳定极限的影响

在静态范围内利用三元分析法阐述了OLTC动作引起的Pmax－、Vcr－曲线的变化,研究了在接入分散,电源的电力系统中,OLTC变比调整对其静态电压稳定极限的影响,并采用两机系统进行了仿真分析得出,OLTC变比变化改变了系统的最大传输功率稳定极限Pmax和临界电压稳定极限Vcr,影响了电力系统稳定运行范围;由于分散电源在配电网的接入,OLTC变比变化对Pmax和Vcr的影响程度不同。     

温度对油纸绝缘扁平气隙缺陷局部放电特性的影响

Partial discharge based risk assessment method has rapidly developed as an effective diagnosis method in oil/pressboard insulation system.The temperature in field power transformer is variable according to loading and the PD properties in insulation are also variable in different temperature.In order to assess the influence of temperature on partial discharge properties in oil/pressboard insulation system,a partial discharge testing system with temperature adjusting function was adopted in this paper.The partial discharge data obtained from the experimental work were presented in phase-resolved PD(PRPD) pattern analysis and pulse sequence analysis(PSA).The single pulse in time domain at different temperature is also analysed in this paper.     

电力变压器的安装及缺陷处理

叙述了电力变压器安装的一般步骤,包括安装前的准备工作、安装程序和安装过程中的试验项目。根据现场经验,对安装过程中可能出现的问题进行了分析,并提出了解决方案,给出了安装过程中变压器缺陷的处理方法。