TiO_2纳米粒子对高水分变压器油中电荷输运的影响

水分是导致运行中变压器油绝缘性能下降的重要因素之一。为了提高高水分含量下变压器油的绝缘性能,本文利用半导体TiO2纳米粒子对变压器油进行改性研究。测量了不同水分含量下改性前后变压器油的工频击穿和局部放电特性。发现当变压器油中含水量较高时,半导体TiO2纳米粒子不仅可以使变压器油的工频击穿电压提高至改性前的2倍以上,而且可以有效抑制油中局部放电现象。利用电声脉冲(PEA)对变压器油中电荷累积和消散特性进行了测量研究,结果发现:TiO2纳米粒子能够提高高水分变压器油中电荷的消散速率,一定程度上抑制了水分对变压器油中电场的畸变,从而提高了变压器油的绝缘性能。     

纳米改性变压器油的破坏特性

变压器等输变电主设备的油纸介质已越来越不能满足特高电压等级对大容量、小型化、高可靠性绝缘系统的严格要求。为了解决输电电压等级提高带来的高性能变压器油及油纸绝缘问题,基于纳米改性技术,开展具有优异电气性能的新型纳米油纸复合绝缘系统的研究。采用变压器油纳米添加改性技术,研究了纳米改性变压器油的制备方法,得到了纳米改性提高变压器油破坏特性的最佳配比,并对纳米改性变压器油在交流、直流、雷电冲击下的破坏特性和局部放电起始电压进行了对比研究。研究发现纳米改性可以提高在较大间隙下变压器油的击穿电压,并且能显著提高其局放起始电压,改善其雷电冲击下50%放电伏秒特性曲线。基于纳米粒子介质球在电场中的极化理论,研究了粒子表面极化电荷密度分布和产生的势阱,并指出纳米粒子界面对载流子的捕获和流注的阻挡作用是较大电极间隙下纳米改性变压器油绝缘性能提高的原因。研究结果说明了纳米改性对于变压器油纸绝缘系统的性能提高提供了新的可能途径。     

冲击电压下3种纳米改性变压器油击穿特性的比较和分析

为了提高变压器油绝缘性能,研究了不同导电性能纳米材料添加改性对变压器油冲击击穿特性的影响。选取铁磁性导电纳米粒子Fe3O4、半导体纳米粒子Ti O2和非导电纳米粒子Al2O3,经不同的表面改性剂处理后,分散于变压器油中,制得纳米改性变压器油,并证实了3种纳米改性变压器油的稳定性。通过操作冲击击穿试验,对比研究了不同浓度纳米改性变压器油的宏观击穿特性,得出3种纳米改性变压器油的最佳质量浓度分别为0.03g/L、0.01 g/L和0.02 g/L。在最佳浓度下进行雷电冲击击穿试验,结果表明:除负极性雷电冲击情况外,纳米改性变压器油的冲击击穿特性较之纯净变压器油均有较大程度的提高,其中操作正极性击穿电压提高幅度较大,可达44.1%、33.3%和35.5%。基于针–板模型的极性效应,结合空间电荷分布分析及结合极化理论,认为纳米改性变压器油冲击击穿特性的改善与添加纳米粒子捕获电子并改变原有空间电场分布相关,而3种纳米改性变压器油击穿特性的不同是由纳米粒子表面极化特性的不同导致。     

TiO_2纳米改性变压器油中正流注测量与机理分析

利用纳米粒子进行绝缘改性能够提高变压器油的正极性冲击击穿性能,但是其改性机理善不清楚。为此基于阴影测试技术搭建了变压器油中流注测量平台,测量研究了正极性标准雷电冲击电压作用下TiO_2纳米改性前后变压器油中流注的产生和发展特性。研究结果表明,TiO_2纳米粒子提高了变压器油中正极性的流注起始电压,且能显著抑制正流注的发展。与纯油相比,TiO_2纳米油中正流注的直径大,流注分支多。据推测TiO_2纳米粒子引入的更多浅陷阱对电子捕获作用,以及流注分支之间的屏蔽作用导致前端电场强度降低,是TiO_2纳米油正极性冲击击穿电压提高的原因。     

基于AlN纳米粒子改性的纳米变压器油雷电冲击特性研究

为了提高变压器油的绝缘特性,选取直径为40 nm的Al N纳米粒子,制备出不同浓度的Al N纳米改性变压器油,通过透射电子显微镜对变压器油中纳米粒子的尺寸展开研究,并对不同浓度纳米改性变压器油的雷电冲击特性进行分析。结果表明:纳米改性变压器油中的纳米粒子直径主要分布在20~500 nm之间。随着纳米粒子浓度的上升,纳米变压器油的正极性雷电冲击击穿电压比纯净变压器油提升了约50%,并随着纳米粒子浓度的上升呈现先上升后下降的趋势。根据雷电冲击特性的实验结果,利用"势井模型"解释了纳米变压器油正极性雷电冲击击穿电压比纯净变压器油升高的原因,并提出随着纳米粒子浓度的升高,由纳米粒子数密度增加导致的总捕获电荷数量增加和纳米粒子等效半径增加导致的总捕获电荷数量减少是一对竞争关系的理论,从而解释了纳米变压器油的正极性雷电冲击击穿电压先升高后下降的原因。     

直流电压下TiO2纳米改性变压器油中电晕放电特性及机理

电晕放电作为变压器油中局部放电主要形式之一,危害换流变压器的安全运行。纳米改性可以提高变压器油的绝缘性能,但直流电压下电晕放电的改性效果和机理研究不足。为此制备了二氧化钛纳米改性变压器油,采用针板缺陷进行了电晕放电过程中的图像拍摄、脉冲电流及光脉冲的测量,并测量了油中的电荷输运特性。研究发现,负极性直流电压下,纳米变压器油的击穿电压提高了23.8%;电晕放电强度也明显降低:外施电压50 kV时,纳米变压器油中电晕发光面积减小了86.0%,光脉冲和电流脉冲的频率分别减小75.6%和76.3%,幅值分别减小92.8%和78.6%。纳米粒子抑制电晕放电是因为纳米粒子向变压器油中引入更多浅陷阱促使电子从电离区逃脱,同时其极化捕捉作用削弱了电子碰撞电离,抑制了电子崩起始;此外带负电的纳米粒子的形成相当于增加了负离子数量、降低了其迁移率,从而降低了针尖处电场强度。该研究可为直流电压下变压器油电晕放电的抑制方法和纳米粒子的选型提供依据。     

AlN纳米改性变压器油的电热性能及其应用研究

纳米颗粒改性变压器油由于其具有散热性能和绝缘性能等优势受到越来越广泛的关注。在总结近年来纳米改性变压器油研究现状的基础上,采用氮化铝(Al N)制备了纳米改性变压器油,并对其导热特性、绝缘特性及其应用进行研究。结果表明:Al N纳米粒子最高能使改性变压器油的热导率提高7%。同时,Al N纳米改性变压器油的正极性雷电冲击击穿电压最高提升了50%。此外,通过现场试验验证了Al N纳米变压器油的散热性能。纳米改性变压器油能显著提升变压器的散热能力,在相似环境中,额定负荷下纳米油变压器中的油温比普通油变压器约低12℃。     

变压器纳米改性油纸复合绝缘研究综述

油纸复合绝缘作为电力变压器内部的主要绝缘，其电气性能备受电力设备制造与运行人员的关注。纳米颗粒结构因具有大比表面积、量子尺寸及宏观量子隧道效应等优异性能，常作为提高绝缘材料电气性能的添加物质。该文总结了近年来变压器油纸复合绝缘纳米改性的研究成果，首先概述变压器油纸复合绝缘纳米改性的方法；其次详细介绍三种纳米颗粒改性机理的理论模型，并分析改性前后绝缘油纸内部与表面的电荷行为、局部放电特性和交直流复合及雷电冲击电压下的击穿特性；最后对目前研究成果进行总结，并展望未来变压器油纸绝缘纳米改性的研究和发展方向。这些研究成果的总结将为改性油纸绝缘的发展与电气性能改善提供参考和借鉴。     

纳米改性变压器油制备与绝缘特性研究现状

目前纳米变压器油由于其高散热性和高绝缘性能受到越来越广泛的关注。以近年来纳米变压器油和纳米油纸复合系统的研究现状为基础,分析了纳米变压器油的制备及其导热性能和电气性能(如击穿、局部放电、老化、抗水分特性等),同时分析了纳米油纸系统的制备及其电气性能(如击穿、局部放电等),并分别介绍了目前常用的3种用于解释绝缘油介质和油纸复合系统介质中纳米颗粒改性机理的模型,最后提出了纳米变压器油领域后续研究需要关注的问题,即纳米粒子材料的选择,低成本、高性能纳米粒子的制备,纳米粒子对纳米油电导率和介质损耗的影响,纳米粒子对油纸复合系统的影响。     

纳米改性变压器油工频局部放电特性研究

以不同浓度的TiO_2和SiO_2纳米改性变压器油为研究对象,通过光电检测法采集局部放电脉冲信号,探究纳米颗粒的添加对变压器油局部放电特性的影响。结果表明:纳米颗粒通过增加油中浅陷阱密度、畸变电场分布,可以有效降低载流子迁移率,从而抑制放电的发展。加入纳米颗粒后,变压器油的击穿电压和局部放电起始电压都有所提高,局放脉冲上升沿变化率降低。不同种类纳米颗粒对放电的不同阶段抑制效果不同,SiO_2纳米颗粒对放电的起始发展阶段抑制程度更大,TiO_2纳米颗粒对放电的预击穿阶段抑制效果更佳。     

Development and practical operation of perfluorocarbon immersed 275kV transformers with compressed SF6 gas insulation

A perfluorocarbon (PFC) immersed 275 kV power transformer with compressed SF₆ gas insulation has been under development. This paper clarifies the AC partial discharge inception voltage and time characteristics of PFC immersed insulation and also clarifies that a prototype 275 kV 100 MVA three-phase transformer could be operated without any trouble during long-term overvoltage tests. This prototype proved that it had AC partial discharge inception strength of higher than 1.5 times the AC test voltage and lightning impulse breakdown strength of 1.5 times the test voltage. A 275 kV, 250 MVA three-phase power transformer was developed and operated at the outdoor substation of the Chubu Electric Power Co., Japan. This transformer has been successfully operated to date and a detailed internal inspection of the transformer was carried out after one year and 9 months of successful practical operation. No significant abnormal condition was recognized     

Effect of lightning impulse prestress on AC partial discharge characteristics in cast resin transformer

It is crucial for cast resin transformer to detect void and delamination existing in the cast resin because these defects reduce insulation performance of cast resin equipment. Furthermore, defects are likely to be mixed in cast resin because of several surface boundaries between resin and conductor. It is considered that detecting partial discharge (PD) is effective to diagnose equipment of power equipment. However, it is reported that withstand lightning impulse voltage test may give influence on AC partial discharge test, especially partial discharge inception voltage. This paper deals with accumulated charge in a void under AC voltage to investigate the effect of the impulse voltage prestress on subsequent AC PD characteristics in cast resin transformer. AC PD characteristics were compared before and after impulse application. In addition, recovery of PD characteristics was measured to investigate charge decay. As a result, the mechanism of prestress effect is revealed and surface charge density is estimated quantitatively in cast resin transformer.     

The mechanisms leading to the useful electrical properties of polymer nanodielectrics

Polymer nanocomposites with metal oxide nanoparticle fillers exhibit enhanced electrical breakdown strength and voltage endurance compared to their unfilled or micron filled counterparts. This paper presents the following hypothesis for the mechanisms leading to improved properties. The inclusion of nanoparticles provides myriad scattering obstacles and trap sites in the charge carriers' paths, effectively reducing carrier mobility and thus carrier energy. The result is homocharge buildup at the electrodes, which increases the voltage required for further charge injection due to blocking by the homocharge. The hypothesis is supported by electroluminescence, pulsed electro acoustic analysis, thermally stimulated current measurements, a comparison of AC, DC, and impulse breakdown, as well as absorption current measurements, in silica/crosslinked polyethylene matrix composites with supporting evidence from titania/epoxy composites.     

Dielectric strength of transformer insulation at DC polarityreversal

Among special insulation tests for DC power converter transformers, polarity reversal tests can be substituted by applied AC voltage tests from a stress similarity in oil ducts. In order to investigate equivalent levels of other insulation tests to polarity reversal tests, the dielectric strength of a transformer at polarity reversals was examined by use of a model of typical oil/paper composite insulation. From the comparison of polarity reversal and other insulation tests, it was found that dielectric strength of transformer insulation at polarity reversals is phenomenally similar to that under switching impulse stresses. Equivalent AC test voltage to polarity reversal test was also estimated from the experimental results. The breakdown characteristics of polarity reversal tests showed that partial discharges at polarity reversals impose less harmful stress on insulation barriers than those at usual AC or impulse tests     

Prediction of breakdown voltage of transformer oil from predischarge phenomena

Complex experimental studies of the discharge mechanism in transformer oil under positive lightning impulse were carried out using alternative diagnostic methods. An attempt to establish a connection between the macroscopic features of discharge and the breakdown voltage was made.     

Effects of voltage impulses on extruded dielectric cable life

Laboratory tests were performed to investigate the effects of lightning and thumping impulses on the performance of various 15 kV cable designs. The cable designs were aged in the laboratory using an accelerated water treeing test. Some samples were subjected to lightning impulses, some were subjected to thumping impulses, and some were aged without impulses applied. The impulse effects were evaluated using time to failure data, AC breakdown data, and impulse breakdown data. The results show that lightning or thumping impulses do not affect cable AC or impulse breakdown strength. There is also no visual difference between wafers taken from impulsed and control samples. However, there is a strong indication that lightning impulses can reduce cable life. Almost all the aging failures occurred while the cables were warm. No failures occurred during the impulse application. It is also shown that increased cable wall thickness extends cable life for the tree-resistant high-molecular-weight insulation material tested, but not for the crosslinked polyethylene insulation material tested     

变压器中性点串小电阻抑制直流偏磁的研究

高压直流输电单极或双极不对称运行时,直流入地电流会使接地极附近的交流变电站地电位升高,从而导致直流电流侵入中性点接地的交流变压器,产生的直流偏磁将影响变压器正常运行。针对直流偏磁现象,提出了变压器中性点串接小电阻的方案,利用电力系统仿真软件PSCAD/EMTDC对接电阻后的交流输电网络进行建模,仿真分析了该方案的抑制效果,以及工频过电压和雷电过电压对中性点绝缘的影响,给出了串接电阻的建议值。研究结果表明:变压器中性点串接合适阻值的小电阻可以简单而有效地抑制直流偏磁电流,随着电阻值的不断增大,电流的衰减效果并不明显;故障情况下串接电阻后会抬高中性点的电压,建议取值的小电阻能够承受短时工频过电压和冲击过电压,不会对中性点的绝缘造成影响。     

冲击电压下不同温度油中空间电荷迁移对电场分布影响

冲击电压下油中空间电场分布对于变压器内部绝缘优化设计至关重要,现有设计方法没有考虑温度和空间电荷迁移对电场的影响。为了研究温度对空间电荷迁移特性的影响,搭建温度可控的冲击电压下油中电场测量平台,测量雷电冲击电压下25℃、50℃、60℃均匀电场的油中空间电场分布,分析空间电荷对油中电场的影响。实验结果表明:随着温度的增加,60℃时油中场强峰值比25℃时下降了5.2%;50℃时油中场强分布在0.60μs时提前达到稳定阶段。温度升高后,电子漂移速度最大是25℃下的1.8倍,迁移率最大是25℃下的2.0倍,温度升高引起电子迁移率、漂移速度的增加导致了上述现象。该文研究结果为变压器绝缘优化设计提供了实验依据。     

The electrical breakdown characteristics of oil-paper insulationunder steep front impulse voltages

Disconnecting switch operations in gas insulated equipment cause transient voltages with risetimes as steep as 5 to 20 nanoseconds and magnitudes as high as 2.5 pu. There is very little information on the effect of these transients on oil-paper insulated equipment. There have been reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper insulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and BC Hydro. V ₅₀ (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 μs), lightning and switching impulse waveforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front impulse waveform. The V₅₀ results showed that the breakdown strengths were lower for steep front impulses than for lightning impulses. The multiple impulse breakdown results showed that oil-paper insulation breakdown strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to steep front transients will fail at voltages below the lightning impulse design level (BIL). The volt-time data had a discontinuity. The breakdown process at risetimes below about 50 ns was different from the breakdown process at risetimes above 50 ns     

换流变压器油纸绝缘复合电场击穿特性及其试验方法

为了查明换流变压器油纸绝缘在交流叠加直流电场下的击穿特性,利用交流、直流及交流叠加直流等复合电场,对油隙、浸油纸板和油纸复合绝缘进行了击穿试验研究。结果表明：油隙的击穿场强几乎不随交流电压分量变化;浸油纸板击穿场强随交流电压分量增加迅速下降;受浸油纸板击穿特性影响,油纸复合绝缘击穿场强随交流电压分量减小明显提高。说明油隙的击穿场强与交流电压分量基本无关,但浸油纸板的击穿场强受交流电压分量影响严重。依据试验结果,进一步对换流变压器交流和直流电压试验项目的有效性进行了讨论。经分析发现换流变压器主绝缘的油隙与浸油纸板在运行电压下出现的最高电压仍为交流叠加直流电压。交流和直流电压试验的电压幅值能够满足要求,但电压波形与承受的运行电压相差较大。认为交流电压试验考核油隙绝缘介电强度是有效的,而直流电压试验考核浸油纸板绝缘介电强度并不充分。