非线性绝缘电介质介电特性表征与测试
哈尔滨理工大学工程电介质及其应用技术教育部重点实验室 哈尔滨 150080
Characterization and Measurement of Dielectric Properties of Nonlinear Insulating Dielectrics
Key Laboratory of Engineering Dielectrics and its Application Technology, Ministry of Education Harbin University of Technology Harbin 150080 China
收稿日期: 2018-08-3 网络出版日期: 2018-11-30
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Received: 2018-08-3 Online: 2018-11-30
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作者简介 About authors

李忠华 男 1962年生,博士,教授,博士生导师,主要从事非线性聚合物绝缘理论与相关测试技术、电缆绝缘与诊断技术等方面的研究工作。
非线性是导致问题复杂化的关键因素之一,且存在于几乎所有物理现象之中。电气绝缘介质的非线性介电行为在科学层面提供了更广泛的研究空间,同时在工程技术层面为电气绝缘结构设计、更高电压等级电力设备研发提供了新的破解瓶颈问题的技术途径。具有电场调控功能的非线性绝缘材料,被称为“智能绝缘材料”,已在大型电机端部绝缘和电缆附件防电晕结构得到广泛的应用。随着高压直流绝缘技术的进步,电气绝缘材料和绝缘结构非线性介电特性和机理研究得到越来越广泛的关注。非线性绝缘介质介电特性的表征和测试技术是极具挑战的研究课题,又是基础研究和工程应用技术研究不可回避的关键性基础问题。稳态直流电场下,非线性绝缘介电特性用伏安特性或电导率与电场强度关系来表征,相应测试技术比较简单。稳态交流电场场下,用等效(相对)介电常数和等效电导率与电场强度关系来表征,测试技术的关键是如何准确地分解阻性和容性电流。暂态电场下非线性绝缘介电特性的表征参数体系尚未建立,测试技术最为复杂,相关研究亟待开展。
关键词:
Nonlinearity is one of the key factors leading to the complexity of the problem, and exists in almost all physical phenomena. The nonlinear dielectric behavior of electrical insulating dielectric provides a more extensive research space on the scientific, and it provides the new way to solve the bottleneck problem in the R & D of higher voltage class power equipment. The nonlinear insulating material with the function of electric field regulation is called "smart insulating materials", and has been widely used in the corona protection structures in large motor and cable accessories. With the development of HVDC insulation technology, more and more attentions have been paid to the study on nonlinear dielectric properties and mechanism of electrical insulating materials and insulation structures. The characterization and measurement of dielectric properties of nonlinear insulating dielectrics is an extremely challenging research topic, and it is also the inescapable key fundamental problem. In the steady-state DC field, the dielectric characteristics of nonlinear insulating dielecrics is characterized by volt-ampere characteristics or the relationship between electrical conductivity and electric field strength, and the corresponding measurement technique is relatively simple. In the steady-state AC field, the dielectric characteristics of nonlinear insulating dielecrics are characterized using the relationships of the equivalent (relative) dielectric constant and the equivalent conductivity with the electric field, and the key technique of measurement is how to decompose accurately the resistive current and the capacitive current. The parameters system for characterizing the dielectric characteristics of nonlinear insulating dielectrics under transient electric field has not been established, and the measurement technology is the most complex, so the related research needs to be carried out urgently.
Keywords:
本文引用格式
李忠华.
Li Zhonghua.
1 引言
非线性绝缘材料具有简化绝缘结构设计、减薄绝缘厚度的效果,在高电压复杂绝缘结构设计和制造领域具有广泛的应用前景。事实上,非线性绝缘介质在高电压绝缘领域已得到了较为广泛的应用,如高压电机定子线棒端部防电晕结构中的防电晕带[8,9]、电缆终端用电场应力控制片(或管)等[10,11]。非线性绝缘介质在高压直流电缆主绝缘和附件绝缘结构[12]以及高压复合绝缘子[13,14,15,16,17]中具有潜在应用前景。在世界范围内,高压直流输电技术得到广泛的关注,在我国“十三五”期间也将得到更快发展[18]。高压直流输电技术的迅猛发展对电力设备绝缘技术是一个巨大挑战,因为直流绝缘技术相比交流绝缘技术要复杂,其原因之一就是非线性问题。
2 国内外研究进展
2.1 非线性绝缘直流稳态介电特性
非线性绝缘介质直流稳态介电特性则用电导率与电场强度的函数关系(电流密度与电场强度曲线或伏安特性曲线)来表征[35]。从原理上讲,非线性直流稳态性能测试技术相对比较简单。从技术角度看,非线性绝缘直流稳态介电特性面临三个技术难点:其一是在高电场下测试时如何消除试样因焦耳热引起的温升,已有研究者设计了特殊形式的试样室,通过注入冷却的氮气来排除因焦耳热产生的热量[8];其二是弱电流信号测试时干扰问题,除了选用高稳定度直流电源和提供良好的屏蔽外,同时也可以采用时域数字信号插值滤波技术;其三是消除绝缘介质极化过程吸收电流的影响问题,个别绝缘介质极化建立过程十分缓慢,测试时间达到105s仍未达到真正意义的稳态,具体的方法是采用极化电流与去极化电流求和运算求得。采用极化电流与去极化电流求和运算实现稳态传导电流测试仍存在一定的问题,因为求和运算获取稳态电流技术是建立在极化充分建立和极化与去极化过程是对称这两个基本假设基础上的。在实际测量过程中,这两个假设条件并非真正意义上的满足。
由于非线性直流稳态介电特性表征和测试技术相对成熟,相关机理研究也趋于完善,如空间电荷限制电流理论、热激跳跃电导理论、肖特基效应和普尔-费兰凯尔效应等成功地建立直流稳态电导率与温度和电场强度的非线性函数关系[36]。
本课题组通过大量实验研究,得出LDPE/SiC复合材料的非线性稳态电导影响因素及其规律;结合非线性复合材料的微观结构和经典电介质理论,提出了复合材料的三步输运电导模型;根据所建立的三步输运电导模型,理论推导得出了聚合物基非线性复合材料稳态直流电导率与表观平均电场强度和温度的近似理论[37],即
该理论公式包含聚合物基体属性、半导电无机填料属性和界面厚度、界面势垒等微观信息,并定性地解释了复合材料非线性电导与电场强度、温度、填料粒径和填加浓度的关系。
2.2 非线性绝缘交流稳态介电特性
关于非线性绝缘电介质交流稳态介电性能表征问题,国内外从事工程电介质研究的学者普遍借鉴线性绝缘介质交流稳态介电特性表征途径,即用并联等效电路模型表征介于平板电极间的绝缘介质,不同之处在于采用电压控制性电阻和电容。与此对应的表征非线性绝缘交流稳态介电特性参数体系为电场依赖型电导率和相对介电常数。
非线性绝缘交流稳态介电特性测试,同样可沿用线性绝缘介电特性的测试方法,但可能面临一些技术问题。
高压交流电桥法通常用于线性绝缘介质介电常数和损耗因数(或复介电常数)的测量。采用高压交流电桥研究非线性绝缘交流稳态介电特性时,通过不同幅值交流电压的多次测量可以得到介电常数和损耗因数与电场强度的关系(或复介电常数实部和虚部与电场强度的关系)。常用高压交流电桥均为工频高压电桥,只反映工频条件下交流稳态特性,所获的信息局限性很大;更重要的是高压交流电桥平衡条件与频率有关,非线性绝缘电介质在标准正弦交流电压作用下响应电流含有高次谐波,因此电桥不可能实现平衡。
图1
图1
B.R.Varlow实验室非线性绝缘交流稳态介电特性测试系统原理图
Fig.1
Schematic diagram of testing system for the nonlinear insulation steady-state AC dielectric characteristics of B.R. Varlow laboratory
B.R.Varlow等在忽略阻性电流前提下,通过不同工频交流电压幅值下电压等效电容的测量得到电容–电压(介电常数–电场强度)曲线,进而得到了静态和动态介电常数与电场强度的关系。
图2
图2
非线性绝缘电介质高频、高压伏安特性测试系统
Fig.2
The testing system for high frequency and high voltage volt-ampere characteristics of nonlinear insulated dielectrics
美国桑迪亚国家实验室测试系统实验能力最强,最高频率为1GHz,最高电压可达20kV,最大电流可达10kA,可实现电容值在1~50nF范围内的非线性电介质试样极化强度与电场强度(P-E)电滞回线的测量,代表着当今非线性绝缘介质交流稳态介电特性测试领域在硬件方面的最高水平。
图3
图3
本课题组非线性绝缘交流介电特性测试平台原理图
Fig.3
Schematic diagram of the measuring platform for the AC dielectric properties of non-linear insulation developed by our team
表1 11 非线性绝缘材料及绝缘结构介电特性表征参数体系
Tab.1
表征对象 | 基本特性曲线 | 静态参数特性曲线 | 动态参数特性曲线 | 等效参数特性曲线 | 表征行为 |
---|---|---|---|---|---|
非线性绝缘结构 | 阻性伏安(IR-U)特性 | Gs及Gs-U特性曲线 | Gd及Gd-U特性曲线 | Geff及Geff-U特性曲线 | 阻性 |
Rs及Rs-U特性曲线 | Rd及Rd-U特性曲线 | Reff及Reff-U特性曲线 | |||
电荷–电压(Q-U)特性 | Cs及Cs-U特性曲线 | Cd及Cd-U特性曲线 | Ceff及Ceff-U特性曲线 | 容性 | |
静态复阻抗及其与电压 特性曲线 | 动态复阻抗及其与电压 特性曲线 | 等效复阻抗及其与电压 特性曲线 | 综合 | ||
非线性绝缘材料 | JR-E特性曲线 | γs及γs-E特性曲线 | γd及γd-E特性曲线 | γeff及γeff-E 特性曲线 | 阻性 |
ρs及ρs-E特性曲线 | ρd及ρd-E特性曲线 | ρeff及ρeff-E特性曲线 | |||
D-E特性曲线 | εs及εs-E特性曲线 | εd及εd-E特性曲线 | εeff及εeff-E特性曲线 | 容性 | |
P-E特性曲线 | χs及χs-E特性曲线 | χd及χd-E特性曲线 | χeff及χeff-E特性曲线 | ||
静态复介电常数及其 与电场特性曲线 | 动态复介电常数及其 与电场特性曲线 | 等效复介电常数及其 与电场特性曲线 | 综合 |
综合国内外研究发现,几乎所有研究者均仿照线性绝缘介质的等效电路建立非线性绝缘交流稳态特性非线性并联等效电路,没有将极化过程松弛极化行为和非松弛极化行为分开,导致同一试样不同频率下的测试结果不同[52]。松弛极化行为和非松弛极化行为混在一起,所得表征参数无法反映非线性绝缘介质极化行为的物理本质。由于交流稳态介电参数表征体系没有反映非线性绝缘介质极化行为的物理本质,非线性绝缘交流稳态介电机理研究必然存在相当大的局限性。
2.3 非线性绝缘介质暂态介电特性
非线性绝缘介质暂态介电特性是指非线性绝缘介质在脉冲、阶跃以及直流叠加脉冲等非周期瞬变电场下表现出的宏观介电响应特性。
图4
图4
Boggs实验室冲击电压下非线性电介质暂态介电性能测试系统
Fig.4
The measurement system for transient dielectric Properties of nonlinear dielectrics under impulse voltage in Boggs laboratory
图5
图5
Boggs非线性绝缘暂态等效电路
Fig.5
Boggs transient equivalent circuit for nonlinear insulation
3 关于非线性绝缘介质暂态介电特性表征和测试技术的思考
3.1 关于暂态介电特性的表征的思考
为了克服现有非线性绝缘介质暂态介电特性等效电路模型在表征非线性松弛极化行为的不足,参照线性绝缘暂态特性等效电路模型,可采用图6所示的非线性绝缘暂态介电特性等效电路模型。
图6
图6
非线性绝缘暂态介电特性等效电路模型
Fig.6
Equivalent circuit model for transient dielectric characteristics of nonlinear insulation
该等效电路模型中,GDC(U)表征非线性直流稳态电导特性;C∞(U)表征高频非松弛极化非线性极化特性;CS(U)和RS(U)串联支路表征非线性绝缘松弛极化特性,非线性松弛时间用τ(U) = CS(U)RS(U)来表征,其中,CS(U)表征非线性绝缘松弛极化对直流静态电容的增量。表征松弛极化特性的支路可以有多个支路的并联,支路个数取决于松弛极化机制的个数。模型中除RS(U)外,GDC(U)、C∞(U)和CS(U)均具有明确的物理意义,但RS(U)与CS(U)共同表征松弛极化的松弛特性。暂态激励电压U是时间的函数,所以上述所有参数均是时间的函数。
在平板均匀电场条件下,消除试样几何尺寸因素后可得到表征非线性绝缘介质暂态介电特性参数 χ∞(E)、χs(E)、γDC(E)和τ(E)。由于暂态过程松弛极化行为是出于动态随时间变化的,故用极化率 和τ(E)来表征,而不是εs(E)和τ(E)。各参数与电场强度的函数形式可采用多项式,由此可定义类似非线性光学中的高阶电导率和极化率。
非线性绝缘暂态介电特性参数体系全面反映其介电属性和物理本质,换言之,通过暂态介电参数进行理论计算可得到任意激励电压形式的稳态及暂态介电响应特性。
3.2 关于非线性绝缘暂态介电特性测试技术的思考
非线性暂态介电特性的表征涉及多个物理量与电场强度的函数关系,测试技术就是通过实验的途径获取这些参数及其函数形式。对测试技术而言,可测得不同形式波形和幅值暂态电压(电场)时域波形和对应响应电流时域波形。
如何以测试得到的激励电压和响应电流时域信号为基础数据获得非线性绝缘电介质介电特性表征参数是非线性绝缘介质暂态介电特性测试技术的关键。这一过程和自动控制领域的非线性模式在原理上是相同的,非线性参数数量越多,自由度越高,模式识别难度越大,或鲁棒性越差。
为了克服非线性模式识别因自由度高、鲁棒性差的问题,从理论上讲需要适当增加约束条件。每一种测试条件下测得一组激励电压和响应电流时域信号数据为一个约束条件,这就要求进行多种激励电压波形的组合测试。电压波形组合的合理优化,可有效减少测试次数和提高非线性绝缘介质介电参数的测试精度和准确性。要实现这一目标需要开展大量基础理论研究和实验研究。
非线性绝缘介质暂态介电特性参数测量的基础是如何准确测得激励电压和响应电流时域信号。这就要求测试系统能提供各种可能的暂态激励电压,同时满足响应电流信号的宽频段、宽量限精准测量。往往仅靠硬件电路的优化设计很难满足宽频段、宽量限精准测量,需要通过软件进行测试系统的校验和响应电流信号时域波形的重构。
3.3 关于非线性绝缘暂态介电特性表征和测试技术后续需要开展的研究
非线性绝缘介质的暂态介电特性表征和测试技术研究现状十分落后,甚至处于空白状态。测试技术是支撑相关基础理论研究和工程应用技术研究的手段。综合分析非线性绝缘介质暂态介电行为,建议开展以下几方面的研究工作:
(1)非线性绝缘暂态介电特性表征参数体系的建立。可借鉴非线性光学的表征途径,建立高阶电导、高阶极化率和极化松弛特性形状函数等参数体系。表征参数体系中,非线性松弛极化的形状函数的确定难度最大,需要通过大量实验规律的总结和理论分析来确定。
(2)非线性绝缘暂态介电特性等效电路的建立。等效电路的作用是建立非线性绝缘介质介电参数体系和激励电压、响应电流时域信号关联的桥梁,即首先通过激励电压和响应电流信号作为基础数据,通过适当的算法求取非线性等效电路非线性器件参数函数,而后消除试样几何尺寸因素得到介质表征参数。需要通过理论和实验综合研究确定等效电路的形式,以确保等效电路的有效性。有效性的评价应通过验证性实验进行验证。
(3)非线性等效电路非线性器件表征函数和参数的求取。即以激励电压和响应电流信号作为基础数据,通过适当的算法确定非线性等效电路非线性器件参数函数和求取函数中的具体参数。该过程可借鉴非线性模式识别技术得以实现。
(4)激励电压波形组合的优化。每一种激励电压与响应电流时域测试数据视为一个约束条件,至少需要多少组测试数据可保证非线性模式识别的鲁棒性是值得深入研究的问题。
(5)非线性绝缘暂态介电特性测试平台的建立。搭建测试平台的硬件系统和开发软件系统。硬件系统主要由电源与激励电压和响应电流测试系统组成。电源系统可采用可编程信号发生器和高精度线性高压放大器组合构成。激励电压和响应电流测试系统应满足宽频段、宽量限精准测量的要求,可通过硬件设计和软件修正两个环节得以实现。
4 结论
非线性绝缘暂态介电行为十分复杂,涵盖直流稳态和交流稳态及极化动态松弛行为。非线性绝缘介质暂态介电特性表征参数体系将是反映材料介电属性最完备的参数体系。采用暂态介电表征参数体系开展相关基础理论研究和应用技术研究可实现研究过程的全面性、系统性和彻底性。然而,因非线性和暂态双重因素,非线性绝缘介质暂态介电特性表征和测试技术面临极大的挑战。非线性绝缘介质暂态介电特性测试技术研究不仅为非线性绝缘电介质理论研究和工程应用研究奠定基础,同时可促进包括铁电体在内的非线性功能电介质介电特性研究。
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