Comparison of techniques for solving the Laser Intensity Modulation Method (LIMM) equation

The Laser Intensity Modulation Method (LIMM) is a technique for the determination of polarization and/or space charge profiles in the thickness direction of ceramic and polymeric samples. The experimental data are analyzed by means of a Fredholm integral equation of the first kind. This equation admits to multiple and very different solutions. A number of techniques have been developed in order to obtain the most physically reasonable profile. Several techniques based on the regularization method have been proposed. A recent version, polynomial regularization, was developed by Lang. A completely different approach is the Monte Carlo method of Tuncer and Lang. Several sets of both simulated and experimental data are analyzed by the two methods in this paper. Conclusions concerning speed and accuracy are presented.     

Determination of the polarization distribution inelectron-beam-poled PVDF using heat wave and pressure pulse techniques

In the present paper, heat wave (LIMM) and pressure pulse measurements are reported of the polarization and electric field distribution in nonuniformly electron-beam poled PVDF. In order to obtain the spatial distributions from the LIMM spectra, the regularization technique is applied to the deconvolution. In this connection nonlinear grid distributions are discussed. The polarization profiles obtained by the two methods are compared over the whole thickness range, and a satisfactory agreement is found     

Polarization and electric field distribution in thermally poledPVDF and FEP

In the present paper, heat wave (LIMM) and pressure step (PPS) measurements of the spatial polarization and electric field distributions in nonuniformly thermally poled α-PVDF and Teflon FEP films are reported. The spatial distributions obtained by the two methods are compared over the entire thickness range, and a satisfactory agreement is found. For PVDF the results of both methods are in accordance with literature data for similarly poled specimens of similar material, the so-called `thermal profile', a polarization peak near the anode. Furthermore, in the LIMM experiments we found a small peak near the cathode, which can be explained by a positive compensation charge layer extending ≈2 μm in depth. For the first time, LIMM spectra of FEP are published. The space charge distribution in the FEP sample is nearly homogeneous inside the sample. Near the anode side an accumulation of negative charges appear. Near both surfaces a positive compensation charge was found with a thickness of ~1 μm. It is demonstrated that by means of our deconvolution the spatial distribution can be determined simultaneously with two thermal parameters, the diffusivity of the sample material and the heat transfer coefficient between sample and sample holder     

An analysis of the integral equation of the surface laser intensitymodulation method using the constrained regularization method

The SLIMM (surface laser intensity modulation method) is used for the determination of polarization distributions near the surfaces of ferroelectric samples. The experimental data measured are pyroelectric currents as functions of frequency. The data analysis requires the solution of a Fredholm integral equation of the 1st kind. This is an ill-posed problem which has multiple and very different solutions. The constrained regularization method can be used to solve this problem. However, the choice of the proper regularization parameter is normally somewhat arbitrary and this can affect the solution drastically. In the technique used here, an optimal regularization parameter is found using a set of simulated data. These data are based on a polarization distribution constructed from a simple frequency-to-space transformation. It is shown that the conventional minimization of the deviations between experimental data and the predicted values according to the principles of least-squares gives meaningless results in an ill-posed problem of this type. Experimental data on lanthanum-doped lead-zirconate-titanate ceramics and LiNbO₃ are analyzed     

Current practice in space charge and polarization profile measurements using thermal techniques

Thermal techniques for probing space charge and electric field distributions in dielectric materials became available approximately 30 years ago. The techniques have reached maturity and they have been employed not only for the primary purpose of electric field or polarization profiling, but also in a wide range of problems posed by materials research. The present survey provides an overview of the historical development, the experimental implementation of the different techniques, the theoretical foundation, methods for the data analysis and a comparison of thermal and acoustic techniques. The thermal wave technique LIMM is used as an example among the thermal techniques, for a discussion of data analysis techniques and for the spatial resolution that can be achieved with thermal wave techniques. A tour d'horizon is provided through recent applications of thermal techniques, in order to demonstrate their capabilities for dielectric material characterisation.     

High Frequency LIMM - A Powerful Tool for Ferroelectric Thin Film Characterization

In this work, the laser intensity modulation method (LIMM) is applied to the investigation of sputtered self-polarized PZT thin films. A previous analytic solution of the LIMM Fredholm integral equation of the first kind by use of the Mellin transform is generalized and limitations of this approach are discussed. The numerically reconstruction of the pyroelectric coefficient profile is based on a eight-layer thermal model. The profile reconstruction was performed using MATLAB software containing algorithms for the inverse solution of the appropriate Fredholm integral equation and a Tikhonov regularization method for stable numerical solutions. Optimized algorithms for thermal parameter determination from the low frequency part of the pyroelectric current spectrum are presented. The impact of thermal parameters on the reconstructed profile was investigated. Monte-Carlo simulations were used for a comparison of different approaches for the regularization parameter estimation.     

Dielectric properties and spatial distribution of polarization inpolyethylene aged under ac voltage in a humid atmosphere

This paper reports the results of an investigation of the dielectric properties, growth of water tree density and spatial distribution of polarization in samples of non-stabilized LDPE. Samples were initially sand-blasted to accelerate initiation of water trees, and subsequently aged in salt solution for approximately three weeks under two regimes of temperature (room temperature and 65°C). Aging both with and without the presence of electric field stress was studied. The capacitance and dielectric loss of each sample were measured using a HV capacitance bridge, and compared to theoretical models over the frequency range 10^-5 to 10⁴ Hz. The results indicate a real movement of charges/ions in the polymer bulk which causes a deterioration of the electrical insulation properties even in the absence of AC fields. The presence of a field further degrades the insulator. Samples were studied using the LIMM (laser-intensity-modulation method) to determine the distribution of polarization, assuming mean polarization is zero and space charge is absent. In the absence of the external field there is a presence of ionic charges of opposite polarity, which increases with increasing aging time and is highest at room temperature. The length of the twenty longest vented water trees was measured after various aging times, and it was found that trees grown at room temperature are longer than those grown at 65°C. Also, the tree density was higher in specimens tested at room temperature     

基于自适应极化滤波的地下浅层P/S波解耦方法

针对地下爆炸近场横纵波场混叠,造成P波震动场难以重建的问题,提出基于自适应极化滤波的横纵波分离解耦方法。首先,通过分数阶Hilbert变换获取波场中P/S波瞬时极化分布,提高混叠特征信息精细化分析的能力。其次,利用ACM提取出P/S波瞬时极化角度信息,并以极化度实现P波、P/S波识别;再次,设计利用角度信息构建空间极化滤波器。最后,进行仿真验证并讨论分析不同调制因子和不同特性组合的分离解耦效果。通过实验仿真结果表明：本方法能够有效实现波场分离解耦,在方向性调制因子为10时,振幅性调制因子为3时,分离准确度达到96.5%,分离解耦效果最佳。     

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。     

变压器油道内冲流电流的一种计算方法

对紊流态下变压器油道内冲流电流的计算进行了研究。描述油内电荷分布的输运方程是一个非线性方程,不存在精确的解析解。应用文中所给定的绝缘油/纸板界面的边界条件并经过合理的简化,推出了该方程的近似解析解和冲流电流的计算公式。公式能描述油流速度、电导率、油温和外加交流电场对变压器油流带电的影响,所引入的修正量可通过实验数据确定。实验证明,计算公式可以用于预测冲流电流的大小,从而避免了繁重的数值计算,为变压器油流带电的计算提供了一种方便的手段。     

基于虚拟阻抗的低压配电网拓扑识别方法

当前拓扑识别技术难以反映潮流特性对拓扑识别的影响,基于配电网现有量测数据,通过分析节点间的电气距离,提出了虚拟阻抗的概念。将节点间具备电气意义的且与电气距离成正相关的连续变量定义为虚拟阻抗,并提出了一种基于虚拟阻抗的低压配电网拓扑识别方法。首先,构建以节点间虚拟阻抗为因变量的多元线性回归方程。然后,通过岭回归计算每一个单相电表与关口电表构成的回归方程的虚拟阻抗,根据计算结果快速判别出拓扑关系异常的电气设备。最后,建立基于导数动态时间弯曲(derivative dynamic time warping, DDTW)距离的校验模型,重新构建得到电气设备的正确拓扑关系,实现低压配电网拓扑关系的修正。以实际的低压配电网台区样本数据为依据,验证了所提方法的有效性。     

A detection method of differences of polarization angles between three‐component signals

I describe a new method of detecting differences of polarization angles, azimuth and elevation angle, between two three‐component signals. The differences of polarization angles are calculated directly from nine cross‐correlations between two three‐component signals. I compare the performance of this method to a conventional method based on a polarization filter, which uses Principal Component Analysis, in a computer simulation. When data length is shorter than a period of a sinusoidal signal, which is used as a signal in the simulation, the new method shows both less dispersion and less bias of errors than the polarization filter does. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 141(1): 45–53, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10044     

Laser Doppler velocimetry with polarization-bistable VCSELs

We present a rate equation model of a single mode vertical-cavity surface-emitting laser (VCSEL) with two states of linear polarization, subject to Doppler-shifted optical feedback. Recent experimental work has shown that the polarization bistability often observed in VCSELs can be exploited to enhance the responsivity of these semiconductor lasers in speed-sensing applications. We review these experimental results briefly and use the rate equation model to explain the increased signal-to-noise ratio.     

Simulation of corona in wire-duct electrostatic precipitator bymeans of the boundary element method

A new approach for modelling the space density and the electric field distribution in electrostatic precipitators is presented in this paper. Two numerical techniques are used iteratively until a self-consistent solution is obtained: the boundary element method, for evaluation of the electric potential; and the characteristic method, determining the charge distribution. This hybrid approach is applied to simulate the electrical conditions in a wire-duct electrostatic precipitator and obtained results are compared with the experimental data     

基于电势法求解稀土永磁电机的同步电抗

本文运用有限元方法分析一台800W稀土永磁同步电动机内部电磁场的分布情况,并在传统的计算电枢反应电抗的基础上提出了一种基于电势平衡方程式的计算方法,本文称之为"电势法".文章还运用直接负载法测定出样机在不同负载下的同步电抗参数,验证了"电势法"的正确性.     

Space charge profile measurement techniques: recent advances and future directions

Some of the advances in space charge measurement techniques over the last 5 years are described. It is concluded that significant further improvements in spatial resolution are unlikely in the next 5 years, except in the case of the LIMM technique implemented using very short laser pulses, where spatial resolution approaching 100 nm might be achievable. It is suggested that existing space charge techniques be combined with conducting atomic force microscopy (CAFM), with the aim of correlating 3-dimensional space charge profiles and 2-dimensional conductivity maps and thus providing a more complete picture of charge transport through dielectrics, particularly in future work on nanodielectrics. Since CAFM samples are typically 5-10 nm thick, it would be necessary to operate the equipment at higher voltage (say 1000 V) so that samples not less than 10 /spl mu/m thick, e.g. polymers, could be investigated. This seems feasible.     

Analytical modeling of subthreshold swing in undoped trigate SOI MOSFETs

A new analytical model for the subthreshold swing of nanoscale undoped trigate silicon-on-insulator metal–oxide–semiconductor field-effect transistors (MOSFETs) is proposed, based on the channel potential distribution and physical conduction path concept. An analytical model for the potential distribution is obtained by solving the three-dimensional (3-D) Poisson’s equation, assuming a parabolic potential distribution between the lateral gates. In addition, mobile charges are considered in the model. The proposed analytical model is investigated and compared with results obtained from 3-D simulations using the ATLAS device simulator and experimental data. It is demonstrated that the analytical model predicts the subthreshold swing with good accuracy for different lengthes, thicknesses, and widths of channel.     

Novel fault‐location method for overhead electrical distribution systems

The implementation of a fault‐location methodology in an electrical utility is dependent on the accuracy and ability of the method in locating all fault types. However, meeting these requirements is not an easy task. This paper presents a novel fault‐location algorithm based on a new formulation for faults in overhead distribution systems. A single fault‐location equation has been derived to provide a unified solution for any fault type. This is in contrast with other methods that use one equation for each fault type. The proposed method does not require the identification of the fault type; only the fundamental values of the voltages and currents that are measured at the substation terminal are required to solve the proposed fault‐location equation. An overhead 11‐kV, 21‐bus, real‐life radial distribution feeder is simulated to evaluate and test the viability of the proposed method using the PSCAD/EMTDC software. The results indicate that the proposed method gives high accuracy and can be implemented with the available technology. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

An express method of measurements of parameters of technological electron beams

An express method of direct measurements of the current density distribution of axially symmetric electron beams is proposed. It is shown that use of the technique is urgently needed to improve the reproducibility of the results of electron-beam processing. Comparative analysis of existing methods to measure the parameters of electron beams used for technological purposes is carried out. A technique to restore a radial distribution of electron beam current density as well as a device to implement the technique is developed. The results of radial distribution recovery are analyzed in the case in which the initial distribution is given in the form of a model function of normal distribution. An algorithm to determine estimated sizes of the area within which spatial distribution discretization is carried out is developed. Based on analysis of the experimental dependences, it is shown that the suggested numerical method can be applied to all types of axial-symmetric distributions. The nature of errors occurring after distribution function recovery is analyzed. A technological projection of express diagnostic systems for the processing electron beams is made taking into account parameters of the existing element base.