A generalized dielectric polarization evolution equation

In this paper a non-equilibrium statistical-mechanical theory of dielectric relaxation is developed. This approach differs from previous work in that a generalized nonlocal evolution equation for the polarization is constructed. General equations of motion are presented for the polarization, internal energy, and entropy which include effects of memory. These equations can be expressed in terms of reduced-correlation functions, and are valid for non-equilibrium and arbitrary field strengths. Expressions for an effective local field also are developed. The Fourier transform of the evolution equation yields a general compact expression for the Fourier transform of the memory function and a specific form for the susceptibility. The kernel, Fourier transform of the memory function are developed, and relaxation-time functions for special cases. In the limit of a single relaxation time, a Debye response is obtained. In the subsequent special cases exponential and Gaussian forms for the memory functions are assumed. The final special case relates a power-law circuit transfer function to the theory of Dissado and Hill. In this case the memory kernel and relaxation times are derived from the Dissado-Hill response function     

Transforms, Generalized Functions, and All That

Generalized functions, e.g., Dirac delta functions and derivatives of the same, are currently in vogue. In this paper there is developed a practical approach to generalized functions and the many uses thereof in Fourier and Laplace transform theory. A novel concept, the " line of convergence" concept, is central to the approach. By use of it, generalized steady-state analysis (e. g., sinusoidal and exponentially weighted sinusoidal analysis) of linear time-invariant systems is tied with transient analysis to a common methodology in transform theory.     

基于小波变换与傅里叶变换相结合的电能质量监测方法

介绍了至今为止国家标准中规定的各项电能质量指标的计算方法,提出了一种将小波变换与短时傅里叶变换相结合的方法对电能质量的暂态和稳态问题监测的方法,其中小波变换选用Mallat算法,短时傅里叶变换窗函数选用Hanning窗。利用小波变换Mallat算法对暂态电能质量问题进行实时监测,并且区分稳态扰动与暂态扰动,再对各频段进行短时傅里叶变换去分析,从而能够很好的对时域和频域进行分析。MATLAB的仿真结果验证了该方法的准确性和有效性     

小波变换及其在电力系统瞬态分析中的应用

通过对Fourier变换、短时Fourier变换的回顾,介绍小波变换的原理,并给出小波变换作为一种新的分析工具在电力系统瞬态分析中的应用方法。通过分析、比较和示例我们可以清楚地看到小波变换在分析电力系统瞬态时非常实用且非常有效,是传统的Fourier变换无法比拟的。     

小波变换在配电网单相接地故障选线中的应用

小波变换克服了傅里叶分析的缺陷，能同时对时频局部化，具有很强的信号特征提取功能，可将其应用到小电流接地电网单相接地故障检测中。对故障后暂态电气量进行分解，利用小波变换系数作为判据实现故障选线，仿真实验表明该方法可以快速准确地检测出故障线路。     

基于特定频带的短时傅里叶分析

短时傅里叶分析(STFT)和小波分析都具有分析暂态谐波的能力,但两者都存在不足之处,STFT需要有一个合适的窗才能有满意的分析效果,小波分析不能得到单个的谐波频率和幅值。文中通过比较STFT和小波分析在电力系统暂态信号分析中的优缺点,结合两者的优点,提出了基于特定频带的STFT的方法。该方法利用多分辨率分析确定暂态故障发生的时刻和暂态信号的主要频率范围,在此基础上确定STFT的时窗中心和合适的窗宽,可有效地提高STFT的精度和效率,快速、准确地得到暂态信号中’主要谐波频率分量的频率和幅值,通过MATLAB仿真证明了该方法的有效性。     

The damping effect of the flywheel dampers installed at different shaft couplings on torsional oscillations

This paper eamines the damping effect of flywheel dampers installed at different shaft couplings on torsional oscillations on the 951 MW turbine-generator shafts due to network faults. A studied system is described and torsional fatigue on the turbine shafts following various electrical system disturbances is analyzed and evaluated. Moreover, the influence of the safety factor selected and set at the design stage to the shaft life expenditure due to fatigue damage is also presented. Useful additional insight into their modes of operation is obtained from the fast Fourier transform of the transient response results. All computer simulations thus show that the proposed dampers are effective in suppressing the torsional oscillations on the shafts. In this paper, time-domain torque evaluation is also combined with the fast Fourier transform method to derive a selection procedure for obtaining suitable flywheel dampers to depress the amplitude of these critical shaft oscillations to acceptable levels.     

消除暂态过程影响的滤波算法及其在故障测距中的应用

电力系统故障暂态过程中,电压、电流信号包含着衰减直流等暂态分量,这必然对故障暂态过程电气量的获取精度产生影响.通过构建精确求解基频及整次谐波分量的非线性方程以及将其转化为线性方程进行求解,并针对离散化积分引入的误差进行修正,提出一种能完全滤除衰减直流分量的高精度、高稳定的滤波算法.通过对各种典型信号及幅频特性的分析,验证了该算法计算精度高、稳定性好,且对非整次谐波等故障暂态分量具有较强的抗干扰能力和抑制作用,其在故障测距中的应用表明该算法能有效提高故障暂态过程中的测距精度.     

Digital Signal Transmission on Power Line Carrier Channels: An Introduction

The possibility of transmitting digital signals on power line carrier channels is investigated. An accurate model of the multiconductor transmission line and a rigorous propagation algorithm are used. A computer aided procedure is presented to evaluate channel transient response to any input digital signal code. The discrete convolution method uses the inverse Fourier transform of the channel frequency response in amplitude and phase. The eye diagram approach permits evaluation of channel perfomance. The results obtained in an application on a three-phase long line, confirm the feasibility of the proposed method.     

一种提取电力系统谐波的快速算法

正确提取电力信号的谐波分量是提高供电质量的前提，根据电力信号主要分量为周期信号的特点，将离散周期正交小波变换应用于暂态谐波信号分量的提取，形成了快速简捷的分解及重构算法，经算例验证，这种方法在提取暂态谐波分量时较傅里叶变换具有更好的时频分辨率，分析表明，合适的小波基及符合实际情况的代价函数是确定最优分解二叉树的关键，从而能够极大地提高计算速度和分解精度。     

小波分析在汽轮机振动故障诊断中的应用研究

针对常见的汽轮发电机组频率突变的振动故障,采用傅立叶变换和小波变换对同一汽轮发电机组振动信号进行傅立叶变换和小波分解。结果表明傅立叶变换无法检测出信号的瞬态变化,只适用于分析平稳信号;而小波分析可以有效的提取信号的瞬态变化特征,能准确的检测出信号的奇异点位置,利用小波分析有利于提取汽轮发电机组振动信号的故障特征。此外,对汽轮发电机组的混合信号进行小波分解,可以将信号有效的分离,为确定故障种类提供了依据。     

A procedure for tuning automatic controllers with determining a second-order plant model with time delay from two points of a complex frequency response

The problem of obtaining the mathematical model of a plant in the course of adaptively tuning the operating automatic closed-loop control systems is considered. A new method is proposed for calculating the parameters of a model with four free coefficients represented by two inertial sections with a time delay. The model parameters are calculated from the data of experiments on determining two points of a plant??s complex frequency response. The results from checking the performance of the method in combination with obtaining information on the plant dynamics by applying the Fourier transform to the impulse transient response of the system are presented. The PID controller is tuned using a parameter scanning algorithm with directly checking the amplitude-frequency response of the closed-loop system, using which the stability margin can be calculated and different quality criteria can be applied.     

Impulse sampling sequences in time-convolution electromagnetic transient analysis in power systems

The paper refers specifically to the process by which numerical solutions for transmission line step responses and indicial-impedance functions, as formed from inverse Fourier transform evaluations, are used in time-convolution electromagnetic transient analysis in power systems. Previously, curve-fitting and linear-segment matching procedures have been adopted, whereas the present paper develops a form of representation based on sequences of impulse samples. By avoiding curve-fitting altogether, the proposals further advance time-convolution methods in power systems, and improve the procedures required in applying them in practice. Typical solutions in the paper confirm the solution accuracy to which the proposals can lead, together with their computing time requirements.     

基于EMD的Hilbert变换应用于暂态信号分析

将一种新的非平稳信号处理方法--基于经验模态分解(EMD)的希尔伯特(Hilbert)变换方法,应用于电力系统暂态信号分析中。通过EMD方法提取信号的固有模态函数(IMF),再进行Hilbert变换,求瞬时频率、瞬时振幅,得到信号的Hilbert谱,进而得到Hilbert边际谱,对故障暂态和扰动信号进行了分析。通过瞬时频率进行故障暂态和扰动时刻的准确检测;通过Hilbert边际谱与傅里叶幅值谱的比较,表明Hilbert边际谱在分辨率上具有明显的优越性。该方法为电力系统暂态信号分析提供了一种新的分析手段。仿真结果验证了该方法的有效性。     

Periodic functions and the discrete Fourier transform: atime-domain view

The discrete Fourier transform (DFT) is often used to compute the Fourier series coefficients of periodic functions. Most explanations of this process rely on advanced concepts from Fourier transform theory; as an alternative, it is shown by the author that the DFT may be derived solely in the time domain, and that this derivation leads to the well-known relation between sampling rate and aliasing. Also included is an extension of the DFT to multiple dimensions     

A new method for decaying dc offset removal for digital protective relays

Voltage and current signals used in protection devices may contain harmonics and decaying dc offset in transient states. Because of this, protection devices must contain filters to remove the part of the signal which is not of interest and estimate the value of the component or components which are to be used for protection functions.Traditionally, the discrete Fourier transform (DFT) filter has been used in digital protection devices. This filter has the advantage of being easily applied, but the disadvantage that when an exponentially decaying component is present the results obtained are incorrect. Because of this, different algorithms based on the DFT have been developed in order to remove the decaying dc offset and reduce digital processing errors.This paper describes a new method for removing the exponential component associated with electrical signals in transient states. Its ease of application results in a low computational load as compared with other methods. This characteristic, together with the accuracy of results, makes this new method suitable for application in real time on protection devices.     

FAST S-PARAMETER COMPUTATION OF A MICROSTRIP INTERDIGITAL FILTER USING TLM,PRONY'S AND DIGITAL FILTERING TECHNIQUES

A spectral estimation technique based on Prony's method has been used to analyse a five section microstrip interdigital filter with the TLM method. The time response of the filter is expressed as the sum of only five damped sinusoids, yielding an analytical expression for the time response. Furthermore, since an analytical Fourier transform exists for this expression, the frequency response is obtained without applying the fast Fourier or discrete Fourier transform. This typically reduces the time required for a TLM analysis by a factor of five. The approximation agrees very well with a full TLM time response. It is also shown that the numerical noise due to single precision computation of the absorbing boundary algorithm can be eliminated by digital filtering techniques. The effect of the via holes used for grounding the quarter-wave resonators of the filter has been included by discretizing them along with the rest of the circuit. Scattering parameters for the filter compare well with measurements published by Swanson.     

结合短时傅里叶变换和谱峭度的电力系统谐波检测方法

针对电能质量分析中的谐波检测问题,提出一种结合短时傅里叶变换和谱峭度的电力系统谐波检测方法。采用短时傅里叶变换作为时频分析工具对采样信号进行时频分解,同时引入频谱标准差和基于短时傅里叶变换的谱峭度作为辅助分析。通过频谱标准差和谱峭度对谐波模态进行识别,并根据识别结果从频谱矩阵中提取出相应的谐波分量,然后采用基于奇异值分解的扰动定位方法对暂态谐波的起止时刻进行准确定位。仿真实验结果验证了算法的有效性,在低信噪比的情况下仍具有较高的检测精度,具有较好的抗噪性和鲁棒性。     

交流旋转机械的状态监测

在综述了各种交流旋转机械的状态监测方法后,对小波变换新技术进行了介绍,并通过小波变换对暂态非平稳信号的分析,阐明了小波变换在分析暂态信号方面,比传统的Ｆｏｕｒｉｅｒ变换更具优越性。交流旋转机械随着运行状态的劣化,会产生一些暂态信号。因此,利用小波变换来监测交流旋转机械的状态,具有良好的应用前景。