抽水蓄能电站输水系统水力振荡可能性分析

抽水蓄能电站输水系统布置复杂,机组工况多且转换频繁,因水泵水轮机不稳定流动导致输水系统振荡甚至产生共振的可能性较大。本研究旨在分析水泵水轮机各种不稳定因素与输水系统发生共振的可能性。首先总结了水泵水轮机尾水管涡、旋转失速等不稳定流动的发生工况和频率范围;其次以某抽水蓄能电站为例,采用传递矩阵法计算输水系统自振频率特性,发现尾水管涡和旋转失速频率与系统频率重叠,有引起共振可能性;进一步以尾水管涡扰动作为振源对输水系统进行强迫振荡分析,得出系统沿线的压力振荡幅值分布。研究表明,尾水涡带和旋转失速最可能导致系统水力共振,某些电站出力波动可能与此有关;S区振荡也可能引起强烈的水力振荡,造成机组过渡过程事故;动静干涉、卡门涡是高频振动,主要影响水轮机压力脉动,可导致转轮结构共振和破坏,与输水系统共振的可能性小。     

配电网超低频振荡的仿真计算研究

通过数学分析得出系统等值对地电容较大时 ,接地故障消失激发的超低频振荡会产生危及系统安全的过电流 ;EMTP仿真计算表明超低频振荡最大过电压 <2 .0p .u .,而其过电流最大倍数却可高达 5 2 8p .u .;通过分析提出了TV高压侧中性点串联电阻、系统装设消弧线圈等 ,抑制超低频振荡过电流的方法     

Key factors affecting voltage oscillations of distribution networks with distributed generation and induction motor loads

The existence of critical voltage modes in distribution systems and the key factors responsible for them are presented in this paper. The analysis is carried out over a distribution test system for different types of induction motors. Eigenvalues and participation factors are used to investigate the modal interaction within the system. This study shows that load voltage dynamics significantly influence damping of the voltage modes. These modes have frequencies of oscillations between the electromechanical and subsynchronous oscillations of power systems. Significant parameters of the system which affect the damping and frequencies of the oscillations are identified to provide a clear understanding of the problem. A bifurcation study is performed to determine the dynamic loadability limit of the system. Time-domain simulations are carried out to verify the validity of the modal analysis and provide a physical feel for the types of oscillations that occur in distribution systems. The impact of various distribution network configurations on these modes is also demonstrated through nonlinear simulations.     

锅炉燃烧系统的声学固有频率分析

为更有效地排除锅炉燃烧系统中的声耦合燃烧振荡问题,理论分析了锅炉简化模型的声学固有频率,同时结合实例研究了较复杂系统的耦合模态,指出锅炉燃烧设备声学固有频率的复杂性,总结了计算锅炉固有频率和模态的一般方法。     

On the resonance effects due to ground wires in transmission lineswith non-uniform soil conductivity and non-uniform towerresistances

High-frequency resonance effects due to shield wire grounding may affect carrier transmission performance in the vicinity of certain critical frequencies. The authors investigate whether nonuniformities in soil conductivity and in tower footing resistances along the power line may lead to the suppression of such resonance effects. Simulation results show that fluctuations in soil conductivity and in lower footing resistance along the line do not by themselves destroy resonance effects     

Evaluation of torsional oscillations in paper machine sections

Torsional oscillations greatly affect performance and determine the bandwidth (BW) and damping of speed loops. Backlash due to gear reducers can also contribute to the triggering of oscillations, especially when the drive runs at very low load torque. This paper presents a detailed evaluation of these effects in typical electromechanical drive trains applied to paper machine sections. The cases evaluated consider torsional oscillations in two-mass and three-mass systems, and the effect of shaft diameter and length on the resonant frequencies of three typical paper machine sections. Time-domain response plots are evaluated to show the effect of speed response overshoot, reducer backlash, and step or ramp speed commands. Based on these results, mechanical design guidelines are given for the most significant drive train components in order to minimize torsional oscillations of the speed-controlled drive system.     

Synchronous weld pool oscillation for monitoring and control

A novel approach for inducing and monitoring oscillations in a molten weld pool is presented. Recent research efforts have illustrated that the weld pool resonates at natural frequencies that are related to its dimensions and state of penetration. This phenomenon may be used to monitor the weld pool, particularly its depth of penetration, in a closed-loop feedback control system. The approach used to induce pool oscillations was to excite the weld pool with current pulses synchronized to the natural oscillations of the pool. Implementation of this synchronous weld pool pulsing technique was based on the use of a phase-locked loop (PLL) system. The natural weld pool oscillations are used as the reference frequency source and a pulsing circuit is controlled by the PLL oscillator, so that the arc current pulses repeatedly impact the pool after a fixed number of reference oscillation periods. An optical sensor detects the pool oscillations which are amplified, filtered, and limited to eliminate amplitude variations from the optical signal. A model of the weld pool is developed which uses a fluid droplet formulation for the relation of weld pool geometry and other physical parameters to the natural frequencies of the weld pool. Comparison of the weld pool's actual resonant frequency with the expected resonant frequency as predicted by weld pool geometry models and measurements of the pool width (or area) allows assessment of the state of penetration of the weld pool into the workpiece     

Dependence of the reactivity compensation degree of a synchronous compensator on its parameters

The problems of asynchronous self-excitation of a synchronous machine with two windings on the rotor and a series capacitance in the circuit have been considered. A formula for calculating free oscillations has been given. It has been shown that, for a system consisting of a symmetric synchronous machine and a series capacitance, there are two natural frequencies, each corresponding to a certain zone of self-excitation. The equation of the curve that describes the region of self-excitation has been obtained. The effects of different parameters of machine on the lower part of the self-excitation zone have been investigated. It has been shown that this part of the zone is determined by the ratio between the active resistances of the machine circuits, but not their absolute values. A method to determine parameters providing the required compensation of the machine reactance has been given.     

Torsional Interactions between Synchronous Generators and Long Transmission Lines

The dangers of subsynchronous resonance in series capacitor compensated transmission lines have led some utilities to consider the alternative of shunt capacitive compensation, since the electrical resonant frequencies in this case are usually considered to be supersynchronous. However, in heavily compensated or very long lines, at least one of the resonant frequencies may become subsynchronous. This paper examines the torsional interactions between electrical resonant frequencies (which can be supersynchronous or subsynchronous) and the mechanical oscillations of a generator connected to a long transmission line. Examples are given and the electrical damping is calculated in order to evaluate the risk of self-excitation occurring.     

一种基于固有频率的超高压线路相间保护方案

在分析影响固有频率因素的基础上,提出了一种利用固有频率的超高压输电线路相间故障保护方案。理论分析表明,故障后产生的各种频率信号中,固有频率信号是明确的,且信号较强,便于采集。该方案利用线路两侧的固有频率主频差和阻抗判据区分区内、外相间故障,可以有效提高区内故障时保护的动作速度,同时保证区外故障时保护不误动。该方案不受系统振荡、负荷电流、故障时刻、过渡电阻、故障点位置和系统运行方式的影响,受电流互感器饱和的影响较小。ATP仿真和MATLAB分析验证了该方案的有效性。     

Generation of Picosecond Pulse Train With Alternate Carrier Frequencies Using Dispersion Oscillating Fiber

The train of picosecond pulse pairs with alternate carrier frequencies was observed when the train of picosecond pulses was launched into the fiber with sine-wave variation of the core diameter. The effect occurs due to dispersion oscillations along the fiber length, which result in splitting high-order solitons. In the experiments, the splitting of second-order soliton was observed. The separation between carrier frequencies of output pulses depends on the phase of periodical oscillation of the fiber dispersion. Numerical simulations were made and show good agreement with the experimental results.     

低压输水单元水力振荡的数值研究

在长距离分段低压输水系统中,为防止过大的流量波动,系统中不能产生过大的水力振荡,尤其不能发生水力共振,因此必须对其振荡特性进行分析,以保证系统的正常稳定运行。本文采用基于压力管道内水体水力振荡的线弹性模型描述输水管道中水流的控制方程,利用状态方程来描述输水单元。基于系统控制理论,分析了采用不同阶数的线弹性模型时输水单元的频率特性。同时,分析了三阶线弹性模型的特征值与输水单元参数的相关性,以及各特征频率对输水单元的影响。其结论可为输水系统的设计和优化提供依据。     

核电主泵电机推力轴承平衡块支撑的运动稳定性

虽然相互"搭接"的推力轴承平衡块能够较好地平衡作用在各推力瓦乊间的轴承载荷,但若平衡块的设计参数不当,可能会出现平衡块支撑的不稳定运动现象,导致推力轴承功能失效。根据平衡块摆动与轴承载荷的关系,建立了平衡块支撑运动的非线性微分方程,利用多尺度方法给出了平衡块支撑的运动稳定性及其边界条件,它与轴承载荷中动态分量的激励频率和静态分量的共振频率密切相关。     

Predicting osseous changes in ankle fractures

A quantitative assay of subtle morphologic changes that are induced in the trabecular structure of weight-bearing bones as a result of immobilization prescribed as part of routine clinical management is investigated. It is postulated that a clinically meaningful structural measure can be inferred from comparatively small, relatively uniform regions of trabecular bone, which are easily isolated from conventional transmission radiographs. This approach derives from the observation that common transmission radiographs of alveolar bone contain trabecular regions with spatial attributes that are statistically independent of image magnification over a reasonable range of spatial frequencies. Selecting these regions for analysis is advantageous because certain attributes of the resulting data are theoretically independent of the focal-object distance used to produce the original radiograph. Therefore, the size and shape of the irradiated tissue mass do not influence the measurement, as they do in morphometric-based analysis     

PSS2A低频段振荡抑制效果验证方法

随着区域电网逐渐互联,系统发生在1 Hz以下的低频振荡所占比例越来越高,并且发生了发电机励磁系统投入电力系统稳定器（PSS）后在一些运行方式下产生负阻尼的现象。针对目前常见PSS试验方法不能对低频段振荡的抑制效果进行验证,提出了在机端电压上叠加正弦扰动信号的试验方法。现场试验验证表明,该试验方法合理有效。     

Hippocampal gamma oscillations in rats

Previous studies suggested that γ oscillations in the brain are associated with higher order cognitive functions, including selective visual attention, motor task planning, sensory perception, working memory, and dreaming rapid eye movement (REM) sleep [1]--[7]. These oscillations are mainly observed in the cortical regions and also occur in neocortical and subcortical areas and hippocampus. These oscillations may occur under certain pathological conditions, such as epilepsy, and are mainly observed in the cortical regions and hippocampus. The previous studies have suggested that epilepsy may be associated with disturbances of autonomic nervous system(ANS) andwith changes in autonomic cardioregulatory function [8]--[13]. In this article, we investigate the influence of acute exposure to 2-aminoethoxy-diphenylborate (2-APB), a membrane-permeable inositol 1,4,5-trisphosphate (IP₃) receptor, and store-operated Ca²⁺ channel (SOC) blocker on the complexity of hippocampal γ oscillations. Our central hypothesis is that acute exposure to 2-APB significantly reduces the hippocampal γ oscillations. To test this hypothesis, we use brain-slice recordings and the advanced nonlinear dynamical analysis method based on the Lempel-Ziv (LZ) estimator. Our nonlinear dynamical analysis results estimated from brain-slice recordings suggested that 2-APB exposure significantly reduces the hippocampal γ oscillations.     

An accurate method for estimation of series and parallel resistance in detailed model of transformer winding using impulse test results

In this paper, a novel and accurate method is presented for evaluating the series and parallel resistances for detailed modeling of power transformers. In the proposed method, the black box model of power transformers as well as the natural frequencies of its winding are estimated by the impulse test. Then the series and parallel resistances can be evaluated for most upper and lower natural frequencies with reasonable accuracy to be implemented in the detailed model. The exact estimation of these resistances is important for the damping determination of transformer winding oscillations. The proposed model enables the designers to correct the design data sheets before mass production by performing the impulse test on the prototype. In the present study, a comparison between the simulation and test results verifies the validity of the proposed method.     

Synchronous and asynchronous operation in dual‐band VCOs

We present a comprehensive analysis of the asynchronous and synchronous operations of fourth‐order oscillators underlying dual‐band voltage‐controlled oscillators. We analyze the occurrence of the self‐synchronization phenomenon (internal resonance) if the ratio of normal frequencies is nearly a ratio of integers, which is 1:3 in the cubic approximation of the nonlinear oscillator characteristic. In this case, we have the simultaneous presence of 2 oscillations with a frequency ratio 1:3, which was demonstrated to be very effective in generating high‐frequency signals in mm‐wave range. The analysis is carried out by developing an analytical approach relying on the averaging principle, as it follows the van der Pol method. The averaging equations, derived simply by a quadrature, allow us to analyze easily the stationary and transient oscillations, and their stability, both in asynchronous and synchronous operations. Expressions for the amplitudes and phases were derived for a cubic nonlinearity and verified by Spice simulations.