Voltage oscillatory instability caused by induction motor loads

Isolated power systems which fit the structure of a single equivalent generator supplying an equivalent composite load can experience a voltage oscillatory instability entirely traceable to the effects of induction motor loads interacting with automatic voltage regulators. This scenario is one where an AVR properly tuned for open circuit conditions is not necessarily well tuned for the on load condition. The phenomenon is explained and a simple solution through AVR compensation is suggested     

Examination of vibration control of linear oscillatory actuator

Linear oscillatory actuators (LOAs) are widely used in linear motor shavers, electric toothbrushes, and mobile phone vibrators as high‐speed oscillatory driving devices. The maximum amplitude of the motor is obtained at a resonant frequency that is determined from the spring constant and the mass of the motor. However, there is a problem with these actuators: the amplitude decreases sharply when the driving frequency differs from the resonant frequency in a mechanical vibration system with minimal damping. In this paper, we propose a control method for the resonant frequency in which the thrust, which shows the characteristics of a spring, is generated on the basis of the displacement of the mover and the damping factor, in which the thrust, which shows the characteristics of damping, is generated based on the velocity of the mover. The proposed method assures that the resonant frequency always corresponds to the driving frequency and that rapid changes in the amplitude are avoided. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(4): 55–62, 2012; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21172     

Assessment of oscillatory stability constrained available transfer capability

This paper utilizes a bifurcation approach to compute oscillatory stability constrained available transfer capability (ATC) in an electricity market having bilateral as well as multilateral transactions. Oscillatory instability in non-linear systems can be related to Hopf bifurcation. At the Hopf bifurcation, one pair of the critical eigenvalues of the system Jacobian reaches imaginary axis. A new optimization formulation, including Hopf bifurcation conditions, has been developed in this paper to obtain the dynamic ATC. An oscillatory stability based contingency screening index, which takes into account the impact of transactions on severity of contingency, has been utilized to identify critical contingencies to be considered in determining ATC. The proposed method has been applied for dynamic ATC determination on a 39-bus New England system and a practical 75-bus Indian system considering composite static load as well as dynamic load models.     

SVC控制引起的电压振荡失稳研究

在发生电压崩溃的单机-动态负荷系统中，采用传统SVC控制器（静止无功补偿器）提高其电压稳定性：研究表明SVC控制可以延迟系统鞍结分岔点，大大提高系统电压稳定性。但新状态变量的引入改变了系统特性，使得系统在发生鞍结分岔之前，先经历一个Hopf分岔。时域仿真表明，此Hopf分岔诱导系统发生振荡型电压失稳，系统的负荷极限由Hopf分岔参数决定。因此，在使用FACTS控制器提高系统电压稳定性时，要详细考虑其对系统中各种分岔的影响，综合优化控制器的设计和安装。     

Selective modal analysis of power system oscillatory instability

The dimensionality problems posed by the analysis of the oscillatory stability of large power systems remains an open issue. An extension to the original selective modal analysis (SMA) approach that has been inspired by the oscillatory stability problem is presented. SMA is a physically motivated framework for understanding, simplifying and analyzing complicated linear time-invariant models of dynamic systems. The proposed method avoids dealing with the large system matrix and shows significant reductions in storage and computation requirements as compared to straight eigenanalysis. It can also exploit a priori knowledge on the potential instability case to be studied. This reduced-order eigenanalysis approach has been checked for examples of realistic size and can be used as a basis for an efficient production-grade code for analysis of oscillatory stability in large power systems     

动磁式横向磁通直线振荡电机建模分析

针对目前已有动磁式直线振荡电机结构复杂、漏磁较大等问题,提出一种动磁式横向磁通直线振荡电机,其具有动子质量轻,加工难度小,力特性好等优点,适合应用于冰箱压缩机等领域.基于建立的电机等效磁路模型推导出了电磁推力表达式,阐述了该电机电磁推力产生的机理.解析公式显示,电磁力系数与电机结构参数及永磁体材料属性等有关,而与动子位...     

Phase compensated gas turbine governor for damping oscillatory modes

With market deregulation, there is constant pressure to utilise existing assets in more effective ways in order to achieve high levels of performance and as governor technologies mature the ability of governors to achieve much more than the standard power–frequency regulation function increases. Thus, this paper has focused on a more active use of governor control for a gas turbine to provide improved system stabilisation and performance via the inclusion of phase compensation in the governor control loop. Due to the decoupled nature of the mechanical power and excitation control loops, performance improvement via governor control does not interfere with generator voltage regulation, which is a drawback of conventional generator damping provision via a power system stabiliser (PSS). In addition, the mechanical power control loop is also less affected by the operating condition of the power system and is hence more robust. It is shown that inclusion of appropriate phase compensation in the governor control loop can improve dynamic and transient stability, either alone or in conjunction with a PSS in the exciter control loop, without adversely interfering with voltage control or changing steady state power–frequency regulation.     

PWM整流器虚拟电网磁链振荡性能改善

针对PWM整流器虚拟磁链定向矢量控制中存在因积分初始值选取不当造成的直流偏移和磁链振荡问题,提出了一种磁链改进控制策略.以分析虚拟磁链振荡产生的因素为基础,建立了虚拟磁链振荡模型,并通过对系统有功、无功电流特性的研究,提出了将虚拟磁链与PWM整流器控制结合起来综合考虑,利用虚拟磁链微分负反馈和有功电流共同作用来抑制虚拟磁链振荡的控制策略.理论分析表明该方法能有效抑制虚拟磁链、直流母线电压的振荡,仿真和实验验证了所提控制策略的正确性和可行性.     

Static thrust analysis of a cylindrical moving-core linear oscillatory actuator

This paper considers materials for the movers, the volume ratio of the stator and of the mover, and the structure of the mover in a cylindrical moving iron-type linear oscillatory actuator (LOA) having a permanent magnet. As a result, we obtain the following:

• (1) the static thrust characteristics are evaluated on two movers: movers P and S are made of a permendur and a stainless steel, respectively. It is clarified that the static thrust of the mover P is 30 percent larger than that of the mover S;
• (2) it is suggested that the structure of the mover P should be improved to eliminate a “lump” in the static thrust-displacement curve due to the step on the mover, and which disturbs the smooth motion of the LOA.

     

电源滤波器抑制阻尼振荡波的试验研究

针对变电站一次回路开关操作产生的暂态干扰,分析电源滤波器的插入损耗和特征阻抗,进行电源滤波器抑制阻尼振荡波抗扰度试验,研究了电源滤波器在抑制开关操作引起的电磁干扰方面的作用,为在变电站电磁环境内选用电源滤波器提供了参考。     

变压器振荡型冲击电压下的局部放电测量

为了提高发现油浸式变压器潜伏性绝缘缺陷的能力,研究振荡型冲击电压下典型油纸绝缘缺陷的放电特性,掌握振荡型冲击电压下局部放电的特点和规律,提出了振荡型冲击电压试验同时进行局部放电检测的方法,开发了基于振荡型冲击电压的变压器局部放电检测系统,解决了局部放电脉冲的完整测量问题,初步实现了对局部放电的辨识和绝缘状态诊断。应用结果表明:该系统对于发现油浸式变压器绝缘缺陷是有效的。     

Slow-coherency based composite mode oscillatory stabilization bymeans of a hybrid PSS

The authors address the problem of the stabilization of composite mode electromechanical oscillations of electric generators. A hybrid PSS (power system stabilizer) with a two-level structure composed of a local PSS and a global PSS is proposed. A weighted combination of local and global control signals is used as the supplementary signal for the improvement of the damping characteristics of both local and interarea modes. Time-scale decomposition is applied to the decoupling of the local and interarea modes. Determination of the global control law is based upon a slow-coherency aggregated model     

A mathematical model of a three-axis electromechanical converter of oscillatory energy

The article suggests a design of an electromechanical converter of oscillatory-motion energy with three degrees of freedom of an inductor, the implementation of which will increase the power density and efficiency of electric machines of this class. A mathematical model for a test three-axis electromechanical oscillatory converter reflecting interaction between the mechanical, electromagnetic, and thermal parameters of the proposed technical solutions has been suggested. A simulation model of the three-axis electromechanical energy converter is developed and investigated with the Matlab Simulink software package. The results can be used in designing one-, two-, and three-axis converters of oscillatory motion into electromechanical energy.     

Damping of system oscillatory modes by a phase compensated gas turbine governor

The proliferation of gas turbines in power systems increases the scope for taking advantage of mechanical torque control for improved network damping. This paper describes a phase compensated governor for a gas turbine and explores its potential contributions to system damping in a multimachine context. It is shown that the inclusion of phase compensation in the governor control loop is capable of achieving dynamic stability for the system without the need of a power system stabilizer (PSS) in the generator excitation control loop and without adversely influencing terminal voltage control. In addition, it is demonstrated that a phase compensated governor (PCG) is also capable of significantly improving transient stability and by complementing the effectiveness of a conventional PSS, enables a superior overall contribution to network damping.     

双定子直线振荡电机谐振特性分析

双定子直线振荡电机采用谐振弹簧式设计,电机工作特性与运行频率直接相关.为了使双定子直线振荡电机能够获得较好的工作性能,基于气体力负载等效线性化,对于电机进行了数学建模,从理论上分析了电机运行频率对于其工作特性的影响,并通过实验进行了验证.实验结果表明,直线振荡电机的等效机械谐振频率不仅与动子质量及弹簧刚度有关,还会受负载影响而变化,随排气压力的增大而增大;而电机功率因数则在空载状态下存在两个为1的最大值.当工作频率接近机械谐振频率时,电机达到同样的排气压力所需的电流及输入功率最小,效率最高;但此时功率因数并不为1,其大小与负载及驱动频率有关.为保证电机高效稳定的运行,应根据电机工作特性曲线确定额定负载所对应的机械谐振频率作为电机的额定运行频率.     

基于振荡分量时频特性的次同步振荡早期预警

针对目前次同步振荡辨识方法是在振荡已经较大程度发生时才能辨识出来的缺陷,提出了一种次同步振荡危险分量的早期辨识方法。利用EMD重组滤波的方法提取出在次同步振荡范围内的分量,将其重组后作为分析信号,对信号做希尔伯特黄变换得到各振荡分量的振荡参数和时频变化关系,根据振荡分量频率与特征频率的走势关系和阻尼比,对振荡分量的危险性做出早期判断。用理想信号验证了所提方法对振荡参数辨识的精度,并以哈密地区次同步振荡的实际录波数据验证了此方法能够对次同步振荡做出早期预警。     

Resonant behavior of EHV transformer windings under system originated oscillatory transient over voltages

Grid connected EHV transformers experience various terminal disturbances when in service. The present work attempts to investigate the voltage stresses that may develop on the transformer insulations under a variety of terminal disturbances. A number of standard and non-standard wave shapes like lightning impulse, chopped lightning impulse, steep-front long tail switching surge and oscillatory transient over voltages have been simulated and impressed on the terminals of a 400 kV EHV power transformer operating in the Indian power grid to ascertain how the winding insulations are stressed under these disturbances. Relevant section of the Indian power grid and the transformer has been modeled using Alternative Transient Program (ATP). It has been established that oscillatory system transients can trigger natural resonate frequencies of the transformers causing high voltage stresses on the insulations. Short Time Fourier Transform (STFT) analysis of the oscillatory voltage response of the windings confirmed the presence of resonant frequencies indicating forced resonance. Some remedial measures involving winding design modifications have been suggested in the paper to overcome the problem.     

A linear programming method for the scheduling of pumped-storageunits with oscillatory stability constraints

Scheduling of pumped-storage units at the Taiwan Power Company (TPC) is investigated in this work. In addition to economic considerations, oscillatory stability of the power system as affected by the operation of pumped-storage units is of major concern. To achieve a secure as well as economical dispatching schedule for the pumped-storage units, differential dynamic programming is first employed to reach a preliminary schedule in which total fuel cost of the thermal units over the study period is minimized. Oscillatory stability analysis of the power system under the preliminary schedule is then performed by computing the eigenvalues and eigenvectors for all electromechanical modes. Sensitivities of the eigenvalues for the poorly-damped inter-area mode with respect to individual generator outputs are evaluated. Then a systematic approach based on linear programming is presented to refine the preliminary schedule in order to improve the damping for the inter-area mode. The effectiveness of the proposed approach is demonstrated by short term operational planning of the Taiwan power system     

Fuzzy logic controller for enhancing oscillatory stability of AC/DC interconnected power system

A novel control strategy is developed for High Voltage DC (HVDC) links to improve oscillatory stability of interconnected power systems. An ‘area’ principle is proposed for controller design to damp AC tie power oscillations by increasing the oscillatory ‘deceleration area’. Exploiting fast control capability of HVDC link, a fuzzy-rule is adopted to smooth the power transition for reducing power ‘shock’ to power system. The strategy also incorporates adaptive control techniques to prevent tie-link power ‘chattering’. Simulation results using the proposed control scheme on two typical power systems are presented. The results demonstrate that significant improvements in both oscillatory and transient stability are obtained.