Damping enhancement by robust tuning of SVC controllers in thepresence of load parameters uncertainty

The paper explores the influence of power system loads on static VAr compensator (SVC) damping effectiveness. Equipped with properly tuned additional stabilizers, SVCs have been successfully used for improving the damping of electromechanical oscillations in power systems. The damping improvement is usually achieved through indirect voltage influence through voltage dependent loads. Tuning of an additional compensator stabiliser (ACS) traditionally assumes that loads are statically voltage dependent. However, load parameters are generally uncertain and loads often exhibit same dynamic response. This uncertain dynamic behaviour of loads can introduce an additional phase shift between voltage and load response and such can detune the ACS. Examples of detuning effects of dynamic loads as well as robust tuning of ACS are presented and discussed     

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.     

Analysis and control of the inter-area mode phenomenon using selective One-Machine Infinite Bus dynamic equivalents

New insights into the application of One-Machine Infinite Bus (OMIB) equivalents to the analysis and study of the inter-area mode phenomenon in stressed power systems are provided. First, the notion of a time-varying OMIB equivalent is briefly reviewed and details of the adopted analytical model are introduced. On the basis of this representation, a systematic technique is then proposed to extract the underlying dynamics of the process directly from time-domain transient stability simulations. The analysis tool takes into account both, the dynamics of the slow inter-area oscillations and the dynamics of the fast inter-machine oscillations. Criteria to interpret inter-area oscillations in terms of the kinetic energy transfer between the most disturbed machines, and the rest of the system following the onset of a major contingency are given and methods to implement the proposed approach are discussed. A case study on a 5-area dynamic equivalent model of the Mexican interconnected system in which several static VAR compensators are used to support system voltage is presented to illustrate the developed procedures. Studies are conducted to identify and characterize the energy transfer process that accompanies the inter-area mode separation as well as to evaluate the effect of voltage support on system damping. The results show that time-varying OMIB dynamic equivalents can be used to capture the dominant features of the inter-area mode behavior, which are needed in the analysis and control of system dynamic performance.     

对中长期电压失稳机理的定性探讨

为理解中长期电压失稳现象的本质,采用小扰动分析法在系统特性曲线上展示静态特性不同的具体负荷的静态电压稳定域的确定过程,解释了失稳过程中系统电压稳定于低水平的现象,指出不同负荷在系统特性曲线下半支的稳定情况由负荷的静态特性所决定;在此基础上还讨论了目前两类截然不同的中长期电压失稳现象的机理解释,即无功功率和有功功率不平衡解释,指出这两种机理之间存在着紧密的联系和统一之处。     

Low Frequency Oscillations in Longitudinal Power Systems: Experience with Dynamic Stability of Taiwan Power System

Sustained low frequency oscillations have been observed in Taiwan power system which is of longitudinal structure. It is the purpose of this paper to examine the various factors affecting the damping characteristics of these oscillations which caused dynamic instability problem in the operation of Taiwan power system. It is observed that the amount of power flow on the EHV transmission line and the characteristics of load have a significant effect on the damping of the system while the speed-governing system and the gain of automatic voltage regulator have only a minor one. Detailed investigation using both the frequency domain and time domain approaches also reveals that power system stabilizers can be employed as an effective means for improving dynamic stability of Taiwan power system.     

电力系统稳定器影响频率振荡的机理及阻尼分析方法

频率振荡是有功频率控制过程的小扰动稳定问题。已有研究集中于发电机调速器和原动机环节的分析。电力系统稳定器也可用于抑制频率振荡。分析了发电机励磁系统影响频率振荡的机理,当负荷具有电压调节效应时,则发电机励磁系统通过影响负荷电压进而影响负荷功率,从而对频率振荡产生影响。给出了频率偏差通过电力系统稳定器、励磁、网络、负荷等环节影响电磁功率的过程,利用阻尼转矩法计算电磁功率阻尼系数并分析了电力系统稳定器的影响。提出了多机系统中不同发电机电力系统稳定器对频率振荡阻尼影响大小的评估方法,选择影响大的发电机进行参数优化可更加有效地提高频率振荡阻尼。利用IEEE的4机2区系统对分析结论进行了仿真验证。     

Identification of power system load dynamics using artificialneural networks

Power system loads are important in the planning and operation of an electric power system. Load characteristics can significantly influence the results of synchronous stability and voltage stability studies. This paper presents a methodology for the identification of power system load dynamics using neural networks. Input-output data of a power system dynamic load is used to design a neural network model which comprises delayed inputs and feedback connections. The developed neural network model can predict the future power system dynamic load behavior for arbitrary inputs. In particular, a third-order induction motor load neural network model is developed to verify this methodology. Neural network simulation results are illustrated and compared with the actual induction motor load response     

电压稳定分析中异步电动机动态负荷建模探讨

探讨电压稳定分析中异步电动机(IM)负荷的建模问题。分析一般的动态负荷模型(GDLM)不能捕捉IM动态行为的机理。分别以IM的滑差模型和滑差磁链模型为基础，严格地导出了适用于电压稳定分析的简化一阶(SIM)和详细三阶(DIM)动态负荷模型，同时还说明了SIM和DIM模型实质上是对GDLM模型进行了不同程度的改进。对SIM和DIM模型在暂态电压稳定分析和静态电压稳定分析中的适用性进行分析比较，仿真结果表明：SIM和DIM分别以不同的准确度适用于不同场合的电压稳定分析。     

基于源-荷功率动态匹配的海上平台电力系统静态电压稳定分析

对海上平台电力系统进行静态电压稳定分析是保证其安全性、可靠性的前提.针对海上平台电力系统多平衡节点特征,基于透平发电机组和旋转负荷群的源-荷特性建立计及源-荷功率动态匹配特性的扩展连续潮流模型;针对海上平台电力系统的机组小容量特征和旋转负荷的生产要求,基于机组出力和节点电压的限值确立静态电压稳定分析边界约束;推导扩展连续潮流增广雅可比矩阵,提出基于牛顿-拉夫逊法的静态电压稳定分析方法.对某海上平台电力系统的静态电压稳定分析结果表明,与传统牛顿-拉夫逊法相比,所提方法不仅能找出随负荷增加的电压薄弱节点,且能给出各机组适应负荷增加的出力值.     

含大型风电场的电力系统概率最大输电能力快速计算

针对含大型风电场的电力系统概率最大输电能力(total transfer capability , TTC)展开研究,建立了加入异步风力发电机模型的含参潮流模型,推导了含风电场注入功率项的全注入空间静态电压稳定域边界局部切平面解析式,在此基础上提出了将Monte Carlo仿真和电压稳定域方法相结合的综合考虑风电场风速、负荷、发电机出力和设备故障不确定性因素的概率TTC分层快速计算方法。利用该方法进一步分析了风速概率分布参数对TTC的影响,结果表明,准确获取风电场风速分布参数是准确计算概率TTC的前提。     

基于模糊逻辑的电力系统电压崩溃警报系统

文中建立了一个既考虑静态传输能力极限又考虑动态负荷特性的电压崩溃警报系统,给出不同级别的警报,指出电压崩溃将有可能发生。警报级别越高,电压崩溃发生的可能性越大。系统故障和负荷缓慢增长都可引发电压崩溃,文中针对后者。该系统通过模糊逻辑实现,擅长处理不确定性和非线性问题。考虑了引起电压崩溃的2个主要因素:传输能力极限和负荷动态特性,前者由线路稳定因子LQP表示,后者由dQ/dt和dP/dt指示。经模糊系统得到动态电压稳定指数(DVSI),并划分成不同的警报级别,即电压稳定水平VSL。其可行性和有效性由IEEE 39节点系统验证,动态负荷为电动机。     

Static VAR system auxiliary controllers for improvement of dynamic stability

A study of the improvement in system dynamic stability through effective use of auxiliary control of a static VAR system (SVS) is presented. The efficacy of various controls signals for reactive power modulation of an SVS installed at the midpoint of a long transmission line is investigated with the objective of damping low-frequency oscillations which are critical in limiting power transfer. A new auxiliary signal designated as computed internal frequency (CIF) is proposed which synthesizes the frequency of the generator internal voltage from quantities available at the SVS bus. It is demonstrated that this signal is far superior to other conventional auxiliary signals in that it allows full utilization of the network transmission capacity.     

Design and Implementation of Adaptive Neuro–Fuzzy Inference System Based Control Algorithm for Distribution Static Compensator

This article focuses on the design and implementation of a distribution static compensator using an adaptive neuro–fuzzy inference system based controller. The distribution static compensator is controlled to provide power quality improvement, such as power factor correction, harmonics compensation, load balancing, and voltage regulation. Active and reactive power fundamental components of load currents are extracted using d-q theory. A distribution static compensator is realized using a voltage source converter. Both simulation and experimental results prove the effectiveness of the control algorithm under non-linear loads. The adaptive neuro–fuzzy inference system based controller works satisfactorily for power factor correction and harmonics reduction under balanced as well as unbalanced load conditions. Test results clearly depict the dynamics of the performance of the system under steady state as well as dynamics under load change and load unbalancing.     

Dynamic stability improvement via coordination of static var compensator and power system stabilizer control actions

Power system controllers such as the static var compensators and the power system stabilizers are receiving a wide interest since many technical studies have proven their effects on damping system oscillations and stability enhancement. This paper is mainly concerned with coordinating the control actions of static var compensators and power system stabilizers to achieve improved dynamic performance of the power system using the newly developed concept of ‘domains of influence’. Novel sensitivity-based algorithms are presented for the purpose of effectively identifying the domains of influence of various control parameters on critical dynamic system modes. Applications to the Saudi Consolidated Electric Company (SCECO) power system show that significant improvement in power network stability can be achieved via coordinating the control actions of both static var compensator and power system stabilizer instead of using them individually in an uncoordinated manner.     

Drawbacks of impedance networks

This paper presents the influence of voltage‐fed impedance networks, known as Z‐source and quasi–Z‐source, as well as some more sophisticated networks on the static and dynamic properties of voltage source inverters. The impedance networks increase output voltage distortions with the second harmonic of the fundamental harmonic and decrease the power efficiency. The distortions of the output voltage increase for the discontinuous current mode of the impedance network. The DC voltage boost factor depends on the impedance network power losses. The impedance networks add 2 resonant frequencies that are very close each other to the control transfer function of the inverter in the frequency range that is close to the fundamental frequency. The influence of the impedance network on the control transfer function depends on the effective damping resistance in the impedance network. The impedance network operation during the “shoot‐through” time increases the damping of the inverter output filter. The effective damping resistances and the inductance of the coils depend on the power losses in the magnetic materials. Theoretical models along with experimental verification can help to estimate the real influence of impedance networks on the inverter properties.     

负荷模型动态特性不确定性对低频振荡的影响

对电力负荷的动态特性对低频振荡的影响进行研究。采用由1个感应电动机与1个静态负荷并联而成的综合负荷模型，考虑在建模过程中存在负荷参数辨识误差及负荷特性估计的不准确造成的不确定性，研究动态负荷所占比例和电动机惯性时间常数对电力系统低频振荡频率和阻尼的影响。通过对一个4机2区域系统的典型实例进行研究分析，得出了一些有意义的结论。     

On influence of load modelling for undervoltage load sheddingstudies

This paper explores the influence of load models on decisions of undervoltage load shedding in power systems. A controlled load rejection can be used as an emergency countermeasure to avoid widespread blackout when system voltages are unstable. In this paper, dynamic simulations of a small power system using both static and dynamic load models are presented. When using a static load model, the system includes an explicit model of a transformer with load tap changer. The aim is to demonstrate how different load models influence the analysis and calculation of the amount of load needed to be shed to stabilise the system voltage     

交直流混合系统电压稳定问题综述

采用交直流输电已成为必然趋势,但高压直流输电(HVDC)也给系统带来了不良的负荷特性,使得换流站交流母线的电压稳定问题成为困扰直流输电系统正常运行的一个重要问题,特别在受端系统较弱时更是如此。为了寻找更好的方法来评价交直流系统电压稳定性,对交直流系统静态电压稳定性分析方法进行归纳,评价了它们的优缺点,提出从交直流系统潮流计算方法和电压崩溃点计算方法两方面来改进分析方法。在交直流系统动态电压稳定性分析中,对建模的现状和存在的问题以及动态负荷对电压稳定的影响方面进行了描述,明确了建模要考虑动态负荷,既能描述换流器换相失败的动态特性,又要能用于求解大规模交直流系统。最后从交流系统和直流系统两方面讨论了防止电压失稳的措施。     

负荷特性对江苏电网电压稳定性影响的仿真分析

江苏电网具有电源分布不均、负荷增长迅速、无功储备不足等问题,是典型的易发生电压稳定问题的系统。在仿真中考虑了动态负荷模型和参数的影响,分析了2006年江苏电网负荷增长和负荷中心大扰动后的电压稳定性,并与采用原有数据的计算结果进行了比较。结果表明,现在一些电网电压稳定分析常沿用以往角度稳定分析所用的静态负荷数据的作法存在较大的不足,不能完全满足电压稳定分析和制定应对措施的要求。还就采用静止无功补偿器(SVC)提供动态无功支持,提高江苏系统电压稳定性进行了研究。结果表明,现阶段采用SVC提供江苏电网扰动时负荷的动态无功支持,提高电压稳定性是一项可行的措施。     

Investigation and Improvement of Transient Response of DVR at Medium Voltage Level

An area of interest for dynamic voltage restorer (DVR) research is the damping of transient oscillations initiated at the start and at the recovery instant from a voltage sag. Nonlinear loads, with harmonic currents close to the DVR filter resonance frequency, can also excite the resonance oscillations. To compensate voltage sags and dampen high-frequency oscillations simultaneously, an investigation of the transient response of DVR is first carried out. Possible control schemes and their effects on the oscillation attenuation are also studied. Such studied control schemes include the commonly used single voltage loop control, voltage feedback plus reference feedforward control, and double-loop control with an outer voltage loop and an inner current loop. Subsequently, an effective and simple resonance damping method is proposed by employing a closed-loop control with an embedded two-step Posicast controller. The proposed control methods have been extensively tested on a 10-kV DVR system. It is shown that the proposed damping methods improve both the transient and steady-state performance of the DVR.