Evaluation of control sequence for improving voltage stability in power systems

This paper addresses an investigation of control sequence for improving voltage stability in power systems. The controllers include transformers (step-up, EHV and on-load-tap-changer (OLTC)), generators, and shunt capacitors. Dynamic simulations are performed first for a typical five-bus system covering the above mechanisms: (1) the time constants for the EHV transformer and the OLTC transformer are evaluated first; (2) the generator voltage and the step-up transformer are utilized to evaluate the performance; (3) the effectiveness of the shunt capacitors at load buses and at EHV buses is assessed; (4) the effectiveness of lock of the OLTC is examined; and (5) finally, the system voltage evolutions with different starting times for both EHV/OLTC taps are compared to ensure a proper control action. Based on the above results, a set of 12 dynamic simulations is used to explore the control sequence for effectiveness in improving voltage stability. It is found that the best control sequence is as follows: generator voltages, shunt capacitors at load buss, EHV transformer taps and then OLTC taps.     

基于H-MMC的直驱式永磁同步风力发电系统的运行与控制

提出了一种基于六边形模块化多电平交交变流器(H-MMC)的直驱式永磁同步风力发电系统,可以将三相风力发电机通过一级交交变换后直接并入三相交流电网,具有功率损耗低、变流器电容电压纹波小、无升压变压器并网的优点。详细分析了该系统的功率特性及变流器子模块电容电压波动的情况,提出了基于环流控制的电容电压平衡控制和基于双比例谐振控制器的电流跟踪控制,最终实现了风力发电机最大功率点跟踪下系统的稳定运行。MATLAB/Simulink仿真和RT-LAB硬件在环实验结果验证了基于H-MMC的风电系统具有良好的稳态运行特性和动态响应特性。     

基于双dq变换软件锁相的动态电压恢复器研究

动态电压恢复器是一种新型的电能质量调节装置,它能有效地抑制电网电压波动对敏感负载的影响。针对在电网畸变以及不对称跌落的情况下,动态电压恢复器需要准确地捕获基波电压正序相位。采用了一种双dq变换软件锁相环(DDSRF-PLL),不仅能够快速有效地检测电网电压在对称、不对称、不对称且畸变情况下正序基波电压的相位,为动态电压恢复器提供准确的锁相角,同时也具备较好的动态性能,使动态电压恢复器获得较好的补偿结果。在Matlab/Simulink中建立了带超级电容的动态电压恢复器的仿真模型,仿真结果验证了双dq变换软件锁相在动态电压恢复器的有效性和可行性。     

考虑光热光伏混合电站内外双重电压安全的两阶段无功优化控制

针对实际运行的大规模光热光伏混合电站并网后面临的“内忧外患”电压问题，既受电站内部节点电压状态的影响，也受外部电网系统静态电压稳定状态的影响，提出一种考虑光热光伏混合电站内外双重电压安全的两阶段无功优化控制方法。首先，通过静态电压稳定裕度、节点电压均衡度和无功储备裕度3种指标表征混合电站电压安全状态。然后，结合小时级长时间尺度的光伏出力预测信息，在第I阶段实施有载变压器和电容器慢速调整。针对分钟级短时间尺度内的光伏出力波动，以混合电站内的光伏发电单元、光热发电单元和静止同步补偿器为第II阶段的优化控制对象，实现快速精细化电压调节。所提方法能有效避免全站设备频繁参与优化调整，从而提高控制效率。最后，通过测试系统验证分析结果表明：通过两阶段无功控制协调，所提方法能够抑制光伏出力的波动冲击，有效降低站内电压安全越限和失稳风险。     

An optimal allocation of distributed generation and voltage control devices for voltage regulation considering renewable energy uncertainty

Distributed generation (DG) may result in voltage fluctuation by changing line flow and reactive power injection, especially DG that generates power from renewable energy resources. To cope with this problem, this paper proposes an optimization process to optimally regulate the system voltage profile to lie close to the desired values by using the adaptive Tabu search (ATS) algorithm. The system voltages will be regulated by using dispatchable DG and voltage control devices, i.e. voltage regulator and capacitor. Moreover, probabilistic load flow calculation by using Monte Carlo simulation is chosen to evaluate the uncertainty of DG powered by renewable energy resources. The number of switching operations of the voltage regulator and capacitor are also accounted for in the optimization constraints, as excessive frequent switching operations can damage these devices. The optimal sizes and locations of dispatchable DGs and capacitors are considered as the optimization variables. The proposed method is demonstrated in an IEEE 34‐bus distribution test system and a modified 21‐bus Provincial Electricity Authority (PEA) system (Thailand). © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

治理同步辐射装置电压波动的TSC设计方法

为了减少同步辐射装置负荷冲击特性引起的配电网电压波动,采用以交流过零型固体继电器为开关的晶闸管投切电容器(TSC)无功补偿装置对其进行无功补偿,在借助于M atlab的仿真工具simu link获得负荷侧无功电流幅值后,求得该网络实现无功补偿所需投切的电容量;然后,用以Intel 80C196KB单片机为核心的微机控制系统来控制TSC电容器的投切,最终达到实时补偿网络负载的无功功率的目的。     

Secondary voltage control: analysis, solutions and simulationresults for the Spanish transmission system

Secondary voltage control (SVC) was introduced in some European countries as a means to improve power system voltage security and operation. It is a closed loop system dedicated to keep the voltages of some load buses, called pilot buses, constant. In this paper, a new approach covering all the main aspects of SVC-pilot buses and control generators selection; the control law; and the decentralization problem-is presented. This approach has been applied to the Spanish power system. Some of the results obtained are also presented     

含同步调相机的直流换流站稳态无功协调控制策略

网源稳态调压方式会对系统动态无功支撑能力产生直接影响。针对特高压弱受端电网电压稳定性下降的问题,提出了一种直流换流站内滤波电容欠补偿、同步调相机迟相运行的稳态无功协调控制策略。同步调相机自动电压控制(AVC)子站作为控制中枢,与直流控保系统进行运行信息、动作指令信息和动作报文信息通信,以交直流系统无功交换量为控制目标,通过同步调相机与滤波电容无功置换,达到减少滤波电容投入组数、增加同步调相机稳态无功出力的目的,从源头上减少交流电压跌落导致的直流换流站无功补偿缺额。基于雅中—江西特高压直流投运后的江西电网规划数据,在PSASP仿真平台上开展潮流计算与暂态稳定分析。不同工况下的仿真结果表明,所提协调控制策略可以有效降低弱受端电网电压失稳的风险,提升直流功率的消纳能力。     

连接弱交流系统的高压直流换流站无功补偿协调控制策略

在馈入端交流系统强度较弱、直流传输功率频繁波动的高压直流系统中,换流站无功补偿设备的协调控制对改善换流母线电压的稳定性具有重要意义。提出了一种基于静态同步补偿器(STATCOM)和电容器组相互协调的多模式协调控制策略,主要包括换流母线电压无静差跟踪额定电压的稳态调压模式、换流站参与近区交流系统电压调节的自动电压控制(AVC)模式,以及换流母线发生短路故障时抑制电压波动与换相失败的暂态控制模式。基于实际工程参数,在电磁暂态仿真软件(PSCAD)建立了±800kV酒泉—湖南特高压直流输电系统仿真模型,并在直流功率波动、AVC中心下发指令电压和馈入交流系统短路故障等多种工况下对该多模式控制策略的性能进行了仿真验证。仿真结果论证了所提出的换流站无功补偿设备协调控制策略的合理性和有效性。     

高渗透率逆变型分布式发电对 电力系统暂态稳定和电压稳定的影响

为研究大规模逆变型分布式发电对电力系统暂态和电压稳定性的影响,借助PSCAD/EMTDC仿真软件,以一个12母线测试系统为研究对象,建立电力系统和逆变型DG的动态模型。通过暂态仿真和理论分析,重点研究了不同渗透率DG对主同步发电机最大转速偏差和转速偏离频率的影响,分析了不同控制模式的DG在故障前(稳态)、故障期间和故障后对母线电压的影响。研究结果表明:渗透率越高,逆变型DG对系统暂态稳定性的负作用越大,但电压控制的逆变型DG能明显改善系统电压的稳定性。     

二极管箝位型四电平逆变器的中点电压平衡控制方法

二极管箝位型四电平逆变器具有开关器件少、输出电压高、结构紧凑、功率密度高的优点,在中压变频调速领域有很好的应用潜力。针对其母线中点电压平衡问题,提出一种新型载波交叠PWM调制策略,满足伏秒平衡原则。在此基础上进一步提出了一种可平衡母线中点电压的解耦控制方法,将3个母线电容的电压平衡控制分为2个解耦的控制目标:1)上、下母线电容电压的平衡控制;2)中间母线电容电压平衡控制。首先通过零序电压注入的方式实现上、下母线电容电压的平衡控制,其次通过微调PWM脉冲占空比的方式实现母线中间电容的电压平衡控制。研制了一台二极管箝位型四电平逆变器的小功率实验样机并实验证明了该调制策略以及电压平衡控制方法的正确性和有效性。     

配电网串联电容补偿对电机机端电压调节性能的影响

电力系统配电网中一般有较多的波动性或冲击性电动机负载,造成电压偏低、波动严重,严重影响电能质量和电机的运行性能。针对这一问题,提出采用串联电容就近对电动机进行补偿的方法,计及电机的负载特性,通过对比计算确定了在配电变压器高压侧进行串补对机端电压调节的效果最好。仿真计算了波动性负荷情况下串补装置不同补偿方式对电机机端电压调节性能的影响。结果表明:采用低度过补偿方式,可以在避免自激的情况下,在较大的范围内提升线路末端电机的电压性能,消除由于波动性负荷引起的电压波动,为配电网中改善电动机的运行性能和机端电压质量提供了一条新的思路。     

基于有源箝位的功率器件串联均压技术

功率器件串联的关键技术挑战是如何实现串联器件的动、静态均压。提出一种基于有源箝位的器件串联均压电路及其控制方法。所述均压电路仅由1个辅助开关管和1个箝位电容串联而成,该电路与各主功率管并联。主功率管的关断电压可自动被箝位电容电压箝位,从而将器件串联均压问题转化为各箝位电容电压均衡问题。提出了一种箝位电容均压方法,利用负向电流回收箝位电容电能,并能保证辅助开关管实现零电压开通。所提均压技术具有结构简单、损耗低、模块化等优点。此外,通过调整箝位电容放电时间差异,形成阶梯型桥臂电压,以减小桥臂电压的瞬时变化率dv/dt。搭建一台 3 kV/750 V 30 kW 谐振型直流变压器样机,每个桥臂采用6个SiC MOSFET串联。实验结果表明所提均压方法在各个工况下均能实现串联器件的动、静态均压,不均压度小于3 %,且在3 kV输入电压下峰值效率达98.4 %。     

A voltage source PWM rectifier-inverter with feedforward control of instantaneous power

In general, diode rectifiers with electrolytic capacitors on the dc side have been used as dc power supplies for voltage source inverters. This type of rectifiers, however, causes many problems such as poor power factor and harmonics. Recently, voltage source PWM rectifier-inverters have been studied to provide the following advantages; (1) harmonic-free on both ac sides; (2) unity power factor on the input ac side; (3) power flow of either direction or power regeneration; (4) reduction of the dc capacitor. It is, however, difficult for a conventional one to regulate the capacitor voltage on transient states because of having only a voltage feed-back loop. This paper describes a voltage source PWM rectifier-inverter with feed-forward control of instantaneous power. Based on the pq theory, the instantaneous power which is calculated in the control circuit of the inverter is fed forward to the control circuit of the rectifier. The feed-forward control of instantaneous power makes a great contribution to sufficiently suppress voltage fluctuation of the dc capacitor on transient states. Transient characteristics are discussed and some interesting experimental results of a laboratory model are shown.     

A new approach for voltage harmonic distortion minimization

The problem of harmonic reduction is alternatively approached as an energy user’s problem. The user’s objective is to eliminate his voltage harmonic distortion problem locally without considering the effects of voltage distortion at neighboring buses. The remedy is insertion of a reactor in series with the local compensating capacitor. A method is presented for finding the optimum fixed LC combination to minimize voltage harmonic distortion at a load bus while holding the power factor at a desired value and constraining the compensator values which would create resonant conditions and the manufacturer’s standard values for power shunt capacitors.     

基于电容电压波动的模块化多电平换流器电容电压均衡策略

针对模块化多电平换流器(MMC)的子模块电容电压均衡问题,为了在较低的开关频率下抑制电容电压波动,提出了一种标记排序电容电压均衡策略。标记排序均衡策略能够根据设定的电容电压波动率边界与每个子模块的投切状态和电容电压,将同一桥臂内的子模块标记为两种类型,分别进行排序与投切控制。其中电容电压波动率边界可以根据系统运行的不同条件进行调整。提出了用于评价电容电压均衡控制策略性能的两个通用性指标,分别为器件平均开关频率和桥臂的电容电压波动率。在PSCAD/EMTDC中搭建了一个21电平MMC的测试系统,将标记排序均衡策略的控制性能和已有的两种电容电压均衡策略进行了仿真对比,并计算了不同均衡策略下MMC的损耗分布。仿真结果证明了标记排序均衡策略的有效性。最后,对电容电压波动率边界取不同值时的情况进行了仿真测试,给出了电容电压波动率边界的选取建议。     

A weighted voltage model predictive control method for a virtual synchronous generator with enhanced parameter robustness

To address the problem of insufficient system inertia and improve the power quality of grid-connected inverters, and to enhance the stability of the power system, a method to control a virtual synchronous generator (VSG) output voltage based on model predictive control (MPC) is proposed. Parameters of the inductors, capacitors and other components of the VSG can vary as the temperature and current changes. Consequently the VSG output voltage and power control accuracy using the conventional MPC method may be reduced. In this paper, to improve the parameter robustness of the MPC method, a new weighted predictive capacitor voltage control method is proposed. Through detailed theoretical analysis, the principle of the proposed method to reduce the influence of parameter errors on voltage tracking accuracy is analyzed. Finally, the effectiveness and feasibility of the proposed method are verified by experimental tests using the Typhoon control hardware-in-the-loop experimental platform.     

A combined artificial neural network-fuzzy dynamic programmingapproach to reactive power/voltage control in a distribution substation

Reactive power/voltage control in a distribution substation is investigated in this work. The purpose is to determine proper capacitor on/off status and suitable load tap changer (LTC) positions for the 24 hours in the next day. To reach this goal, an artificial neural network (ANN) is designed to reach a preliminary dispatch schedule for the capacitor and LTC. The inputs to the ANN are main transformer real power and reactive power and primary and secondary bus voltages and the outputs are the desired capacitor on/off status and LTC tap positions. The preliminary dispatch schedule is further refined by fuzzy dynamic programming in order to reach the final schedule. To demonstrate the effectiveness of the proposed method, reactive power/voltage control is performed on a distribution substation in Taipei, Taiwan. Results from the example show that a proper dispatch schedule for capacitor and LTC can be reached by the proposed method in a very short period     

基于超级电容器控制策略的含风电微电网电压波动的抑制

在分析含风电及储能的微电网电压—功率特性基础上,提出一种基于超级电容器控制策略的电压波动抑制方法,可对风电功率的随机波动进行快速补偿,保证风电与超级电容器储能共同向外输出的功率稳定,实现系统母线电压的稳定控制。对该方法进行了PSCAD仿真,仿真结果表明:风速变化时,该方法可有效抑制风电功率变化引起的电压波动;大型阻感性负载投入、外部故障时,该方法亦能减小电压波动和电压恢复时间,从而验证了所提方法的有效性与可行性。     

基于电容电压波动补偿的混合型模块化多电平变换器

为了降低模块化多电平变换器的电容体积,文中提出一种基于电容电压波动补偿混合子模块的模块化多电平变换器。首先,介绍所提混合型模块化多电平变换器的系统结构与减小电容体积的机理,同时对混合子模块的投切原则进行详细分析;然后,通过分析混合子模块内部不同电容的能量波动,给出所有电容的容值设计方法,并与传统的半桥型模块化多电平变换器进行对比;最后,通过仿真与实验验证了所提方案的有效性。结果表明,文中提出的混合型模块化多电平变换器能够以较低的开关器件成本,大幅度提高电容能量利用率,降低模块化多电平变换器子模块电容的体积,实现系统功率密度的提高。