Optimal load-shedding using local information

This paper describes an optimal load-shedding policy based on quadratic programming using only some power system state variables after severe generation outages. When generation outage is severe, the imbalance between supply and demand causes a declining frequency. Utilities generators cannot be operated excessively at frequencies above normal. Some of the load must be shed to prevent system damage. Optimal load-shedding policies have been studied using all state variables of the power system. However, in real power system operations, it is difficult to obtain remote information in that fraction of a second following generation outages. An optimal load-shedding method is constructed in this research using quadratic programming (QP) under the assumption that all power system state variables can be available. However, these state variables cannot be easily accessible (as already mentioned). A suboptimal load-shedding scheme based on the local state variables is then studied. The proposed load-shedding method is based on the aforementioned optimal one. The change in line-power flows and the amount of generation power outage are used as accessible data at each load point. Incorporating this local information into the optimal loadshedding method based on the QP method, the proposed load-shedding method is established here. The effectiveness of this proposed method is illustrated by two examples and simulation results on a model power system show that the method is encouraging.     

Estimation of power outage size based on the dominating differential equation

This paper describes a method to estimate how much power will drop after severe generation outage. When a large generation outage occurs, system frequency plummets to an unacceptable frequency level. Frequency drop affects both customers and generation systems. Thus adequate and quick load-shedding must be done to prevent problems. To institute any load-shedding policy effectively, the size of the generation outage must be precisely estimated in a very short time. So far, several methods have been proposed to estimate the power outage amount by measuring the decaying frequency variations that are obtained at each local bus. These methods are easily applied, but cannot be expected to provide good results in real power system operations because of the noises that are a part of decaying frequency variations. To cope with this problem, a new estimation method based on one that uses the dominating differential equation is proposed in this paper. The most precise estimation at this point is obtained by using additional factors, such as the sine wave included in the decaying frequency fluctuations and considered as a part of the equation. Simulation studies on a model power system consisting of five generating stations and four load points show that the newer method is encouraging. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (3): 39–49, 1997     

电网规划中最小切负荷费用计算方法

电力市场环境下的电网规划需要综合考虑电网的投资费用、运行费用和切负荷费用,所以有必要计算各规划方案的最小切负荷费用.通过建立以切负荷费用最小为目标函数的数学模型,提出了逐步经济切负荷法.该方法是一个逐步消除系统过负荷的迭代过程.每一次迭代中引入经济有效度指标来综合考虑候选节点的切负荷有效度和单位切负荷费用,并以此确定一个切负荷最佳节点以及该节点的切负荷量.当系统再无过负荷时,就终止迭代.此时,所有切负荷节点上的切负荷费用之和即为系统最小切负荷费用.算例证明了该方法的有效性和可操作性, 与电力市场下电网的实际运行情形相吻合.     

电网运行风险在线评估中基于灵敏度分析的负荷削减模型

传统负荷削减模型通常采用全局寻优得到负荷削减范围,难以满足电网运行风险在线评估的时效性要求。为解决现有的负荷削减模型存在的问题,提出了一种基于灵敏度分析的负荷削减模型,通过计算各支路对母线节点的灵敏度,筛选对越限支路潮流影响显著的母线节点作为负荷削减范围,将全局寻优转换为局部寻优。综合考虑负荷重要程度和计及设备电气耦合关系的临近原则,以各类负荷削减量加权求和最小为目标,采用原对偶内点法计算负荷削减量。所提模型可在保证负荷削减结果正确的前提下,大幅提高计算效率。以IEEE 300节点等系统为例,验证了所提负荷削减模型的正确性和高效性。     

静止同步补偿器抑制电压跌落的研究

随着电力系统敏感设备的不断增加及用户对电能质量要求的提高,电压跌落的抑制问题成为电力系统研究的一项重要内容。传统方法应用静止同步补偿器(STATCOM)抑制电压跌落,文章在此基础上提出了新的观点。首先,基于改进故障点法,结合STATCOM的补偿原理,对电网各节点电压跌落频次进行了评估。进而,分析STATCOM对抑制电网其它节点电压跌落的影响,提出STATCOM的分布式配置方案。最后,结合敏感负荷节点的电压跌落经济性评估,提出了配电网中STATCOM的优化配置概念。通过IEEE-30节点标准测试系统,分析验证了文章所提出的理论。     

复杂电网多重故障条件下的电压暂降分析

针对复杂电网发生多重故障时的电压暂降问题,提出一种新的分析方法。该方法将所有故障点视为虚拟节点并引入多重故障点位置参数。通过计算虚拟节点与非故障节点、虚拟节点之间的互阻抗和自阻抗,将节点阻抗方程进行扩展。结合各故障点的故障边界条件计算得到各故障电流相量,进而推导出电网中任意节点故障电压相量关于多重故障位置参数的函数,最后结合阀值电压进行电压暂降分析。在标准IEEE 30节点系统中验证所提出的电压暂降分析方法的有效性。     

考虑边界母线电压无功特性的地区电网无功优化

地区电网的电源是连接上下级电网的变电站,将REI等值用于地区电网无功优化,以解决常规模型中无法反映边界母线(即连接上下级电网的母线)间电压无功特性(边界母线电压幅值受自身以及其他边界母线下级电网无功变化的影响情况)等问题。推导了能有效反映电压无功特性的等值电抗与边界母线电压无功灵敏度矩阵的关系,分析了等值电抗的变化特性以及潮流状态在等值前后的一致性,提出首先根据等值前后电压无功特性保持一致的原则利用本地数据求取等值参数,然后在优化计算中根据优化前后电压偏差的不变性对优化电压进行修正。通过对四川某地区电网的仿真计算,验证了该方法可以有效反映边界母线间的电压无功特性,使优化潮流计算电压更加准确。     

一种新型低频减载方案的研究

在突然恶性事故情况下,由于发电厂的退出,电力系统有可能产生严重的频率下降,导致系统崩溃和大面积停电事故。低频减载装置无疑是一种抑制频率下降的有效方法。它依据测量到的频率值,按轮次分步骤地减负荷。显然,此时的电力系统特性已经变化,而低频减频（UFLS）装置却没有做出相应的修正,仍然是在全系统范围内切除预接负荷。作设计了一种新的减负荷方案,它利用频率和电压的变化,针对每一种事故情况,特别是在承受较大扰动的地区切负荷,以求达到供需平衡。对互联系统的严重功率缺额的模拟研究表明,新的减负荷方案明显优于现有的低频减载（UFLS）方案。     

基于配电网先验信息的谐波状态估计量测点最优配置

针对现有谐波状态估计量测点数量较多的问题,提出利用独立分量分析(independent component analysis,ICA)和配网先验信息进行量测点优化配置。以负荷的非线性程度和负荷容量的乘积度量节点的谐波发射水平,大于设定阈值的节点作为可疑谐波源。根据量测量维数不少于谐波节点数量,且量测点可提供电压和电流量测的特点,确定量测点数量。再以谐波发射水平、母线电压等级的加权和为目标函数计算可疑谐波源节点的重要度,选择较高重要度的母线配置量测设备。IEEE14节点配电网和某地区实际电网的算例证明了该方法的有效性和实用性。     

基于潮流转移因子的电力系统连锁跳闸风险评估

提出了一种基于潮流转移因子的电力系统连锁跳闸风险评估方法。在基于最优潮流交易的基础上,应用潮流转移因子对支路切除后的潮流转移情况进行预测,考虑潮流转移后支路电流不确定性建立后备保护动作概率模型,用首条支路故障概率和下条支路后备保护动作概率之积描述连锁跳闸的可能性指标,以连锁跳闸后是否切负荷将风险严重性进行分级,然后分别以连锁跳闸后系统购电费用增幅和切负荷费用为严重性指标,从而得到特定交易下支路连锁跳闸风险值,获得高风险连锁跳闸的支路路径信息,为制定交易计划和防范风险提供决策支持信息。IEEE10机39节点系统算例仿真结果表明了该方法的有效性。     

海上风电经柔性直流联网系统受端交流故障穿越协调控制策略

针对海上风电经柔性直流联网系统受端交流故障导致的直流过电压问题,提出了直流过电压协调抑制策略。针对单极直流过电压,通过合理切换双极MMC控制模式,可使故障极MMC主动维持直流电压稳定。并设计了风电场精确减载控制策略,以保证非故障极MMC满载运行,从而降低单极MMC退出对受端交流电网的影响。针对双极直流过电压,设计了一种基于本地直流电压测量信息的风电场减载控制策略,即根据直流电压变化率及偏差量主动降低风电场有功出力,以抑制直流电压上升率及幅值。并提出了附加桨距角控制及其参数选取原则,使风电场与各换流站内电容共同维持直流电压稳定,提高系统故障穿越能力。最后,基于RTLAB OP5600实时数字仿真平台搭建了系统仿真模型。不同受端交流故障情况下的仿真结果表明,所提直流过电压协调抑制策略可保证直流电压在安全运行范围,维持系统安全稳定运行。     

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.     

电压薄弱节点的在线识别与控制方法

快速准确地识别电压薄弱节点并执行相应的控制措施,是防御电力系统电压失稳的一项重要工作.首先,在对电力网络进行等值和解析的基础上,建立了识别节点电压薄弱性的解析指标,该指标具有明显的物理意义,反映了负荷节点电压与所有负荷电流在相应网络阻抗上引起的累积电压降的比值.然后将节点电压薄弱性指标分解为与各负荷节点相对应的贡献度指标,利用贡献度指标可以解析地识别导致节点电压薄弱的关键因素,并能给出相应的控制措施.IEEE 118节点系统仿真结果表明,与模式分析方法相比,所提方法计算结果准确,能有效识别出与所有关键模式相对应的各个薄弱节点,并能给出减轻节点薄弱性的控制措施.     

A two-point estimate method for uncertainty modeling in multi-objective optimal reactive power dispatch problem

Due to nonlinear and discrete variables and constraints, optimal reactive power dispatch (ORPD) is a complex optimization problem in power systems. In this paper, the purpose is to solve multi objective ORPD (MO-ORPD) problem considering bus voltage limits, the limits of branches power flow, generators voltages, transformers tap changers and the amount of compensation on weak buses. The objectives of this paper are real power losses and voltage deviations from their corresponding nominal values, which are conflicting objectives. Because of the stochastic behavior of loads, the MO-ORPD problem requires a probabilistic approach. Hence, in this paper, a two-point estimate method (TPEM) is proposed to model the load uncertainty in MO-ORPD problem. Moreover, the proposed method is compared with some other methods such as deterministic approaches and Monte Carlo simulations (MCS). The obtained results approve the efficiency of the proposed methodology. The proposed models are implemented and solved using GAMS optimization package and verified using IEEE 14-bus and IEEE 30-bus standard test systems.     

基于电流变化量的自适应母线保护

提出了母线差动保护的自适应判据和母线外部故障时TA饱和的判别方法,用于快速反应母线故障和解决母线区外故障保护误动问题。根据母线外部故障时工频电流变化量差动电流主要表现为母线分布电容电流的变化量,母线内部故障时特别是经高阻故障时可能有一部分穿越性负荷电流流经母线,流进母线的工频变化量电流在流出电流支路的阻抗和故障点等效的过渡电阻之间分配的特点,得到自适应母线保护的动作判据。基于饱和的TA在瞬时电流过零时有一线性传变区,瞬时电流变化量制动电流时间先于瞬时电流变化量差动电流这种特点,得到母线区外故障时TA饱和的判别方法。提高了保护的可靠性和灵敏度,不受系统运行方式和过渡电阻的影响。     

220kV变电站高、中压侧母线最佳接线方式探讨

当前广州地区电网的变电站母线接线形式多种多样。针对这种情况,从电网运行实际情况出发,通过对220kV变电站高、中压侧母线和500kV变电站中压侧母线出线间隔设备进行了为期1．5年的停电次数统计跟踪,指出了当前广州地区部分220kV变电站母线接线形式对电网存在运行风险。为此,从电力系统安全稳定出发,并结合相关技术原则,提出了广州地区220kV变电站高、中压侧母线最佳接线形式。按最佳接线方式设计、运行,可提高电网输电的可靠性,减少电网主设备全停运的次数,避免超稳定极限事件的发生或大面积停电事故的发生。     

Application of active power sensitivity to frequency and voltage variations on load shedding

The occurrence of a large disturbance in a power system can lead to a decline in the system frequency and bus voltages due to a real and reactive power deficiency or due to the formation of islands with generation–load imbalance. Load shedding is an emergency control action that can prevent a blackout in the power system by relieving the overload in some parts of the system. This paper shows that rate of change of frequency can be utilized to determine the magnitude of generation–load imbalance, while the rate of change of voltage with respect to active power can be utilized to identify the sensitive bus for load shedding. The frequency, voltages and their rate of change can be obtained by means of measurements in real-time from various devices such as digital recorders or phasor measurement units or these parameters can be estimated from the voltage data by other means such as an optimal estimation method like Kalman filtering. The rate of change of system frequency, along with the equivalent system inertia may be used to estimate the magnitude of the disturbance prior to each load shedding step. The buses with a higher rate of change of voltage may be identified as the critical ones for load shedding and load can be first shed at these buses, depending on the change in the power flow at each bus. This application is tested on the IEEE 30 bus system and the preliminary results demonstrate that it is feasible to be used in load shedding to restore system voltage and frequency.     

基于聚类分析方法的电力系统负荷节点分区策略

针对传统分区方法无法给出区域内关键负荷节点的缺点,基于局部电压稳定指标提出一种新的负荷节点分区及关键负荷节点确定的新方法。依据局部电压稳定指标灵敏度的物理意义,将系统中负荷节点映射到多维负荷空间中,将多维负荷空间中负荷节点间的空间距离定义为负荷节点间的电气距离,建立起与负荷节点空间相对应负荷子图模型。引入谱聚类算法,对负荷节点进行分区,层次递推的给出负荷节点的合理分区,将分区结果与局部电压稳定指标灵敏度矩阵相结合,确定各分区中的关键负荷节点。将该方法应用到IEEE-14节点系统与IEEE-118节点系统进行分析比较,仿真结果验证了所提算法的可行性和有效性。     

基于最小奇异值灵敏度的电压稳定薄弱节点研究

最小奇异值作为静态电压稳定指标已广泛应用于电力系统的稳定分析之中。在最小奇异值分析的基础上,给出了最小奇异值对负荷功率的灵敏度,提出将最小奇异值灵敏度用于薄弱节点的分析及无功补偿点的确定。以IEEE57节点系统为例,在考虑负荷平均增长的情况下,分析了负荷变化对最小奇异值的影响,确定了系统薄弱区域及最弱节点,算例表明在所确定的最弱节点处进行无功补偿,可更有效地提高系统电压水平和电压稳定性。