A stochastic based voltage collapse indicator

The voltage collapse of a power system is analyzed as the result of the eventual push of the system operating point beyond the boundary of voltage security due to random fluctuations in the system load characteristics. The effect of different fluctuation intensity levels at load buses on the eventual voltage collapse of a power system is studied with the help of a stochastic based voltage collapse indicator. It is shown that this indicator, will provide further insights into the power system's vulnerability due to the high degree of uncertainty that exists in present day voltage collapse load models     

防止交直流联合系统电压崩溃措施的研究

在应用崩溃点法求得交直流联合系统电压崩溃点的基础上,利用左特征向量W和电压崩溃边界曲面Σ之间的关系计算出提高交直流联合系统负荷功率裕度的最优控制方向。在此基础上,研究可提高交直流联合系统负荷功率裕度、防止电压崩溃发生的交流系统措施和直流系统措施。     

电力系统电压崩溃临界状态和快速算法

本文利用潮流方程的二阶非线性特点，从满足电压崩溃临界状态的边界条件出发，再由潮流方程的一对多根解（高电压解ｘ０和低电压解ｘ１）组成的新变量的一些性质，提出了一种快速，简便又满足精度要求的计算临界状态的模型和算法。     

Look-ahead voltage and load margin contingency selection functionsfor large-scale power systems

Given the current operating condition (obtained from the real-time data), the near-term load demand at each bus (obtained from short-term load forecast), and the generation dispatch (say, based on economic dispatch), we present in this paper a load margin measure (MW and/or MVAr) to assess the system's ability to withstand the forecasted load and generation variations. We also present a method to predict near-term system voltage profiles. The proposed look-ahead measure and the proposed voltage prediction are then applied to contingency selections for the near-term power system in terms of load margins to collapse and of the bus voltage magnitudes. We evaluate the proposed look-ahead measure and the voltage profile prediction on several power systems including a 1169-bus power system with 53 contingencies with promising results     

电压崩溃临近指标的小波神经元网络拟合算法

电压崩溃临近指标能够有效、快速地对电力系统电压安全进行评估。为此, 提出了一种电压崩溃临近指标的小波神经元网络模型。这个模型以非线性小波基为神经元函数, 通过优化伸缩因子和平移因子确定对应各神经元的小波基函数, 从而合成小波神经元网络, 达到全局最优拟和效果。经过训练的小波神经元网络能在线计算电压崩溃临近指标, 并且具有快速、准确等优点。文中对该模型与人工神经元网络模型进行了比较, 结果证明, 利用小波神经元网络模型进行电压崩溃临近指标预测比利用人工神经元网络模型具有更高的拟合精度, 计算速度更快。仿真结果表明, 该方法能有效地对电力系统电压崩溃做出早期预测, 是一种对系统电压安全进行快速、实时评估的有效工具。     

基于Walve负荷模型典型电力系统多参数分岔分析

以通用非线性系统分岔分析软件Auto97为工具,对基于Walve综合负荷模型的典型3节点电力系统进行了多参数分岔分析。分析过程表明多参数分岔分析相对于单参数分析更能揭示系统参数对电力系统电压稳定性的影响情况。结果显示:选取较高的参考电压Vref与励磁增益KAVR不仅有利于提高功率传输极限、增加稳定裕度,而且有利于避免系统电压振荡失稳;同时表明Vref、KAVR之间具有一定的互补特性,可通过Vref和KAVR的协调运用,避开Hopf分岔,保证系统安全运行。另外表明,大的励磁极限将更有利于电力系统电压动态稳定     

考虑电压约束裕度的无功优化及其内点解法

常规无功优化算法的目标函数一般为满足电压限制下的有功损耗最小,这将使优化后个别母线的电压非常接近其合格范围的上限,使电压合格率下降,成为系统安全运行的隐患。根据实际运行中对这一安全裕度的方法仍存在一些问题。根据实际运行中对这一安全裕度的模糊要求,该文提出了在常规算法中引入模糊约束以解决这一问题。采用原-对偶内点算法详细地推导了带有模糊安全约束裕度的逐次二次规划数学模型。算例的结果表明：带有模糊约束的逐次二次规划内点法可以实现在满足电压安全约束下的网损最小,其网损的损失很小,而优化后的电压合格裕度却有较大的增加。     

Voltage security evaluation based on perturbation method

This paper proposes a new algorithm for estimating voltage security margin. The algorithm is based on the perturbation method and has significant computational efficiency. The proposed algorithm can be used for on-line voltage security evaluation. It has been validated using IEEE-14, IEEE-30 and IEEE-57 bus systems. Results from the tests show higher efficiency and smaller error margins compared to continuation power flow (CPF) method.Voltage collapse is a serious threat to the security of stressed power systems; therefore, voltage security (VS) has become a major challenge for management of power systems. The motivation for this research is a direct consequence of the deregulation of electricity industries and markets worldwide.     

基于Walve负荷模型的励磁系统多参数分岔分析

文中以通用非线性系统分岔分析软件AUTO97为工具，以负荷功率、AVR控制参考电压、励磁增益和励磁极限为分岔参数，对基于Walve综合负荷模型的典型3节点电力系统进行了多参数分岔分析。文中以负荷功率、AVR控制参考电压Vref、励磁增益KAVR和励磁极限Efdlim为分析参数，研究了Vref、KAVR以及Efdlim对系统电压稳定与运行情况的影响，得到了一些更接近实际的结论。分析过程表明：多参数分岔分析相对于单参数分析更能揭示系统参数对电力系统电压稳定性的影响情况。分析结果表明：不考虑励磁极限时，选取较高的参考电压Vref与励磁增益KAVR，不仅有利于提高功率传输极限、增加稳定裕度，而且有利于避免系统电压振荡失稳Vref、KAVR之间具有一定的互补特性，可通过Vref和KAVR的协调运用，避开Hopf分岔，保证系统安全运行：大的励磁极限将更有利于电力系统电压动态稳定。     

An approach for real power scheduling to improve system stability margins under normal and network contingencies

In the present day power system planning and operation, considerable interest is being shown in system security and stability analysis. Pattern of load sharing/generation scheduling that results in heavy flows tend to incur greater losses, threaten stability, security and ultimately making certain generation patterns undesirable. Generation schedules mainly based on economic criteria may lead to lower reserve margins and therefore diminished reliability is a serious concern for the systems. With increased loading of existing power transmission systems, the problem of voltage stability and voltage collapse has also become a major concern in power system planning and operation. While the voltage stability is more dependent on the reactive power sources/voltage profile in the system, it is also a function of real power flows. In this paper, network sensitivity between load voltages and source voltages to compute voltage stability index (L), is used as the basis to evaluate desirable load sharing for improving stability margins. The proposed method has been tested on typical sample systems and also on a practical 24-bus equivalent power system, and results are presented to illustrate the proposed approach.     

开放电力市场环境下的电压稳定性评估

传统垄断结构下电力输电系统的设计和运行原则之一是运行点不常接近安全边界。然而在新的开放电力市场环境下,电力系统的运行点倾向于越来越接近于安全边界;在这一环境下,如果无功功率支持不足,就可能出现电压失稳。文章提出一种计算静态电压稳定极限点的崩溃点(Point of Collapse)算法。由于崩溃点算法特殊的优越性,该算法得到广泛的关注;然而,其方程的维数约为普通潮流方程的两倍,且不易采用稀疏矩阵技术,故其在大系统中的应用存在困难。针对这一问题提出一种算法,其程序可构筑在标准潮流程序基础上。以新英格兰39母线测试系统为例,验证了所提算法的有效性。计算结果表明该算法计算精确且计算高效。     

电力系统动态安全域的实用解法

近年来，电力系统动态安全域（DSR)已越来越为人所接受，它可以提供更为丰富的安全信息，有着广阔的在线应用前景。该文基于暂态能量函数分析，推导出一种新的求取实用动态安全域(PDSR)的方法。通过大量的数值仿真计算表明，实用动态安全域(PDSR)可由描述各节点注入功率上、下限的垂直于坐标轴的超平面和描述暂态稳定性临界点的超平面围成。基于此事实，该文给出了一种快速计算对应于临界暂态稳定的边界超平面的直接法。这种方法是将溢出点处的暂态稳定域边界法矢量近似为常数，并通过线性化把事故后系统轨迹的切向量表示为注入功率的线性函数，然后依据在临界注入下两者之间的正交性，推导出了超平面型式PDSR边界的解析表达式。在新英格兰10机39节点系统上的测试结果表明了这种方法与数值仿真方法的一致性。     

Online VAR support estimation for voltage stability enhancement

The existing resources in present day power systems are inadequate to meet the ever-growing load demand and as a result the grids are operated closer to the voltage stability boundaries. The system is therefore prone to voltage collapse even for a small increase in load demand. VAR support appears to be the only viable remedial measure. This paper develops a fuzzy based online tool to determine the minimum VAR support required for a projected load demand with a view to ensure voltage stability in a power system. The tool incarnates a methodology through which the operator can initiate steps to improve the voltage profile and bring the system far away from the point of voltage collapse. It includes the test results of three IEEE systems to exhibit the viability and effectiveness of the proposed method.     

Formulation, computation and improvement of steady state security margins in power systems. Part II: Results

A steady state security margin for a particular operating point can be defined as the distance from this initial point to the secure operating limits of the system. Four of the most used steady state security margins are the power flow feasibility margin, the contingency feasibility margin, the load margin to voltage collapse, and the total transfer capability between system areas. This is the second part of a two part paper. Part I has proposed a novel framework of a general model able to formulate, compute and improve any steady state security margin. In Part II the performance of the general model is validated by solving a variety of practical situations in modern real power systems. Actual examples of the Spanish power system will be used for this purpose. The same computation and improvement algorithms outlined in Part I have been applied for the four security margins considered in the study, outlining the convenience of defining a general framework valid for the four of them. The general model is used here in Part II to compute and improve: (a) the power flow feasibility margin (assessing the influence of the reactive power generation limits in the Spanish power system), (b) the contingency feasibility margin (assessing the influence of transmission and generation capacity in maintaining a correct voltage profile), (c) the load margin to voltage collapse (assessing the location and quantity of loads that must be shed in order to be far away from voltage collapse) and (d) the total transfer capability (assessing the export import pattern of electric power between different areas of the Spanish system).     

基于稳定域边界二阶逼近的暂态电压稳定分析

电力系统中暂态电压失稳事故时有发生从而引起电力界广泛关注。目前的暂态电压稳定分析方法尚存在不足之处。本文基于非线性动力学的半张量积方法研究电力系统暂态电压稳定问题,以期在电力系统发生大扰动后快速判断故障后系统的电压稳定性,从而有利于及时采取控制措施避免系统崩溃。论文首先建立了暂态电压稳定分析的系统综合模型,主要包括对暂态电压失稳至关重要的发电机、励磁、感应电动机和输电网四个子系统。在此基础上,提出了基于稳定域边界二阶逼近的暂态电压稳定判据;在一个简单系统进行的实例分析表明所提出的暂态电压失稳判据具有速度快、准确度高及易于计算机实现的特点,具有较高的工程实用性,可望有效判断故障后系统的暂态电压稳定性。     

电力市场环境下的电压稳定分析

传统垄断结构下电力输电系统的设计和运行原则之一是运行点不常接近安全边界。然而在新的开放电力市场环境下，电力市场的运行点倾向于越来越接近于安全边界；在这一环境下，如果无功功率支持不足，就可能出现电压失稳，电力市场为电压稳定分析带来了许多新的问题和挑战。为此分析了现有的电压稳定分析方法的机理，以及电力市场环境下的电压稳定分析遇到的新问题以及分析方法。     

On-line detection of power system small disturbance voltageinstability

Small disturbance (SD) voltage stability (or instability) deals with a system's ability to maintain satisfactory voltages following a small disturbance. For an operating condition, a system's SD voltage stability depends on the proximity of the condition to the critical point (or voltage collapse point). A Q angle and Q directional derivatives are proposed for SD voltage instability detection and weak bus identification, respectively. The Q angle index can handle different kinds of loads, e.g., constant P and Q, constant impedance, and constant current, or a combination of them, and is effective in dealing with generator VAr limits. Moreover, the computation speed of the Q angle is fast, which makes it suitable for on-line application. Simulation results using two power systems are provided     

A shot in the dark

Investigations of recent blackouts indicate that the root cause of almost all of these major power system disturbances is voltage collapse rather than the underfrequency conditions prevalent in the blackouts of the 1960s and 1970s. The operation of today's power system with generation frequently being remote from the load centers has made the power system very dependent on the utility transmission system. When transmission lines trip, voltage drops at the load center, whereas frequency may remain normal until a complete system collapse occurs. Utilities have begun to recognize this problem and are installing UVLS programs. Also the industrial customers that have in-house generation that operates in parallel with the utility need to recognize the problem. These industrial cogeneration customers should consider the use of undervoltage separation schemes in addition to their existing underfrequency schemes to address the voltage collapse scenario. This article proposes undervoltage separation logic schemes that can be easily installed within digital relays to enhance the security of an undervoltage separation to prevent false operation due to slow-clearing system faults. The article also indicates the required point of installation of these relays so that they properly measure system voltage.     

Feasible enhancement of a three‐phase soft‐switching inverter with an auxiliary resonant DC link snubber

The purpose of this paper is to improve power conversion efficiency of a three‐phase voltage source type soft‐switching inverter with a single auxiliary resonant DC link (ARDCL) snubber. First, the operating principle of an ARDCL snubber discussed here is described. Second, this paper proposes an effective pulse pattern generation method of the zero voltage space vector of the three‐phase soft‐switching inverter using IGBTs or power modules that can reduce power losses in the ARDCL snubber treated here. In particular, a zero voltage holding interval in the DC rail busline of this three‐phase soft‐switching inverter is to be regulated according to the generation method of the zero voltage space vector. Third, the maximum modulation depth M_max under the condition of correction of the instantaneous voltage space vector can be improved by using a new zero voltage space vector generation method. Finally, the feasible experimental results of this inverter are obtained confirming the operating characteristics such as power conversion actual efficiency, as well as conventional efficiency THD and RMS value of the balanced three‐phase output voltage for an experimentally built three‐phase voltage source type soft‐switching pulse modulated inverter using the latest IGBT modules and evaluated from the standpoint of practical applications in industry UPS and new energy systems. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(1): 89–99, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10234     

A novel collapse prediction index for voltage stability analysis and contingency ranking in power systems

Voltage instability is a serious phenomenon that can occur in a power system because of critical or stressed conditions. To prevent voltage collapse caused by such instability, accurate voltage collapse prediction is necessary for power system planning and operation. This paper proposes a novel collapse prediction index (NCPI) to assess the voltage stability conditions of the power system and the critical conditions of lines. The effectiveness and applicability of the proposed index are investigated on the IEEE 30-bus and IEEE 118-bus systems and compared with the well-known existing indices (Lmn, FVSI, LQP, NLSI, and VSLI) under several power system operations to validate its practicability and versatility. The study also presents the sensitivity assumptions of existing indices and analyzes their impact on voltage collapse prediction. The application results under intensive case studies prove that the proposed index NCPI adapts to several operating power conditions. The results show the superiority of the proposed index in accurately estimating the maximum load-ability and predicting the critical lines, weak buses, and weak areas in medium and large networks during various power load operations and contingencies. A line interruption or generation unit outage in a power system can also lead to voltage collapse, and this is a contingency in the power system. Line and generation unit outage contingencies are examined to identify the lines and generators that significantly impact system stability in the event of an outage. The contingencies are also ranked to identify the most severe outages that significantly cause voltage collapse because of the outage of line or generator.