STRONG CONTROLLABILITY AND OBSERVABILITY AND THEIR EFFECTS ON TRANSIENT STABILITY OF POWER SYSTEMS

ABSTRACT

An algorithm is developed for identifying bus clusters called control areas that have strong local controllability and observability property for measurements and controls at the buses or on generators within the control area. Two types of control areas are identified; one that is based on weak boundaries of the reactive power voltage jacobian and is called voltage control area; the other is based on the jacobian of both real and reactive power balance equations with respect to voltage and angle at network buses. The two control areas are identical under light load conditions, but are different as the system approaches voltage collapse or poorly damped low frequency oscillations. The understanding of strong controllability and observability in terms of real power angle dynamics and reactive power voltage dynamics (voltage control areas) and the strong controllability and observability of voltage and angle dynamics (control areas) helps understanding of voltage instability and inter-area oscillations.     

Weak bus-constrained PMU placement for complete observability of a connected power network considering voltage stability indices

Phasor measurement units (PMUs) are preferred for installation at weak buses in a power network. Therefore, the weak buses need to be located and the strategic locations of PMUs identified to ensure network observability. Thus, the primary aim of this work is to identify the placements of the maximum number of PMUs installed at the weak buses in the electrical network. The voltage collapse proximity indicator, line stability index, fast voltage stability index, and a new voltage stability indicator utilizing load flow measurement are used to determine the weak buses. A novel deterministic methodology based on a binary-integer linear programming model is then proposed to determine the optimal locations of PMUs. The effect of a single PMU outage considering the weak buses is also demonstrated. The effectiveness of the developed approach is tested and validated on the standard IEEE 14-, 118-, 300-, and New England 39-bus systems. The obtained results are also compared to those using different weak bus methodologies.     

A Stability Assessment Methodology

Weak transmission boundaries are shown to cause steady state and transient stability problems. The weak boundaries are defined by a loss of controllability property of the transient stability model. The loss of synchronism for fault contingencies can occur across these weak boundaries [7] if this boundary is weak compared to the other cutsets that encircle but are closer to the fault location. A loss of transient or steady state stability due to loss of generation contingencies will also occur across the elements in these weak boundaries. The inadvertant flows for inertial and governor response to loss of generation contingencies focus back to the region experiencing the loss of generation. These inadvertant flows cross the weak transmission boundaries and are shown to cause loss of generation and the formation of islands across the cutset of transmission elements that constitute the weak transmission boundaries. A method for determining and ranking the weak transmission boundaries of the transient stability model is developed. A method of determining and ranking the vulnerability of transmission elements is also determined. The measures used to determine the weak boundaries and vulnerable transmission elements for loss of generation or fault contingencies can be determined by summing the angle changes for all inertial load flow simulated loss of generation contingencies. The measures and the method for determining the weak transmission boundaries can be proven to detect the loss of controllability property of the transient stability model that define the weak boundaries.     

基于广域量测信息的负荷裕度灵敏度计算新方法

提出一种基于广域量测信息的电力系统负荷裕度灵敏度(LMS)分析方法。根据电力系统的广域量测信息,在线计算电力系统中各负荷节点的负荷裕度;依据所得的负荷裕度,提出一种基于广域量测信息的节点负荷裕度对有功、无功及机端电压的灵敏度分析方法;针对所提的LMS方法,进一步提出了改善系统电压稳定性的相关控制策略;最后将所提方法应用到New England 39节点系统和IEEE 118节点系统中,仿真结果验证了所提方法的正确性和有效性。     

大规模电力系统电压失稳区的确定方法

本文基于扩展潮流方程雅可比矩阵的特征结构分析法，提出了一种确定大规模电力系统电压失稳区的计算方法。该方法可以明确地给出系统在指定运行工况下，全网最有可能发生电压不稳定或电压崩溃的区域以及与之相关的关键发电机和输电支路。从而为诸如低压减载方案的设计、无功补偿的配置和系统的运行调控提供直接的依据。实际电力系统计算结果表明，该方法具有物理概念清晰、计算简单快速，尤其是适合于解算大规模电力系统的特点     

计及分布式电源接入的配电网静态电压稳定性评估方法

为研究分布式电源(DG)接入对配电网静态电压稳定的影响问题,推导了能反映配电网各负荷节点电压稳定程度的电压稳定指标,提出了一种针对配电网的负荷增长策略,对DG接入前后全网最薄弱节点以及整个系统的静态电压稳定性进行了定量分析,结合PV曲线探讨了配电网电压崩溃事故的发生机理,对比分析了DG出力波动性较大时,集中接入和分散接入方式对静态电压稳定性的改善效果。所提的基于系统各节点电压稳定指标计算的静态电压稳定性分析方法可对负荷节点电压稳定裕度进行快速排序,并可实现全网薄弱节点的准确定位。DG接入前后的仿真实验结果表明,合理位置的DG接入可有效改善全网的静态电压稳定性。     

模拟直流发电机特性的储能变换器控制策略

直流微电网的变换器均通过电力电子变换器接入直流母线,而电力电子变换器缺少惯性和阻尼作用,负载功率突变会引起变换器端口电压电流的振荡,给直流母线带来较大的冲击,影响微电网的稳定性。文中参考虚拟同步发电机在并网逆变器控制中的应用,提出了一种模拟直流发电机特性的储能变换器控制策略,使储能变换器具有直流发电机的端口特性,并建立小信号模型,利用阻抗比判据分析了其小信号稳定性。仿真和实验证明所提控制策略可以增强储能单元维持直流微电网内功率平衡的能力,提高直流微电网的供电质量。     

基于凹陷域分析的电压暂降监测点优化配置

提出一种基于凹陷域分析的电压暂降监测点优化配置的新方法,以监测点数目最小为目标函数,以全网敏感性负荷节点电压暂降可观测性为约束条件,利用整数线性规划方法,通过MATLAB编程求取符合要求的最少数目监测点。在构造电压暂降幅值矩阵的计算中,考虑到了每一个节点故障可能引发系统其余节点电压暂降的情况。在IEEE 39节点系统上进行仿真计算,成功求得了保证全网敏感性负荷节点电压暂降可观测的最少数目的监测点。     

暂态稳定断面功率极限区间和关联度指标计算

针对日益复杂的运行情况和系统对安全经济运行的要求,提出了暂态稳定断面功率极限区间和关联度指标计算方法。基于暂态安全稳定量化分析方法获得的预想故障下安全稳定裕度、模式和元件的参与因子,结合功率对输电断面功率的灵敏度计算发电机和负荷对断面功率极限的影响因子,通过发电机、负荷功率调整计算出功率极限的最大值和最小值。基于安全稳定裕度筛选得到输电断面关键故障,采用裕度为权重的加权平均计算发电机和监视母线的综合参与因子,通过发电机和母线参与因子的相关性分析计算两个输电断面的关联度指标。所提出的方法能够为调度运行人员快速掌握当前运行方式下各个输电断面的暂态安全稳定水平,以及彼此之间的交互影响程度提供技术手段,四川电网的算例分析验证了方法的实用性和有效性。     

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 rank correlation-based method for power network reduction

We present a coherency measure and a network reduction algorithm. The rank correlation function was used to determine the mutual couplings of generators. Generators were grouped at two correlation levels. These are grouping tolerances whose values are unique for a given system. The first produces the most strictly coupled generators to be aggregated, and the second identifies the generators to be saved during reduction. The terminals of the generators in each group were extended to include load buses to partition the network. Previously reported techniques were used to reduce generator and load buses.     

Slow coherency based network partitioning including load buses

The authors present a slow-coherency based network partitioning technique which groups generators and load buses simultaneously into several coherent areas. The technique uses the slow eigenbasis matrix which is extended to include load buses. A simple method of calculating the closeness of each row eigenvector to the reference row eigenvectors is used for grouping the load buses with the respective reference machine. The grouping information is useful for system reduction and identification of weak links for calculations involving network decouplings. The grouping method is illustrated using test systems and large practical systems     

振荡中心侵入机群场景下的刚性外推策略

大规模电力外送基地机群失步场景下,振荡中心易侵入机群内部,仍采用现有发电机失步保护应对策略将可能导致机组“雪崩式”切机。从切机切负荷有损控制入手,提出了一种通过调整负荷或机组数量维持系统稳定性的振荡中心刚性外推策略。针对振荡中心落入电力外送基地某机群场景,从理论上分析了该刚性策略的可行性;推导了系统内任意一点处电压幅值的解析表达式,并结合振荡中心处的电压特性,得到中心位置表达式;基于此,在振荡中心外推出机群条件下对受端系统负荷或对侧机群机组切除数量进行预测,制定防止过切的约束,进一步判断系统的稳定性,并确定源荷调整有效范围。最后,基于PSCAD/EMTDC仿真软件建立多机并联外送系统仿真模型,通过仿真验证了系统失步过程中发电机失步保护阻抗轨迹和功角变化,采取外推策略后振荡中心外推出机群且功角趋于稳定,该结果说明了所提策略的有效性。     

给定线路功率的控制潮流求解方法及应用

介绍一种新的控制潮流算法。该算法考虑了发电机有功—频率特性曲线和无功—电压特性曲线的调节特性,避免了常规潮流对PV、PQ及平衡点的假设,有效地解决了电力系统调度运行中通过调整发电机出力使某些重要线路的传输功率维持在给定值的问题。特别是系统受到负荷扰动时,该算法能快速确定各机组的出力。对试验系统的计算表明,该算法是正确有效的。     

Modelling and performance of fixed-speed induction generators in power system oscillation stability studies

This paper describes the modelling and performance effects of induction generator-based wind farms on the oscillation stability of power systems. For groups of 5, 10 and 20 induction generators, it is established that the basic dynamic properties of each group of generators are similar to those of a third-order equivalent machine model whether the generators are evenly loaded or unevenly loaded. The paper analyses the impact of induction-based wind generation on oscillation stability for multi-machine systems as applied to the standard IEEE 30 bus system. The effects of load model representation, system loading, generator output and transmission line network parameters are considered. In general, the multi-machine system is shown to be well damped under all normal operating conditions.     

DETERMINATION OF WEAK BUS ORDERING VECTOR IN MULTIMACHINE POWER SYSTEMS FROM VOLTAGE STABILITY VIEWPOINT

ABSTRACT

It is well known that the increase in load demand with a small increase in transmission capacity of a power system leads to voltage instability problems. Deficit of reactive power in the system leads to poor voltage profiles and with increased loading it leads to voltage collapse. The voltage stability condition of the system can be improved by providing reactive power support at certain buses, called the weak buses of the network. Thus determination of the weakest bus and the Weak Bus Ordering Vector (WBOV) assumes great importance. In this paper, a new and simple method is proposed to identify the weakest bus and obtain WBOV in a multimachine power system. The given system is replaced by the equivalent multiport Thevenin network and then the voltage stability limit condition is derived. The weakest bus and WBOV are obtained from an index called, Voltage Change Index. The method is very simple since it requires only one load flow solution for computing the entire WBOV for a given operating condition and it does not require complex multidirectional optimisation. Also, it is applicable to any pattern of load change in the system.     

基于广域量测的电压稳定在线监测方法

利用电压稳定在线监测系统进行在线预警,是电压稳定在线监控系统的重要组成部分。该文根据单电源简单传输系统模型,提出一种在线监测静态电压稳定性的负荷裕度指标,它可准确地反映节点的负荷裕度,能给出运行点是否位于系统PV曲线上半支或下半支的信息。对多机系统中的负荷节点及联络节点构造单电源功率传输等值系统,等值电源的电压幅值能基本保持恒定;利用广域量测系统,测量计算单电源功率传输等值系统参数及负荷裕度指标。在改进的IEEE 50机测试系统上仿真表明,提出的电压稳定在线监测方法能很好地反映负荷裕度,捕捉到电压稳定薄弱母线。     

Multi-objective economic emission load dispatch solution using gravitational search algorithm and considering wind power penetration

In this paper an economic emission load dispatch (EELD) problem is solved to minimize the emission of nitrogen oxides (NO_X) and fuel cost, considering both thermal generators and wind turbines. The effects of wind power on overall NO_X emission are also investigated here. To find the optimum emission dispatch, optimum fuel cost, best compromising emission and fuel cost, a newly developed optimization technique, called Gravitational Search Algorithm (GSA) has been applied. GSA is based on the Newton’s law of gravity and mass interactions. In GSA, the searcher agents are collection of masses which interact with each other using laws of gravity and motion of Newton. IEEE 30-bus system having six conventional thermal generators has been considered as test system. Two extra wind turbines are also placed at two weak load bus of the system. Two Weak load buses have been selected based on their L-index value. After placing the wind power sources, those buses have been considered as generator bus. Minimum fuel cost, minimum emission and best compromising solution obtained by GSA are compared with those of biogeography-based optimization (BBO). The results show that the GSA surpasses the other available techniques in terms of solution quality and computational efficiency.     

大电网可靠性影响分析的潮流跟踪方法

提出了大电网可靠性评估中削减负荷的潮流跟踪模型, 建立了节点和系统可靠性对元件的分配计算模型,基于此易于分析系统的薄弱环节。在负荷削减的潮流跟踪模型中,按负荷对发电机和线路利用系数的大小顺序削减负荷。基于潮流跟踪削减负荷能建立故障元件与削减负荷节点间的映射关系,该模型的核心在于找出所有元件中对削减负荷节点利用系数最大的元件。算例结果证明了所提出方法的有效性。     

Fixed-order controller for reduced-order model for damping of power oscillation in wide area network

The interconnected power systems are complex and stabilizing control design still remains challenging task. The use of wide area monitoring system (WAMS) offers an integrated measurement-based and model-based control, which suits to the operation of large electric power system (EPS), along with online analysis. This paper presents a study on fixed-order controller design for equivalent network of coherent generator in order to stabilize inter-area electromechanical oscillations in the system. Firstly, the coherent generators in each area of large EPS are determined by mutual information theory, which represents the dynamic equivalence. Then network of each area with input–output variables of the selected generator that participates dominantly is reduced to lower size by square-root variant of balanced truncation algorithm. The dynamics and important oscillation modes are verified in equivalent representation of each area. Finally a local controller (decentralized) in each coherent area and a centralized controller between two coherent areas for selected generator are designed by reducing the H^∞ norm of its closed loop transfer function as much as possible. These controllers feed supplementary control signal in addition to one fed by local conventionally tuned PSS. The decentralized controller for selected generator is fed by local bus power or generator’s speed signal. On other hand, the centralized controller uses difference of power flow/speed of generators as input signal to dampen the oscillations between equivalent networks of two areas. The simulation results reveal effective damping of power/speed oscillations achieved by designed controller with respect to conventional PSS implemented. The robustness of controller is verified for heavy and light load operating conditions.