Method of identifying weak transmission network stabilityboundaries

Methods of determining weak transmission stability boundaries are developed based on the strong controllability and observability properties of power systems. This theory has been previously applied to a dynamical model of the generators and network. It was established previously that the network and generator states associated with a cluster of generator and load buses surrounded by weak transmission boundaries are strongly controllable and observable for a single measurement and control at generators or load buses in that bus cluster. Such a bus cluster is called the control area. Two methods of determining control areas, where strong controllability and observability hold, are developed. Both methods attempt to determine weak transmission stability boundaries that encircle control areas. The groups of generators identified as belonging to each control area is identical to the groups of coherent generators determined using a RMS coherency measure evaluated for the set of all inertial load flow contingencies     

UNIFICATION AND CLASSIFICATION OF ALGEBRAIC TESTS FOR LOSS OF VOLTAGE STABILITY

ABSTRACT

Three distinct types of algebraic bifurcations are shown to occur in both a load flow model and in a transient midterm stability model. Angle bifurcation results in significant angle change and no voltage change at the point of bifurcation. Voltage bifurcation results in significant voltage change and no angle change at the bifurcation point. Voltage and angle bifurcation result in both voltage and angle change at the bifurcation point. Angle bifurcation occurs due to reaching the real power transfer capability across some interface or boundary of the network. Voltage bifurcation is due to inability to transfer needed reactive power to a bus or area across a boundary due to insufficient voltage control and thus reactive supply capability in the region. Voltage and angle bifurcation results when reactive power can' be transferred to a bus or area across a boundary due to insufficient reactive supply and excessive reactive losses (I²X and shunt capacitive reactive withdrawal due to voltage drop) caused by real power transfer. Unique tests for the three types of load flow bifurcation are given. All the load flow voltage instability proximity measures are then classified in terms of which of these three types of load flow bifurcations they test for at the points of collapse. PQ controllability tests are shown to test for voltage, bifurcation and voltage and angle bifurcation. Any proximity test of reactive power versus voltage relationships also tests for voltage bifurcation and voltage and angle bifurcation. Proximity to all three types of load flow bifurcation are measured by either (a) tests of the singularity of the load flow jacobian or (b) proximity measures that test real power transfer or loading effects on voltage. Since equivalent voltage, angle, and voltage and angle bifurcations occur in a transient stability model, the load flow proximity measure tests can be applied on a transient stability model. This paper, thus, classifies and unifies the various proximity measure tests and point of collapse tests for voltage instability on both a load flow and transient stability model.     

计及电压不可行节点的配电网低电压治理方法

提出一种计及电压不可行节点的配电网低电压治理方法。首先给出了配电网电压不可行节点的基本概念,然后在配电网电压无功控制分区的基础上,构建了考虑电压不可行节点的广域无功控制的低电压治理模型。该模型包含无功补偿薄弱区的电压控制模型和无功补偿非薄弱区的无功优化两个子模型。对应的目标函数分别为电压不可行节点的电压越下限量最少和网损最低,约束条件则在传统的无功优化模型上更新电压不可行节点的电压幅值安全下限值。为了提高求解效率,采用一种并行协同进化算法求解所建模型。通过杭州某实际10 kV 55节点配电网进行仿真分析,验证了所提方法的有效性。     

Optimal power flow by BAT search algorithm for generation reallocation with unified power flow controller

Optimal power flow with generation reallocation is a suitable method for better utilization of the existing system. In recent years, Flexible AC Transmission System (FACTS) devices, have led to the development of controllers that provide controllability and flexibility for power transmission. Out of the FACTS devices unified power flow controller (UPFC) is a versatile device, capable of controlling the power system parameters like voltage magnitude, phase angle and line impedance individually or simultaneously. The main aim of this paper is to minimize real power losses in a power system using BAT search algorithm without and with the presence of UPFC. Minimization of real power losses is done by considering the power generated by generator buses, voltage magnitudes at generator buses and reactive power injection from reactive power compensators. The proposed BAT algorithm based Optimal Power Flow (OPF) has been tested on a 5 bus test system and modified IEEE 30 bus system without and with UPFC. The results of the system with and without UPFC are compared in terms of active power losses in the transmission line using BAT algorithm. The obtained results are also compared with Genetic algorithm (GA).     

基于可观测与可控度的直流输电次同步振荡阻尼控制研究

汽轮发电机组轴系在次同步频率范围内往往存在多个扭振模态。目前对高压直流输电引起的多模态次同步振荡尚缺乏行之有效的抑制策略,系统中多次出现应用次同步振荡阻尼控制器后轴系依然存在明显振荡的现象。应用状态空间法对轴系机械系统进行分析,建立了次同步振荡模态的可观测度与可控度的概念。在设计次同步振荡阻尼控制器时,基于次同步振荡的可控与可观测度进行参数优化。PSCAD/EMTDC的时域仿真结果表明,传统的次同步振荡阻尼控制器对于可观测与可控度较低的振荡模态抑制效果较差,而所提出的基于可观测与可控度的次同步振荡阻尼控制     

Power system damping enhancement using unified power flow controller (UPFC)

The Unified Power Flow Controller (UPFC) can inject voltage with controllable magnitude and phase angle in series with a transmission line. It can also generate or absorb controllable reactive power. UPFC is expected to be able to damp power system oscillations more effectively than power electronics devices such as SVG and TCSC. In this paper, a control system design of a UPFC for power system damping enhancement based on the eigenvalue control method is proposed. It is made clear that the best design method for the power system damping enhancement is to determine steady‐state values of the UPFC control variables and the control parameters of the UPFC such as gains and time constants simultaneously, because the controllability of UPFC depends on the steady‐state values of UPFC and the power flow condition. The effectiveness of the proposed control system taking into account UPFC inverter ratings is verified by digital time simulation. Furthermore the effects of the input signals to the UPFC controller on small‐signal stability and transient stability enhancement are studied, and it is made clear that UPFC controllers using global information are more effective for power system damping enhancement than those using local information because global information has stronger observability for power system oscillations than local information. © 2000 Scripta Technica, Electr Eng Jpn, 133(3): 35–47, 2000     

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。     

基于电压薄弱区域监视的PMU非完全观测配置

提出了一种基于电压薄弱区域监视的非完全可观测的同步相量测量单元(PMU)配置方案。采用连续潮流法确定母线的鞍结分岔曲线;在负荷变化率相同的情况下,根据电压幅值变化量来确定电压薄弱母线;考虑低电压水平与电压崩溃的关系,提出将电压幅值变化率作为电压薄弱母线分群所观察的信息,并且给出了两种电压薄弱分群指标,兼顾地域因素,采用模糊最大树聚类法将电压薄弱母线分群;给出了母线非完全可观测程度的度量方法以及PMU的配置流程。最后通过IEEE118节点系统验证了文中方法的有效性。     

考虑无功裕度的电压自适应分区方法及主导节点的选择

针对当前二级电压分区未能较好实现动态无干预调节的现状,基于电压/无功灵敏度及模块度函数,该文提出一种随电网运行工况实时变化的自适应动态分区和主导节点选择方法。以每个无功源为分区中心,根据电压/无功灵敏度大小将负荷节点映射到对应的无功源;构建考虑无功裕度的改进模块度函数,以其为衡量指标进行区域合并自动形成最佳分区。引入其他区域节点对本区主导节点的抗干扰指标,结合主导节点在本区域的可观性和可控性指标,确定主导节点选择方案。从区域内无功源控制灵敏度、无功裕度及主导节点的可观性出发,对分区结果进行评估验证。最后,以IEEE39节点系统为例进行仿真分析,验证了所提方法的有效性和优越性。     

MODES,STATE VARIABLES,AND LOCAL BIFURCATIONS IN POWER SYSTEM DYNAMICS

ABSTRACT

This paper aims at identifying the physical nature of local bifurcations in power system dynamics. Each local bifurcation is associated with a mode of the linearized model around an equilibrium point. The eigenvalue corresponding to this mode can be linked to the state variables of the linearized model. Energy propagation arguments are used to define measures of coupling between a mode and state variables. These arguments are also related to the quantification of observability and controllability of modes. Coupling measures based on the above arguments and calculated from the entries of right and left eigenvectors of the state matrix are used to associate a mode to one or more of the state variables. A troublesome or critical mode crosses into the right half plane as a given parameter is varied. The mode is related to a bifurcation, and is coupled to some state variables. Thus a link is established between the bifurcation and the state variables, and since the state variables of the models of power systems have a physical meaning, the link is made through them to components of the system. We study a fourth order system which exhibits voltage collapse. We show that the real mode associated with the bifurcation is strongly coupled to the voltage magnitude at the load node. We also study a single-machine infinite busbar system which exhibits a Hopf bifurcation. A multimachine power system is also considered where more than one Hopf bifurcation is exhibited.     

含分布式电源的配电网分区和主导节点选取方法研究

为了应对分布式电源及柔性负荷大规模接入给配电网电压控制带来的困难,提出一种基于改进的粒子群优化算法的配电网分区方法,通过分区使分布式电源主动参与到配电网的控制过程中。引入评价分区结果优劣的四个指标,综合考虑分区内无功/有功储备、区域内强耦合和区域间弱耦合,并结合无功/有功可控性和电压可观性,构建主导节点选取公式。该方法将分区转化为优化求解问题,使得分区结果能根据分布式电源运行参数和配电网结构的变化进行灵活调整。通过仿真证明了该分区科学合理,在该方法基础上的分布式电压控制能有效缓解柔性负荷大规模接入所带来的节点电压越限等问题,具有实际应用价值。     

基于系统留数矩阵的广域PSS设计

电力系统低频振荡严重影响电网的稳定运行。传统的电力系统稳定器（PSS）采用本地机组信号,不能有效提高系统阻尼。广域测量系统使得电力系统向广域观测与控制的方向发展。文中使用反馈信号机组和控制机组相分离的设计思想,提出根据转子角频率对振荡模式的能观度选取反馈信号机组,根据PSS输出信号对振荡模式的能控度选取控制机组,从而构成广域PSS控制回路。基于留数矩阵的分析表明了这种广域PSS控制策略具有比本地信号PSS更强的能观性和能控性。最后给出了基于留数的广域PSS参数设计方法。16机系统上的仿真结果表明,基于留数矩阵进行设计的广域PSS能更有效地抑制系统低频振荡,提高互联电网的稳定性。     

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.     

On unreliability of exponential load models

This paper addresses the issue of power system load modelling. In power system and voltage stability studies, only aggregate loads at high voltage buses are needed. Due to difficulties in determining appropriate load models at high voltage buses, the classical exponential static load model is still often used. This paper shows that under certain circumstances, e.g. presence of power factor correcting the condensators, the standard approach of modelling real and reactive power as exponentially dependent on voltage may lead to unrealistic values of reactive power voltage exponents. The development of load models where real and apparent power are exponentially dependent on voltage is proposed to avoid unreliability of the reactive power voltage exponent. The constant power factor relationship between real and reactive power is also discussed. The error that could be made by assuming constant power factor of the load during voltage variation is estimated.     

具有自辨识功能的模糊自动电压无功控制装置

介绍了发电厂高压母线电压及发电厂总无功自动控制装置采用自辨识技术和模糊控制技术后,能在按电压曲线或远方指令运行的同时,有效地控制发电机的运行的运行参数,使发电机组工作在额定范围内。用电网调度自动化系统中的无功、电压优化控制软件对装有这种装置的电厂实行远方控制时,电网可以实现真正的无功电压优化控制。     

ACTIVE AND REACTIVE POWER FLOW FOR MULTIAREA SYSTEMS ON AUTOMATIC GENERATION CONTROL IN THE PRESENCE OF RAPIDLY CHANGING LOADS

ABSTRACT

In this paper, the control area dynamics of power systems on automatic generation control are revisited. The role of reactive power flow between regions under automatic generation control is studied by review of simulations in which the reactive power flow is modeled/not modelled. The effect of transmission line loading is also studied. Reactive power control is accomplished in part by automatic voltage regulation, and reactive power resides in a control “loop” much in the same way as active power resides in conventional automatic generation control. An important observation of the analysis is that short term phenomena (e.g., peaks in tie line flow after a load disturbance ΔP = j ΔQ) require the reactive loop in the model in order to obtain accurate response. Long term phenomena are less sensitive to reactive power modelling. Although these observations are well known qualitatively, examples in this paper help to quantify the role of reactive power in power flow dynamics. The main lesson learned is that there are cases in which reactive power flow must be modelled in order to accurately study active power control and flow. An emphasis in the paper is rapidly changing, heavy industrial loads. The multiarea case is considered.     

变电站电压无功控制范围的整定计算方法

现有变电站电压无功控制装置的控制策略主要是依据九区域图法。文中根据一天24h各负荷点的有功和无功负荷曲线进行全网离散无功优化计算，并以此为基础提出了变电站电压无功控制装置控制范围的整定计算方法。既考虑了对受控变压器低压侧母线电压和高压侧无功功率的最优变化曲线的跟踪，又顾及了减少变压器分接头动作次数的要求。用广州鹿鸣电网的计算结果和变电站电压无功控制装置的模拟试验结果验证了所提出的方法的合理性和有效性。     

Reactive power control for improving voltage profiles: A comparison between two decentralized approaches

This paper is concerned with a local regulation of the voltage profiles at buses where wind power distributed generators are connected. In particular, the aim of the work is to compare two voltage control methods: the first based on a sensitivity analysis and the second on the designing of a fuzzy control system. The two methods are tested by means of simulations on a real distribution system and the results indicate that both methods allow the voltage profiles to be regulated at the wind generator connection bus within voltage standard limits, by taking into account the capability curves of the wind generators. Nevertheless, the fuzzy method presents more advantage in comparison with the sensitivity method. In fact, (i) it provides a gentler action control with a lower reactive power consumption during control operations as the reactive power profile follows better the voltage variations; (ii) the design of the fuzzy controller is independent from the knowledge of network parameters and its topology.     

A new intelligent algorithm for online voltage stability assessment and monitoring

This paper presents a new approach for assessing power system voltage stability based on artificial feed forward neural network (FFNN). The approach uses real and reactive power, as well as voltage vectors for generators and load buses to train the neural net (NN). The input properties of the NN are generated from offline training data with various simulated loading conditions using a conventional voltage stability algorithm based on the L-index. The performance of the trained NN is investigated on two systems under various voltage stability assessment conditions. Main advantage is that the proposed approach is fast, robust, accurate and can be used online for predicting the L-indices of all the power system buses simultaneously. The method can also be effectively used to determining local and global stability margin for further improvement measures.