Security for space power systems

An initial system security monitor (SSM) developed for the power system of a proposed NASA permanently manned space station is presented. Four types of ranked disturbances or faults to the transmission system are successively corrected by remedial action of the SSM. After each corrective action, the SSM verifies the system change that has been implemented, then proceeds to examine lower-priority disturbances if they persist. The SSM authority is limited by the operating state of the system, and only one correction is permitted for each scan cycle. The disturbance signals are verified by the use of a state estimator. The small network size permits use of connectivity searches of the network topology for possible corrective actions and a load flow to predict the effect of corrective actions. The data scan rate is higher than for earth-based systems. Line switching actions are implemented within line cycles because thyristors are used as circuit breakers. The work presents examples of disturbance identification and remedial action that could also apply to earth-based power systems     

On-site safety evaluation for earth fault in mining power systems

Many mining power systems utilize ineffectively grounded sources to restrict the residual current of single-phase earth fault in order to reduce outage and shock hazard. In practice, with the system expansion, topology changing, and insulation aging, the potential residual current and zero-sequence voltage for earth fault vary dynamically and some arcing earth faults can easily cause over-voltage and induce multiple faults. In order to improve the system safety, on-site condition monitoring, safety evaluation, and preventive maintenance for earth fault are proposed in this paper. Some techniques for online parameters measurement based on resonance-measurement principle are presented. Power system operation states are classified into normal secure state, alert state, incipient fault state, and fault state. Some security enhancement methods (preventive action and remedial action) for each state are implemented. The prototype for safety evaluation has been developed and installed in industry power systems for several years.     

基于小波变换和分形理论的电能质量扰动监控系统

介绍了实时电能质量扰动监控系统的结构,详细说明了该系统硬件和软件各个构成模块的工作原理。为实现实时在线监控电能质量扰动,首先需检测出扰动信号,然后进行分析处理。在扰动检测模块中,采用自适应线性神经元实现了对各种扰动的检测,将检测出的扰动信号送入分类模块,采用离散小波多分辨率分析提取不同尺度下的能量分布特征,同时采用分形几何学提取局部方差维数,将二者结合共同构成扰动信号的特征矢量。将提取的特征矢量送入概率神经网络实现网络训练和扰动分类。通过模拟数据测试,该系统的分类率可达到90%。另外,该系统是在CAN总线变电站自动化系统上实现的,通过调整数据的传输格式也可将其应用到其它传输平台的变电站,实现对电能质量扰动的监控。     

A multiple strategic evaluation for fault detection in electrical power system

A new combined formalism is described to detect faults in electrical power systems in this work. The aim of fault detection is to monitor and control the system which guarantees stable supply of electrical energy to consumers. To solve this problem, it has been proposed an integrated use between state space, optimization and analytical method by establishing a methodology for its completion. Initially, state space method performs through deviations signaled by its output, an analysis with relation to direction of fault in each component of power system. Though, state space is in possession of as much information as possible coming from output signals, can identify direction where disturbance has been occurred. Finally, the processing is done by optimization and analytical methods, which analyze the exact fault type and locations, using these signs, corrective action have been taken. By using proposed technique, the tasks of fault detection, classification, and location are accomplished. The common case of faults occurring in the electrical power system has been illustrated to verify the proposed methodology.     

Simulation of load shedding as a corrective action against voltage collapse

The evolution of a lot voltage collapse scenarios on large power systems can be alleviated by controlled load shedding associated with rearrangement of generator outputs. Dynamic simulation of power system mid-term voltage response is required to adequately determine the impact of load shedding actions and to design appropriate control systems. The dynamic simulation must take into account load-voltage behaviour, transformer tap changer controls and limits, generator overexcitation limiters, automatic generation control and system protective devices. This paper demonstrates the impact of load shedding as a corrective action through simulation of the system dynamic response to a disturbance. Whilst load shedding can stabilise a system, rigorous techniques are required to identify the magnitude and location of shedding. The application of voltage modal analysis in combination with the determination of reactive power margins in resolving this problem is demonstrated.     

A corrective control for angle and voltage stability enhancement onthe transient time-scale

This paper deals with the development of a nonlinear programming methodology for evaluating load shedding (LS) as a corrective action to improve the dynamic security of power systems when angle or voltage instability is detected. A centralized corrective control is developed, on the basis of online DSA computations, in order to ensure corrective actions when a threatening contingency actually occurs. The algorithm is implemented and tested on the actual Italian grid managed by ENEL S.p.A     

神经网络电力系统稳定器的设计与实现

本文利用ＢＰ型神经网络逼近受控系统逆动态具体设计实现了一种神经网络电力系统稳定器（ＮＮＰＳＳ）。所设计的ＮＮＰＳＳ以四层ＢＰ网络为主体，采用非线性ＰＳＳ的控制响应构成样本进行训练并通过了单、多机仿真的检验。文章深入分析了ＮＮＰＳＳ的性能和特点，重点探讨了设计过程中存在的各种具体问题。ＮＮＰＳＳ装置采用分层结构实现。动模实验结果证实了ＮＮＰＳＳ应用于真实工业环境的可行性。所设计的ＮＮＰＳＳ装置显著改善了电力系统的动、静态稳定性能并为各种大、小扰动提供了有效的阻尼作用。     

An approach to corrective control of voltage instability usingsimulation and sensitivity

This paper deals with the diagnosis of power system voltage collapse situations, following large disturbances and/or load increases. A method is proposed to identify the set of buses where load restoration is responsible for the collapse and to determine the corresponding corrective actions. It has been implemented in a fast voltage stability simulator, using sensitivity techniques. Tap changer blocking and load shedding are illustrated on a real-life 410-bus system     

Dynamic security corrective control by UPFCs

This paper deals with the development of a nonlinear programming methodology for evaluating corrective actions to improve the dynamic security of power systems when transient instability is detected. Remedial actions are implemented by exploiting the fast response of unified power flow controllers (UPFCs). The algorithm is implemented and tested on the Italian grid     

Grounding transformer applications and associated protectionschemes

The use of grounding transformers on three-phase ungrounded governmental, industrial, and commercial distribution systems is addressed. Ground-fault protection schemes that provide selective and reasonably fast tripping are often incorporated with these grounding transformers. The authors first review the state of the art of grounding transformers to assist electric power systems engineers in the proper understanding of the use and applications of these devices and then discuss two governmental system case studies illustrating improperly applied grounding transformers and/or associated ground-fault protection systems. In addition, they describe remedial actions taken to correct these deficiencies. The objective in presenting improper grounding transformer applications is to highlight protection concerns that are often ignored. It is stressed that a single grounding transformer is not adequate for use with a multibus configuration. A unique protection scheme for use on a multibus arrangement is presented     

Optimal corrective actions for power systems using multi-objective genetic algorithms

In this paper, optimal corrective control actions are presented to restore the secure operation of power system for different operating conditions. Genetic algorithm (GA) is one of the modern optimization techniques, which has been successfully applied in various areas in power systems. Most of the corrective control actions involve simultaneous optimization of several objective functions, which are competing and conflicting each other. The multi-objective genetic algorithm (MOGA) is used to optimize the corrective control actions. Three different procedures based on GA and MOGA are proposed to alleviate the violations of the overloaded lines and minimize the transmission line losses for different operation conditions. The first procedure is based on corrective switching of the transmission lines and generation re-dispatch. The second procedure is carried out to determine the optimal siting and sizing of distributed generation (DG). While, the third procedure is concerned into solving the generation-load imbalance problem using load shedding. Numerical simulations are carried out on two test systems in order to examine the validity of the proposed procedures.     

Two-layer supervisory controller-based thyristor-controlled braking resistor for transient stability crisis

In this paper, a supervisory two-layer control structure is synthesized with network elements as control means for thyristor-controlled braking resistor (BR) of multimachine power system operating in transient emergency state. This creates a multiple local feedback controller that can be realistically implemented using only local measurements and whose performance is consistent with respect to changes in network configuration, loading, and power transfer conditions. Following a major disturbance, the rotor angle and rotor speed of each generator unit are determined and the firing angle of the thyristor switch associated with the BR is computed by the local controllers. By controlling the firing angle of the thyristor, the BR controls the accelerating power in each generator and, thus, enhances the stability margins and damping oscillations. Since the local controllers rely only on measurements particular to their own subsystem, interconnection effects and the nonlinearities introduced by them are accounted for by the supervisory layer. The two-layer controller was tested on the IEEE Western States Coordinating Council (WSCC) test system. Results show that the controller is capable of bringing the system under control when starting with inherently unstable conditions, even when the severity of the disturbances is increased.     

Performance and reliability of electrical power grids under cascading failures

The stability and reliability of electrical power grids are indispensable to the continuous operation of modern cities and critical for preparedness, response, recovery and mitigation in emergence management. Because present power grids in China are often running near their critical operation points, they are especially vulnerable and sensitive to external disturbances such as hurricanes, earthquakes and terrorist attacks, which may trigger cascading failures or blackouts. This paper describes a quantitative investigation of the stability and reliability of power grids with a focus on cascading failures under external disturbances. The 118-bus (substation) power network in Hainan, China is employed as a case study to investigate the risk of cascading failure of the regional power grids. System performance and reliability of the power grids are evaluated under two hypothetical scenarios (seismic impact and intentional disturbance) that could trigger cascading failures. By identifying the most vulnerable (critical) edges and nodes, the robustness of the power network is evaluated under the triggered cascading failures. It is found that the system reliabilities could decline as much as 95% during the triggered cascading failure. This paper also explores the use of concepts from modern complex network theories such as state transition graph and characteristic length to understand the complex mechanism of cascading failures. The findings could be useful for power industries and emergency managers to evaluate the vulnerability of power systems, understand the risk of blackout induced by cascading failures, and improve the resilience of power systems to external disturbances.     

Potential DC system support to enhance AC system performance in theWestern United States

The potentials of utilizing the controllability of DC lines to reduce the complexity of remedial schemes or to increase the stability performance of a system so that the impact of existing and future projects on nonproject systems can be minimized are explored. Less costly and perhaps more environmentally acceptable alternatives of utilizing power transfer overloads of the two existing DC lines of the Western Systems Coordinating Council (WSSC) in the Western US are presented. In addition, a concept to overload one DC line for the outage of the other DC line is investigated. Reducing the complexity of the remedial action scheme would be definite benefit to the WSCC network operating reliability objective     

Online preventive dynamic security of isolated power systems usingdecision trees

In this paper, the application of decision trees (DTs) for online preventive dynamic security of isolated power systems is presented. DTs provide the necessary computational speed for online performance and the flexibility of providing preventive control. Emphasis is placed on the online use of the DTs to test the dynamic security of each generation dispatch scenario and thus to provide corrective advice via generation redispatch. Moreover, the algorithm implemented provides the flexibility of displaying the cost of each redispatch. In this way, the method can help in objective decision-making. Results from the application of the system on actual load series from the island of Crete, where the proposed system is in trial operation, are presented     

关于紧急控制与校正控制优化和协调的评述

闭环离散控制是电力系统在大扰动后保持运行可靠性的主要手段,包括以故障驱动的紧急控制及由受扰轨迹驱动的校正控制。比较这2种方式的控制准则、动作时机、控制精度及代价,以及切机、切负荷和解列措施。若用紧急控制来应对小概率高风险的不确定因素,则在大多情况下将发生过控;若用校正控制来应对概率大的不确定因素,则机会成本很高。强调基于风险概念来协调这2种在物理和经济特性上都互补的控制方式,并提出解耦优化 — 聚合协调的求解方法。     

A study of the application of fast valving and braking resistor for an intelligent SPS

There are some kinds of remedial action schemes that deal with sudden disturbance situations in a transmission network. Generator tripping with an SPS (Special Protection System) is one of the remedial action schemes, and it is a powerful and effective method of solving transient stability problems. When an extra-high-voltage transmission line that is transferring a large amount of power is tripped, generators designated by a SPS are tripped to prevent over-speeding of the generators. However, the tripping of generators can cause difficulties in the power system operation, and if large amounts of power are tripped, load shedding can occur. Therefore, it is desirable that a minimum number of generators is tripped in required stable conditions. For this purpose, an intelligent SPS is currently being developed to reduce the number of generator tripping events. In this paper, an application method involving fast valving and a braking resistor are analyzed as suitable action schemes for a SPS.     

现代电网电力扰动数据分析与主动应用

随着现代电网中反映供用电双方兼容性的电力扰动问题日益突出,如何有效利用监测数据对于科学认知、理解和解决电力扰动相关难题以及电网安全稳定运行与营商环境改善具有重要意义。为此,不同于传统电能质量分析方法在扰动发生后被动采取补救性的分析和治理措施,文中提出在扰动发生、发展和影响之前主动挖掘电力扰动监测数据独特暂态信息以进行"感知-预警-诊断-服务"的主动应用框架。从这4个层面阐述了基于电力扰动数据分析进行敏感负荷辨识、风险预警、隐性故障诊断和优质电力运营模式等不同应用途径的研究价值、主要思路和关键难题。     

Diagnosing Power Quality-Related Computer Problems

Total reliance on sensitive electronic systems for such important functions as data processing, communications, and process control is now a way of life in our commercial, industrial, and governmental activities. This development has necessitated a new concern toward the quality of the electric power supply. Intermittent power disturbances, capable of disrupting electronic equipment are inherent to both commercial and industrial power systems. Any disruption causing downtime and financial loss, power-related or otherwise, is likely to precipitate a study to determine appropriate corrective actions. Unfortunately, analyzing the cause and effect of power disturbances can be difficult, particularly when sophisticated computer systems are involved. The ultimate purpose is to assist the electronic equipment users in reducing power-related downtime.     

一种电力系统稳定性动态分析的统一方法

为了解决电压稳定问题,基于电力系统动态分析的微分代数模型,提出了一种有效的动态稳定分析和失稳类型判别方法。利用带预测-校正步骤的延拓算法追踪平衡解流形,并采取考虑平衡解流形曲率大小的自适应策略控制步长,在平衡解流形曲率较小处采用较大步长,而在平衡解流形曲率较大处采用较小步长;在计及元件动态特性的基础上,利用小扰动法在每个平衡点分析电力系统的动态稳定性,并用数值摄动法计算状态矩阵;利用状态变量的模式参与因子判断系统的动态失稳类型。使用本文所提方法对New England 10机39节点系统进行了仿真分析,实验结果证明了本方法的有效性和实用性。