Cascading failures involve a failure in one system component that triggers failures in successive system components, potentially leading to system wide failures. While frequently used fault tolerant techniques can reduce the severity and the frequency of such failures, they continue to occur in practice. To better understand how failures cascade, we have conducted a qualitative analysis of 55 cascading failures, described in 26 publicly available incident reports. Through this analysis we have identified 16 types of cascading mechanisms (organized into eight categories) that capture the nature of the system interactions that contribute to cascading failures. We also discuss three themes based on the observation that the cascading failures we have analyzed occurred in one of three ways: a component being unable to tolerate a failure in another component, through the actions of support or automation systems as they respond to an initial failure, or during system recovery. We believe that the 16 cascading mechanisms we present and the three themes we discuss, provide important insights into some of the challenges associated with engineering a truly resilient and well-supported system.

京都大学 / 0048 / 新制・課程博士 / 博士(情報学) / 甲第20028号 / 情博第623号 / 新制||情||108(附属図書館) / 33124 / 京都大学大学院情報学研究科社会情報学専攻 / (主査)教授 石田 亨, 教授 多々納 裕一, 教授 山本 章博 / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DFAM

Due to power system deregulations, transmission expansion not keeping up with the load growth, and higher frequency of natural hazards resulting from climate change, major blackouts are becoming more frequent and are spreading over larger regions, entailing higher losses and costs to the economy and the society of many countries in the world. Large-scale blackouts typically result from cascading failure originating from a local event, as typified by the 2003 U.S.-Canada blackout. Their mitigation in power system planning calls for the development of methods and algorithms that assess the risk of cascading failures due to relay over-tripping, short-circuits induced by overgrown vegetation, voltage sags, line and transformer overloading, transient instabilities, voltage collapse, to cite a few. How to control the economic losses of blackouts is gaining a lot of attention among power researchers. In this research work, we develop new Monte Carlo methods and algorithms that assess and manage the risk of cascading failure in composite reliability of deregulated power systems. To reduce the large computational burden involved by the simulations, we make use of importance sampling techniques utilizing the Weibull distribution when modeling power generator outages. Another computing time reduction is achieved by applying importance sampling together with antithetic variates. It is shown that both methods noticeably reduce the number of samples that need to be investigated while maintaining the accuracy of the results at a desirable level. With the advent of microgrids, the assessment of their benefits in power systems is becoming a prominent research topic. In this research work, we investigate their potential positive impact on power system reliability while performing an optimal coordination among three energy sources within microgrids, namely renewable energy conversion, energy storage and micro-turbine generation. This coordination is modeled when applying sequential Monte Carlo simulations, which seek the best placement and sizing of microgrids in composite reliability of a deregulated power system that minimize the risk of cascading failure leading to blackouts subject to fixed investment budget. The performance of the approach is evaluated on the Roy Billinton Test System (RBTS) and the IEEE Reliability Test System (RTS). Simulation results show that in both power systems, microgrids contribute to the improvement of system reliability and the decrease of the risk of cascading failure. / Ph. D.

Understanding the behavior of complex, large-scale, interconnected systems in a rigorous and structured manner is one of the most pressing scientific and technological challenges of current times. These systems include, among many others, transportation and communications systems, smart grids and power grids, financial markets etc. Failures of these systems have potentially enormous social, environmental and financial costs. In this work, we investigate the failure mechanisms of load-sharing complex systems. The systems are composed of multiple nodes or components whose failures are determined based on the interaction of their respective strengths and loads (or capacity and demand respectively) as well as the ability of a component to share its load with its neighbors when needed. Each component possesses a specific strength (capacity) and can be in one of three states: failed, damaged or functioning normally. The states are determined based on the load (demand) on the component. We focus on two distinct mechanisms to model the interaction between components strengths and loads. The first, a Loss of Strength (LOS) model and the second, a Customer Service (CS) model. We implement both models on lattice and scale-free graph network topologies. The failure mechanisms of these two models demonstrate temporal scaling phenomena, phase transitions and multiple distinct failure modes excited by extremal dynamics. We find that the resiliency of these models is sensitive to the underlying network topology. For critical ranges of parameters the models demonstrate power law and exponential failure patterns. We find that the failure mechanisms of these models have parallels to failure mechanisms of critical infrastructure systems such as congestion in transportation networks, cascading failure in electrical power grids, creep-rupture in composite structures, and draw-downs in financial markets. Based on the different variants of failure, strategies for mitigating and postponing failure in these critical infrastructure systems can be formulated.

於歷史事件中我們知道電力網路中些許的傷害會導致大停電事件的發生，而電力網路的安全是學者們一直以來關心的議題，複雜網路發展至今，已有不少研究人員利用複雜網路的方法來分析電力網路，本論文即利用複雜網路的分析方法，分析台灣電力網路脆弱性，並提出一新穎的保護策略。 文章中引用三種分析方法分析台灣電力網路的脆弱性，第一為根據網路效率(efficiency of complex network)，第二為根據靜態負載的連鎖性失效(static load cascading failure)，第三為根據動態負載的連鎖性失效(dynamic load cascading failure)，三種脆弱性分析的方法得到結果的交集處，在文中被認為是電力網路中最具風險的變電站。根據靜態負載的連鎖性失效的結果，我們在不改變網路結構和整體網路的負載量之下，提出一新穎的安全策略，試圖降低靜態負載時連鎖性失效帶來的傷害，並分析此安全策應用至ER隨機網路(ER random network)、BA無尺度網路(BA scale-free network)和台灣電力網路，且量化其結果。 三種脆弱分析的結果皆有共通的部份，表示台灣電力網路在拓樸分析下並不堅固，其結果顯示此安全策略是有效的。本篇論文分析的網路均是無權重也無方向性的網路，如此，未來在網路脆弱性與安全策略的研究還有進一步的研究空間。 / From blackout events in history, we know the damage of a few substations or transmission lines can lead to a big blackout, and the robustness of power grids are always a great concern. Recently the topological analyses of power grid network have developed rapidly and its achievements have become a center of attention. This thesis aims to investigate the vulnerability of Taiwan’s power grid through topological analyses and propose a novel protection strategy. This thesis introduces three methods to investigate the vulnerability of Taiwan’s power grid. The first method is based on efficiency of complex network. The second method is based on static load cascading failure. The third method is based on dynamic load cascading failure. The common results of the three results are considered the high risks in Taiwan’s power grid. According to the static load cascading failure, we propose a protection strategy against cascading failure without changing the structure of network and the whole load of the network, and then demonstrate the effectiveness of this strategy on an ER random network, a BA scale-free network and Taiwan’s power grid numerically. The three methods having the common results, one may say Taiwan’s power grid isn’t robust under the topological analyses, and our protection strategy may be useful for decreasing the damage after cascading failure.

現實社會中，有時會發生大規模的停電事件，一經調查後才瞭解，只是因為一根電纜線、一根高壓電塔或是一個變電站故障導致，令人好奇電力系統的運作模式。因此藉由拓蹼網路的特性去研究電力系統的安全性及穩定性，一直受到人們的關注和討論。本篇論文利用動態負載去模擬現實中的電流，並藉由連鎖性失效的攻擊策略對於網路效率的影響，去判斷其網路的脆弱性分析。 本論文利用網路效率的(efficiency)影響來作為網路脆弱性(vulnerability)的分析，藉由在靜態負載中連鎖性失效的模型(static load cascading failure model)和動態負載中連鎖性失效的模型(dynamic load cascading failure model)來判斷及討論，其中動態負載為不同發電量及消耗量的狀況，藉此觀察台灣的電力網路系統在狀況下的結果。 此外以攻擊方來思考，單一攻擊、多重攻擊和策略性的多重攻擊的狀況底下，網路效率的變化，並將此運用在台灣電力網路系統上，發現台灣電力網路相當脆弱。 / In the real world, sometimes there will be black out (power shortage) happening in large scale areas. After investigating, we found out that it was cause by either a broken transmission line, a high-voltage tower or a substation. We are very interesting to know how the operation scheme of power system works. Therefore, we study the safety and the stability in power system by the characteristics of topology network. This subject is always concerned and discussed by the society. The thesis use dynamic load to simulate current in real world. We foc us effect on network efficiency by the strategy of cascading failure, analyze the vulnerability of network. The thesis use the variation of network efficiency as index to analyze network. According to determining and discussing by static load cascading failure model and dynamic load cascading failure model, dynamic load is the situation in different amount of generating and consumption of electrical energy. We used the data to observe the result of Taiwan power system under the situation. In addition to, changing of internet efficiency by single attack, multi-attack and strategic multi-attack in attacker method of thinking. We found that Taiwan power system is weak after we used the statistics on it.

這篇論文主要可以分成兩個部分。第一部分，我們整理了關於複雜網路的初步研討。最重要的特性有：小世界網路、無尺度度分布。並且介紹了三種模型：BA 模型、EBA模型，以及W-S small world model。接著對於一份實際的社會網路資料—台灣業餘桌球選手對戰網路，做網路的結構分析，試驗其是否具有上述的兩種特性。透過兩種可以模擬出無尺度度分布特性的模型：BA以及EBA模型。我們藉由這兩種模型模擬的結果，以及和競賽網路的比較，試者去闡述模型與理論間為何有些相似，卻又如此不同。並討論了賽制設計對於結構的影響。 在第二部分裡，我們回顧了一些對於網路的拓樸性效率以及可靠度效率的研討，並且討論了兩種不同負載定義下的連鎖故障行為。最後我們使用其中三種方法：拓樸性效率脆弱性、參與中間度(betweenness)過載引發的連鎖性故障行為，以及電力網路的動態電流變化造成的連鎖性故障，對於一個假想的電網做傳輸線的弱點排序。其中由動態電流過載(transient dynamic overload)造成的連鎖性故障可以視為一個簡化後的電力動態網路模型，藉由這三者間排序的不同，我們可以看到複雜網路分析以及基於電力網路傳輸特性所模擬的結果差異。 / This thesis can be divided into two parts. In the first part, we review some basic properties of the complex networks. The most important features are: small world networks and scale-free degree distribution. Then, we introduce three complex models : BA model, EBA model, and W-S small world model. Next, we analyze a real data—CTTC network to test if it has the features we have mentioned above. By the EBA and BA model simulations, we try to illustrate why there are some similarities between the simulations and real data, but they are still so different in most of aspects. In the second part, we review the definitions of the topology and reliable efficiency of a network structure. Next, we discuss two cascading failure model based on different definitions of load of a transmission line in a power grid. Finally, we use three different ways: topology efficiency vulnerability, cascading failure triggered by betweenness overload, and cascading failure triggered by the transient dynamics overload to test the vulnerability of edges in an assuming power grid. The cascading failure triggered by the transient dynamic overload can be viewed as a simplified power flow model. We sort the most vulnerable edges in three different ways. By this, we can observe the difference of the vulnerability analysis based on the complex network and the characteristic of the power transmission..

Water distribution systems, electrical power transmission systems, and other civil infrastructure systems are essential to the smooth and stable operation of regional economies. Since the functions of such infrastructure systems often are inter-dependent, the systems sometimes suffer unforeseen functional disruptions. For example, the widespread power outage due to the malfunction of an electric power substation, which occurred in the northeastern United States and parts of Canada in August 2003, interrupted the supply of water to several communities, leading to inconvenience and economic losses. The sequence of such failures leading to widespread outages is referred to as a cascading failure. Assessing the vulnerability of communities to natural and man-made hazards should take the possibility of such failures into account. In seismic risk assessment, the risk to a facility or a building is generally specified by one of two basic approaches: through a probabilistic seismic hazard analysis (PSHA) and a stipulated scenario earthquake (SE). A PSHA has been widely accepted as a basis for design and evaluation of individual buildings, bridges and other facilities. However, the vulnerability assessment of distributed infrastructure facilities requires a model of spatial intensity of earthquake ground motion. Since the ground motions from a PSHA represent an aggregation of earthquakes, they cannot model the spatial variation in intensity. On the other hand, when a SE-based analysis is used, the spatial correlation of seismic intensities must be properly evaluated. This study presents a new methodology for evaluating the functionality of an infrastructure system situated in a region of moderate seismicity considering functional interactions among the systems in the network, cascading failure, and spatial correlation of ground motion. The functional interactions among facilities in the systems are modeled by fault trees, and the impact of cascading failures on serviceability of a networked system is computed by a procedure from the field of operations research known as a shortest path algorithm. The upper and lower bound solutions to spatial correlation of seismic intensities over a region are obtained.