Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> We study the response of complex networks subject to attacks on vertices and edges. Several existing complex network models as well as real-world networks of scientific collaborations and Internet traffic are numerically investigated, and the network performance is quantitatively measured by the average inverse geodesic length and the size of the largest connected subgraph. For each case of attacks on vertices and edges, four different attacking strategies are used: removals by the descending order of the degree and the betweenness centrality, calculated for either the initial network or the current network during the removal procedure. It is found that the removals by the recalculated degrees and betweenness centralities are often more harmful than the attack strategies based on the initial network, suggesting that the network structure changes as important vertices or edges are removed. Furthermore, the correlation between the betweenness centrality and the degree in complex networks is studied. <s> BIB001 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> We live in a modern world supported by large, complex networks. Examples range from financial markets to communication and transportation systems. In many realistic situations the flow of physical quantities in the network, as characterized by the loads on nodes, is important. We show that for such networks where loads can redistribute among the nodes, intentional attacks can lead to a cascade of overload failures, which can in turn cause the entire or a substantial part of the network to collapse. This is relevant for real-world networks that possess a highly heterogeneous distribution of loads, such as the Internet and power grids. We demonstrate that the heterogeneity of these networks makes them particularly vulnerable to attacks in that a large-scale cascade may be triggered by disabling a single key node. This brings obvious concerns on the security of such systems. <s> BIB002 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> We describe new analytical techniques to help mitigate the disruptions to electric power grids caused by terrorist attacks. New bilevel mathematical models and algorithms identify critical system components (e.g., transmission lines, generators, transformers) by creating maximally disruptive attack plans for terrorists assumed to have limited offensive resources. We report results for standard reliability test networks to show that the techniques identify critical components with modest computational effort. <s> BIB003 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> We present an analysis of the topological structure and static tolerance to errors and attacks of the September 2003 actualization of the Union for the Coordination of Transport of Electricity (UCTE) power grid, involving thirty-three different networks. Though every power grid studied has exponential degree distribution and most of them lack typical small-world topology, they display patterns of reaction to node loss similar to those observed in scale-free networks. We have found that the node removal behavior can be logarithmically related to the power grid size. This logarithmic behavior would suggest that, though size favors fragility, growth can reduce it. We conclude that, with the ever-growing demand for power and reliability, actual planning strategies to increase transmission systems would have to take into account this relative increase in vulnerability with size, in order to facilitate and improve the power grid design and functioning. <s> BIB004 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> The vulnerability of real-life networks subject to intentional attacks has been one of the outstanding challenges in the study of the network safety. Applying the real data of the US power grid, we compare the effects of two different attacks for the network robustness against cascading failures, i.e., removal by either the descending or ascending orders of the loads. Adopting the initial load of a node j to be Lj !" kj# Rm2Cj km$% a with kj and Cj being the degree of the node j and the set of its neighboring nodes, respectively, where a is a tunable parameter and governs the strength of the initial load of a node, we investigate the response of the US power grid under two attacks during the cascading propagation. In the case of a < 0:7, our investigation by the numerical simulations leads to a counterintuitive finding on the US power grid that the attack on the nodes with the lowest loads is more harmful than the attack on the ones with the highest loads. In addition, the almost same effect of two attacks in the case ofa ! 0:7 may be useful in furthering studies on the control and defense of cascading failures in the US power grid. <s> BIB005 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Abstract Power transmission grids play a crucial role as a critical infrastructure by assuring the proper functioning of power systems. In particular, they secure the loads supplied by power generation plants and help avoid blackouts that can have a disastrous impact on society. The power grid structure (number of nodes and lines, their connections, and their physical properties and operational constraints) is one of the key issues (along with generation availability) to assure power system security; consequently, it deserves special attention. A promising approach for the structural analysis of transmission grids with respect to their vulnerabilities is to use metrics and approaches derived from complex network (CN) theory that are shared with other infrastructures such as the World-Wide Web, telecommunication networks, and oil and gas pipelines. These approaches, based on metrics such as global efficiency, degree and betweenness, are purely topological because they study structural vulnerabilities based on the graphical representation of a network as a set of vertices connected by a set of edges. Unfortunately, these approaches fail to capture the physical properties and operational constraints of power systems and, therefore, cannot provide meaningful analyses. This paper proposes an extended topological approach that includes the definitions of traditional topological metrics (e.g., degrees of nodes and global efficiency) as well as the physical/operational behavior of power grids in terms of real power-flow allocation over lines and line flow limits. This approach provides two new metrics, entropic degree and net-ability, that can be used to assess structural vulnerabilities in power systems. The new metrics are applied to test systems as well as real power grids to demonstrate their performance and to contrast them with traditional, purely topological metrics. <s> BIB006 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Modern network-like systems are usually coupled in such a way that failures in one network can affect the entire system. In infrastructures, biology, sociology, and economy, systems are interconnected and events taking place in one system can propagate to any other coupled system. Recent studies on such coupled systems show that the coupling increases their vulnerability to random failure. Properties for interdependent networks differ significantly from those of single-network systems. In this article, these results are reviewed and the main properties discussed. <s> BIB007 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Power system vulnerability is a key concern in modern societies and many efforts have been devoted to its analysis. Recently, the wide domain of complex networks that can be used to assess the vulnerability of network systems has been applied to power grids as well with focus on the topological vulnerability of the transmission systems based on its structure in terms of the nodes and their connection patterns. However, a pure topological approach, as proposed in the current literature, fails in capturing the specific features of power systems. The authors propose an extended topological approach, which can incorporate several important features of the power grids such as flow paths, line flow limits and gen/load bus distribution. Also, a set of new metrics able to provide an assessment of the system vulnerability is defined. ‘Net-ability’ measures the aptitude of the grid in transmitting power from generation to load buses efficiently. ‘Path redundancy’ assesses the available redundancy in terms of paths in transmitting power from a generation to a load bus based on entropy. Based on the previous two metrics, the authors introduce a third metric, the ‘survivability’, as a global metric to evaluate the aptitude of the network in assuring the possibility to match generation and demand in case of failures or attacks. The metrics proposed will be applied to different test systems and to a real-transmission system to illustrate their application and effectiveness. <s> BIB008 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Abstract Vulnerability analysis of power systems is a key issue in modern society and many efforts have contributed to the analysis. Complex network metrics for the assessment of the vulnerability of networked systems have been recently applied to power systems. Complex network theory may come in handy for vulnerability analysis of power systems due to a close link between the topological structure and physical properties of power systems. However, a pure topological approach fails to capture the engineering features of power systems. So an extended topological method has been proposed by incorporating several of specific features of power systems such as electrical distance, power transfer distribution factors and line flow limits. This paper defines, starting from the extended metric for efficiency named as net-ability, an extended betweenness and proposes a joint method of extended betweenness and net-ability to rank the most critical lines and buses in an electrical power grid. The method is illustrated in the IEEE-118-bus, IEEE-300-bus test systems as well as the Italian power grid. <s> BIB009 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Vulnerability analysis in power systems is a key issue in modern society and many efforts have contributed to the analysis. Recently, complex networks metrics, applied to assess the topological vulnerability of networked systems, have been used in power grids, such as the betweenness centrality. These metrics may be useful for analyzing the topological vulnerability of power systems because of a close link between their topological structure and physical behavior. However, a pure topological approach fails to capture the electrical specificity of power grids. For this reason, an extended topological method has been proposed by incorporating several electrical features, such as electrical distance, power transfer distribution, and line flow limits, into the pure topological metrics. Starting from the purely topological concept of complex networks, this paper defines an extended betweenness centrality which considers the characteristics of power grids and can measure the local importance of the elements in power grids. The line extended betweenness is compared with the topological betweenness and with the averaged power flow on each line over various operational states in the Italian power grid. The results show that the extended betweenness is superior to topological betweenness in the identification of critical components in power grids and at the same time could be a complementary tool to efficiently enhance vulnerability analysis based on electrical engineering methods. <s> BIB010 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> This paper addresses the attack modeling using vulnerability of information, communication and electric grid network. Vulnerability of electric grid with incomplete information has been analyzed using graph theory based approach. Vulnerability of information and communication (cyber) network has been modeled utilizing concepts of discovery, access, feasibility, communication speed and detection threat. Common attack vector based on vulnerability of cyber and physical system have been utilized to operate breakers associated with generating resources to model aurora-like event. Real time simulations for modified IEEE 14 bus test case system and graph theory analysis for IEEE 118 bus system have been presented. Test case results show the possible impact on smart grid caused by integrated cyber-physical attack. <s> BIB011 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> While open communication infrastructures are embedded into smart grids to support vast amounts of data exchange, it makes smart grids vulnerable to cyber attacks. In this paper, we investigate the effects of Denial-of-Service (DoS) attacks on load frequency control (LFC) of smart grids. In contrast with existing works, we consider the problem that how DoS attacks affect the dynamic performance of a power system. The state space model of power systems under DoS attacks is formulated as a switched system. By applying switched system theories, the existence of DoS attacks that make the dynamics of a power system unstable is proved. A two-area power system is used to conduct case studies. The dynamic performance of the power system, such as convergence and steady-state errors, is compared under different DoS attack scenarios. It is shown that the dynamic performance of the power system is affected strongly if the adversaries launch DoS attacks before the dynamics of the power system converge. <s> BIB012 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> This paper proposes a maximum-flow-based complex network approach for the analysis of the vulnerability of power systems. A new centrality index is proposed, taking into consideration the maximum flow from the source (generator) nodes to the sink (load) nodes, for assessing the network. The Max-Flow Min-Cut Theorem, also known as Ford-Fulkerson Theorem, is used for evaluating the capacity of links. The proposed methodology is then used to identify vulnerable lines of the IEEE 118 bus system and its effectiveness is demonstrated through simulation studies. <s> BIB013 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Security issues related to power grid networks have attracted the attention of researchers in many fields. Recently, a new network model that combines complex network theories with power flow models was proposed. This model, referred to as the extended model, is suitable for investigating vulnerabilities in power grid networks. In this paper, we study cascading failures of power grids under the extended model. Particularly, we discover that attack strategies that select target nodes (TNs) based on load and degree do not yield the strongest attacks. Instead, we propose a novel metric, called the risk graph, and develop novel attack strategies that are much stronger than the load-based and degree-based attack strategies. The proposed approaches and the comparison approaches are tested on IEEE 57 and 118 bus systems and Polish transmission system. The results demonstrate that the proposed approaches can reveal the power grid vulnerability in terms of causing cascading failures more effectively than the comparison approaches. <s> BIB014 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Most government, industry, and academic efforts to protect the power grid have focused on information security mechanisms for preventing and detecting attacks. In addition to these mechanisms, control engineering can help improve power grid security. <s> BIB015 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Cyber systems play a critical role in improving the efficiency and reliability of power system operation and ensuring the system remains within safe operating margins. An adversary can inflict severe damage to the underlying physical system by compromising the control and monitoring applications facilitated by the cyber layer. Protection of critical assets from electronic threats has traditionally been done through conventional cyber security measures that involve host-based and network-based security technologies. However, it has been recognized that highly skilled attacks can bypass these security mechanisms to disrupt the smooth operation of control systems. There is a growing need for cyber-attack-resilient control techniques that look beyond traditional cyber defense mechanisms to detect highly skilled attacks. In this paper, we make the following contributions. We first demonstrate the impact of data integrity attacks on Automatic Generation Control (AGC) on power system frequency and electricity market operation. We propose a general framework to the application of attack resilient control to power systems as a composition of smart attack detection and mitigation. Finally, we develop a model-based anomaly detection and attack mitigation algorithm for AGC. We evaluate the detection capability of the proposed anomaly detection algorithm through simulation studies. Our results show that the algorithm is capable of detecting scaling and ramp attacks with low false positive and negative rates. The proposed model-based mitigation algorithm is also efficient in maintaining system frequency within acceptable limits during the attack period. <s> BIB016 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Current smart power grids have open communication infrastructures to improve efficiency, reliability and sustainability of supply. However, their open communication architecture makes them vulnerable to cyber-attacks with potentially catastrophic consequences. Here for the first time, we propose a time-delay-switch (TDS) attack by introducing time delays in the dynamics of power systems. Such an attack will have devastating consequences on smart grids if no prevention measures are considered in the design of these power systems. We considered how a TDS attack affects the dynamic performance of a power system. To do this, we first formulated a state space model of a smart power grid system under TDS attack using a hybrid systems approach. Second, we prove by analysis and demonstrate by simulation examples how a TDS attack can be used to sabotage and destabilize a smart grid. <s> BIB017 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> The security issue of complex networks has drawn significant concerns recently. While pure topological analyzes from a network security perspective provide some effective techniques, their inability to characterize the physical principles requires a more comprehensive model to approximate failure behavior of a complex network in reality. In this paper, based on an extended topological metric, we proposed an approach to examine the vulnerability of a specific type of complex network, i.e., the power system, against cascading failure threats. The proposed approach adopts a model called extended betweenness that combines network structure with electrical characteristics to define the load of power grid components. By using this power transfer distribution factor-based model, we simulated attacks on different components (buses and branches) in the grid and evaluated the vulnerability of the system components with an extended topological cascading failure simulator. Influence of different loading and overloading situations on cascading failures was also evaluated by testing different tolerance factors. Simulation results from a standard IEEE 118-bus test system revealed the vulnerability of network components, which was then validated on a dc power flow simulator with comparisons to other topological measurements. Finally, potential extensions of the approach were also discussed to exhibit both utility and challenge in more complex scenarios and applications. <s> BIB018 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> This paper reviews the most relevant works that have investigated robustness in power grids using Complex Networks (CN) concepts. In this broad field there are two different approaches. The first one is based solely on topological concepts, and uses metrics such as mean path length, clustering coefficient, efficiency and betweenness centrality, among many others. The second, hybrid approach consists of introducing (into the CN framework) some concepts from Electrical Engineering (EE) in the effort of enhancing the topological approach, and uses novel, more efficient electrical metrics such as electrical betweenness, net-ability, and others. There is however a controversy about whether these approaches are able to provide insights into all aspects of real power grids. The CN community argues that the topological approach does not aim to focus on the detailed operation, but to discover the unexpected emergence of collective behavior, while part of the EE community asserts that this leads to an excessive simplification. Beyond this open debate it seems to be no predominant structure (scale-free, small-world) in high-voltage transmission power grids, the vast majority of power grids studied so far. Most of them have in common that they are vulnerable to targeted attacks on the most connected nodes and robust to random failure. In this respect there are only a few works that propose strategies to improve robustness such as intentional islanding, restricted link addition, microgrids and Energies 2015, 8 9212 smart grids, for which novel studies suggest that small-world networks seem to be the best topology. <s> BIB019 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> In a cascading power transmission outage, component outages propagate nonlocally; after one component outages, the next failure may be very distant, both topologically and geographically. As a result, simple models of topological contagion do not accurately represent the propagation of cascades in power systems. However, cascading power outages do follow patterns, some of which are useful in understanding and reducing blackout risk. This paper describes a method by which the data from many cascading failure simulations can be transformed into a graph-based model of influences that provides actionable information about the many ways that cascades propagate in a particular system. The resulting “influence graph” model is Markovian, in that component outage probabilities depend only on the outages that occurred in the prior generation. To validate the model, we compare the distribution of cascade sizes resulting from $n-2$ contingencies in a 2896 branch test case to cascade sizes in the influence graph. The two distributions are remarkably similar. In addition, we derive an equation with which one can quickly identify modifications to the proposed system that will substantially reduce cascade propagation. With this equation, one can quickly identify critical components that can be improved to substantially reduce the risk of large cascading blackouts. <s> BIB020 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> Power grids are often run near the operational limits because of increasing electricity demand, where even small disturbances could possibly trigger major blackouts. The attacks are the potential threats to trigger large-scale cascading failures in the power grid. In particular, the attacks mean to make substations/transmission lines lose functionality by either physical sabotages or cyber attacks. Previously, the attacks were investigated from substation-only/transmission-line-only perspectives, assuming attacks can occur only on substations/transmission lines. In this paper, we introduce the joint substation-transmission line perspective, which assumes attacks can happen on substations, transmission lines, or both. The introduced perspective is a nature extension to substation-only and transmission-line-only perspectives. Such extension leads to discovering many joint substation-transmission line vulnerabilities. Furthermore, we investigate the joint substation-transmission line attack strategies. In particular, we design a new metric, the component interdependency graph (CIG), and propose the CIG-based attack strategy. In simulations, we adopt IEEE 30 bus system, IEEE 118 bus system, and Bay Area power grid as test benchmarks, and use the extended degree-based and load attack strategies as comparison schemes. Simulation results show the CIG-based attack strategy has stronger attack performance. <s> BIB021 </s> Cyber-physical attacks and defences in the smart grid: a survey <s> Complex network (CN)-based attacks: <s> This technical note analyzes the effect of stealthy integrity attacks on Cyber-Physical Systems, which is modeled as a Stochastic Linear Time-Invariant (LTI) system equipped with a linear filter, a linear feedback controller and a $\chi^{2}$ failure detector. An attacker wishes to induce perturbation in the control loop by compromising a subset of the sensors and injecting an exogenous control input, without incurring detection from an anomaly detector. We show how the problem can be modeled, from the attacker's standpoint, as a constrained control problem and that the characterization of the maximum perturbation can be posed as reachable set computation, which we solve using ellipsoidal calculus. <s> BIB022
A notable number of investigations have been developed based on CN theories BIB019 . In general, the CN-based attacks have employed the interdiction as the means of attack; however, it is commonly assumed that the attacker does not possess the knowledge of the RT operational information (the analogue measurements); instead, the attacker can only access the information of system topology (the status measurements). Interconnected power grids are modelled as graphs of nodes (substations) and edges (lines) with designated physical properties. Topological and structural information are then utilised to identify the most-vulnerable components in the transmission system, which can be further visualised intuitively BIB014 BIB020 . A comparison has been summarised in Table 1 and illustrated examples on the IEEE 39-Bus New England Test System have been provided in Fig. 4 . The merit of investigations on CN-based attacks is two-fold: (i) the information required to launch a fully informed attack maybe inaccessible or incomplete; the obtained information may also become obsolete; (ii) a devastating attack does not necessarily rely on the full knowledge of the system dynamics; publicly and IET Cyber-Phys. Syst., Theory Appl., 2016, Vol. 1, Iss. 1, pp.    BIB015 BIB022   BIB011 BIB016 BIB012 BIB017 [24]  [26] BIB003 commercially available data can be used to construct effective schemes that cause catastrophic blackouts. Among the CN-based attacks, topological models have been proposed to analyse the risk of cascaded attacks BIB002 BIB001 BIB004 BIB005 BIB007 . On top of the topological models, hybrid models have integrated topological and electrical properties to integrate structural vulnerabilities in the transmission system BIB006 BIB008 BIB010 BIB009 BIB013 . Cascaded attack schemes on substations and transmission lines based on the hybrid models have been proposed BIB018 BIB021 .