Yakup Koç

The impact of the topology on cascading failures in a power grid model

Cascading failures are one of the main reasons for large scale blackouts in power transmission grids. Secure electrical power supply requires, together with careful operation, a robust design of the electrical power grid topology. Currently, the impact of the topology on grid robustness is mainly assessed by purely topological approaches, that fail to capture the essence of electric power flow. This paper proposes a metric, the effective graph resistance, to relate the topology of a power grid to its robustness against cascading failures by deliberate attacks, while also taking the fundamental characteristics of the electric power grid into account such as power flow allocation according to Kirchhoff laws. Experimental verification on synthetic power systems shows that the proposed metric reflects the grid robustness accurately. The proposed metric is used to optimize a grid topology for a higher level of robustness. To demonstrate its applicability, the metric is applied on the IEEE 118 bus power system to improve its robustness against cascading failures.

Structural vulnerability assessment of electric power grids

Cascading failures are the typical reasons of blackouts in power grids. The grid topology plays an important role in determining the dynamics of cascading failures in power grids. Measures for vulnerability analysis are crucial to assure a higher level of robustness of power grids. Metrics from Complex Networks are widely used to investigate the grid vulnerability. Yet, these purely topological metrics fail to capture the real behaviour of power grids. This paper proposes a metric, the effective graph resistance, as a vulnerability measure to determine the critical components in a power grid. Differently than the existing purely topological measures, the effective graph resistance accounts for the electrical properties of power grids such as power flow allocation according to Kirchoff laws. To demonstrate the applicability of the effective graph resistance, a quantitative vulnerability assessment of the IEEE 118 buses power system is performed. The simulation results verify the effectiveness of the effective graph resistance to identify the critical transmission lines in a power grid.

A robustness metric for cascading failures by targeted attacks in power networks

Cascading failures are the main reason blackouts occur in power networks. The economic cost of such failures is in the order of tens of billion dollars annually. In a power network, the cascading failure phenomenon is related to both topological properties (number and types of buses, density of transmission lines and interconnection of components) and flow dynamics (load distribution and loading level). Existing studies most often focus on network topology, and not on flow dynamics. This paper proposes a new metric to assess power network robustness with respect to cascading failures, in particular for cascading effects due to line overloads and caused by targeted attacks. The metric takes both the effect of topological features and the effect of flow dynamics on network robustness into account, using an entropy-based approach. Experimental verification shows that the proposed robustness metric quantifies a power grid robustness with respect to cascading failures.

MATCASC: A tool to analyse cascading line outages in power grids

Blackouts in power grids typically result from cascading failures. The key importance of the electric power grid to society encourages further research into sustaining power system reliability and developing new methods to manage the risks of cascading blackouts. Adequate software tools are required to better analyse, understand, and assess the consequences of the cascading failures. This paper presents MATCASC, an open source MATLAB based tool to analyse cascading failures in power grids. Cascading effects due to line overload outages are considered. The applicability of the MATCASC tool is demonstrated by assessing the robustness of IEEE test systems and real-world power grids with respect to cascading failures.

A topological investigation of phase transitions of cascading failures in power grids

Cascading failures are one of the main reasons for blackouts in electric power transmission grids. The economic cost of such failures is in the order of tens of billion dollars annually. The loading level of power system is a key aspect to determine the amount of the damage caused by cascading failures. Existing studies show that the blackout size exhibits phase transitions as the loading level increases. This paper investigates the impact of the topology of a power grid on phase transitions in its robustness. Three spectral graph metrics are considered: spectral radius, effective graph resistance and algebraic connectivity. Experimental results from a model of cascading failures in power grids on the IEEE power systems demonstrate the applicability of these metrics to design/optimise a power grid topology for an enhanced phase transition behaviour of the system.

A network approach for power grid robustness against cascading failures

Cascading failures are one of the main reasons for blackouts in electrical power grids. Stable power supply requires a robust design of the power grid topology. Currently, the impact of the grid structure on the grid robustness is mainly assessed by purely topological metrics, that fail to capture the fundamental properties of the electrical power grids such as power flow allocation according to Kirchhoffs laws. This paper deploys the effective graph resistance as a metric to relate the topology of a grid to its robustness against cascading failures. Specifically, the effective graph resistance is deployed as a metric for network expansions (by means of transmission line additions) of an existing power grid. Four strategies based on network properties are investigated to optimize the effective graph resistance, accordingly to improve the robustness, of a given power grid at a low computational complexity. Experimental results suggest the existence of Braesss paradox in power grids: bringing an additional line into the system occasionally results in decrease of the grid robustness. This paper further investigates the impact of the topology on the Braesss paradox, and identifies specific substructures whose existence results in Braesss paradox. Careful assessment of the design and expansion choices of grid topologies incorporating the insights provided by this paper optimizes the robustness of a power grid, while avoiding the Braesss paradox in the system.

Distributed monitoring for the prevention of cascading failures in operational power grids

Electrical power grids are vulnerable to cascading failures that can lead to large blackouts. Detection and prevention of cascading failures in power grids is impor- tant. Currently, grid operators mainly monitor the state (loading level) of individual components in power grids. The complex architecture of power grids, with many interdependencies, makes it difficult to aggregate data provided by local compo- nents in a timely manner and meaningful way: monitoring the resilience with re- spect to cascading failures of an operational power grid is a challenge. This paper addresses this challenge. The main ideas behind the paper are that (i) a robustness metric based on both the topology and the operative state of the power grid can be used to quantify power grid robustness and (ii) a new proposed a distributed computation method with self-stabilizing properties can be used to achieving near real-time monitoring of the robustness of the power grid. Our con- tributions thus provide insight into the resilience with respect to cascading failures of a dynamic operational power grid at runtime, in a scalable and robust way. Com- putations are pushed into the network, making the results available at each node, allowing automated distributed control mechanisms to be implemented on top.

Topology-Driven Performance Analysis of Power Grids

Direct connections between nodes usually result in efficient transmission in networks. Such electric power transmission is governed by physical laws, and an assessment purely based on direct connections between nodes and shortest paths may not capture the operation of power grids. Motivated by these facts, in this chapter, we investigate the relation between the electric power transmission in a power grid and its underlying topology. Initially, we focus on synthetic power grids whose underlying topology can be structured as either a path or a complete graph. We analytically compute the impact of electric power transmission on link flows under the normal operation and under a link failure contingency using the linearised DC power flow equations. Subsequently, in various other graph types, we provide empirical results on the link flow, the voltage magnitude and the total active power loss in power grids using the nonlinear AC power flow equations. Our results show that in a path graph, as an assessment based on shortest paths holds, however, the electric power transmission can lead to substantial amount of link flows, active power loss and voltage drops, especially in large path graphs. On the other hand, adding few links to a path graph could significantly improve those performance indicators of power grids, but at a cost: the resulting meshed topology decreases the control over power grids as a direct assessment between the shortest paths and the electric power transformation is lost. Additionally, a meshed topology with loops increases the redundancy in the design to ensure a safe operation under a link failure contingency.

Structural vulnerability analysis of electric power distribution grids

Power grid outages cause huge economical and societal costs. Disruptions in the power distribution grid are responsible for a significant fraction of electric power unavailability to customers. The impact of extreme weather conditions, continuously increasing demand, and the over-ageing of assets in the grid, deteriorates the safety of electric power delivery in the near future. It is this dependence on electric power that necessitates further research in the power distribution grid security assessment. Thus measures to analyze the robustness characteristics and to identify vulnerabilities as they exist in the grid are of utmost importance. This research investigates exactly those concepts- the vulnerability and robustness of power distribution grids from a topological point of view, and proposes a metric to quantify them with respect to assets in a distribution grid. Real-world data is used to demonstrate the applicability of the proposed metric as a tool to assess the criticality of assets in a distribution grid.

On Robustness of Power Grids

Current and future trends in environmental, economical, and human-caused factors (such as power demand growth, over-ageing of assets in power grids, and extreme weather conditions) challenge power grid robustness in the near future, necessitating research to better analyse and understand the notion of robustness in power grids, and ultimately to enhance it. This dissertation investigates the robustness of power grids from a Complex Networks Theory perspective to develop concepts and measures to quantitatively assess power grid robustness. A set of metrics are proposed to quantitatively assess the robustness of power transmission and distribution grids accounting for the impact of the key system characteristics such as the operative state and the topology. The proposed metrics provide means to exploit the relationship between the topology, operation, and robustness performance of power grids. They are experimentally validated using models of power grids, and applied on IEEE power systems, synthetically generated power grids, and real world power grids. The proposed metrics assist grid operators for dynamical optimization of flow and topology of a given power grid, and grid analysts in strategic asset management and network expansion planning processes for the purpose of robustness enhancement of a power grid.