Risk analysis of critical infrastructures emphasizing electricity supply and interdependencies

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Abstract

Failures in critical infrastructures can cause major damage to society. Wide-area interruptions (blackouts) in the electricity supply system have severe impacts on societal critical functions and other critical infrastructures, but there is no agreed-upon framework on how to analyze and predict the reliability of electricity supply. Thus, there is a need for an approach to cross-sector risk analyses, which facilitates risk analysis of outages in the electricity supply system and enables investigation of cascading failures and consequences in other infrastructures. This paper presents such an approach, which includes contingency analysis (power flow) and reliability analysis of power systems, as well as use of a cascade diagram for investigating interdependencies. A case study was carried out together with the Emergency Preparedness Group in the city of Oslo, Norway and the network company Hafslund Nett. The case study results highlight the need for cross-sector analyses by showing that the total estimated societal costs are substantially higher when cascading effects and consequences to other infrastructures are taken into account compared to only considering the costs of electricity interruptions as seen by the network company. The approach is a promising starting point for cross-sector risk analysis of electricity supply interruptions and consequences for dependent infrastructures.

Introduction

Society is critically dependent on a secure electricity supply, and wide-area interruptions (blackouts) have severe impacts on societal critical functions. Beyond the traditional and deterministic N−1 criterion used in electric power systems, there is no agreed-upon framework on how to analyze and predict the reliability of electricity supply, even though the power system is defined as one of society's critical infrastructures [1], [2]. Critical infrastructures are physical and logical systems with major importance for public welfare. In addition to electricity generation, transmission and distribution, other examples of critical infrastructures are transportation systems, electronic communications, financial services, and water supply [3], [4].

There are different kinds of safety and security challenges that critical infrastructures have in common, such as climate changes, natural disasters, ageing of the systems, restructuring of organizations and outsourcing, terrorism, and globalisation (see, e.g., [2]). The infrastructures are also interdependent, because disruptions in one infrastructure may impact the functionality of other infrastructures, for example between electronic communications and the electric power system. The challenges and interdependencies need to be dealt with through in-depth sector studies and interdisciplinary studies across sectors to enable development of methodologies for comparisons and exchange of best practices.

In risk analyses of electric power systems a major challenge is to identify possible chains of events that could lead to wide-area interruptions, and to further identify the consequences of cascading failures, for example in other critical infrastructures. In the last two decades, a simple approach to quantitative risk and vulnerability analysis has been applied and adapted separately for different critical infrastructure sectors in Norway [5]. This has resulted in various independent risk assessment approaches, and insufficient analyses of interdependencies between the different sectors. The simplified approach resembles preliminary hazard analysis (PHA) [6].

Various authors analyze and model infrastructure interdependencies (see e.g., [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]), and some focus on the electric power system, such as [18], [19], [20], [21], [22]. However, since there is no single methodology suitable for risk and vulnerability analysis of extraordinary events in power systems covering all the aspects of causes and consequences, there is a need for combining different quantitative and qualitative methods [23]. The objective of this paper is to provide an approach based on simulations of outages in the electric power system using methods for contingency analysis (power flow) and reliability analysis of power systems. The results can be used as input, for instance to cascade diagrams [7], in the analysis of the risk of cascading failures and consequences of electricity supply interruptions for other infrastructures.

This paper describes an extension of the simplified approach presented in [24], but expands on the risk analysis of critical infrastructures, emphasizing electricity supply and interdependencies between infrastructures. A case study of the city of Oslo, Norway, was carried out to test and improve the approach in a recent research project1 which included the critical infrastructures electricity supply, water supply, transport (road/rail), and information and communication systems (ICT). The main focus was on serious events and interdependencies between the sectors. In Norway, the network companies' revenue caps are adjusted in accordance with the customers' interruption costs, CENS [25]. In this arrangement the individual end-user consequences are represented by average cost rates per customer category [26]. CENS represents an estimate of the societal costs of electricity supply interruptions, however only considering the end-user’s costs. Consequences when loss of electricity supply results in unavailability of dependent infrastructures, public services etc., are not included in CENS.

The structure of the paper is as follows: First, the cross-sector approach for risk analysis is described with focus on interdependencies between critical infrastructures. Secondly, the approach for risk analysis of electricity supply is presented, as well as the case study. The immediate results are expected number of interruptions of electricity supply, expected interruption duration, interrupted power and energy not supplied for each delivery point. It is shown in a case study that the costs of electricity interruptions for end-users (CENS) might be considerably less than the total costs when cascading effects and consequences to other infrastructures are taken into account.

Section snippets

Risk analysis of interdependencies in critical infrastructures

Risk analyses of critical infrastructures across sectors may be far more complicated than traditional analyses, but provide important information for identification of vulnerabilities, emergency preparedness, and prioritization of risk reducing measures. The earthquake in Japan on March 11th 2011, followed by tsunamis and devastation of the coast in the country's north eastern region is a disastrous example of the effect of cascading failures in critical infrastructures: The electricity supply

Risk analysis of electricity supply

The electricity system is an extremely complex and comprehensive infrastructure. Despite the numerous components and the complexity of the system it is very robust and reliable. However, power system failures occur occasionally in the main grid, as well as in the regional and local networks, most often with minor consequences. While the electricity system on the main grid level is usually dimensioned and operated according to the N−1 criterion, meaning that the system should withstand loss of a

Case study – Loss of electricity supply

A case study according to steps 1–5 described in Section 2, was carried out in collaboration with the Emergency Preparedness Group (EPG) of the city of Oslo. Previous risk and vulnerability analyses of Oslo [35], [36] were used as a basis for the case study which involved serious events in several infrastructures, covering events of technical character, malicious acts, as well as natural hazards. One of the events subject to detailed analysis was “loss of electricity supply to Oslo central

Discussion and conclusions

This paper presents a cross-sector approach for risk analysis of critical infrastructures emphasizing electricity supply and interdependencies between infrastructures. A case study of the city of Oslo was carried out to test and improve the method in a recent research project comprising electricity supply, water supply, transport (road/rail), and information and communication systems. Analyses of interdependencies may either focus on the causes, the consequences, or both. In this paper, the

Acknowledgments

This paper is based on results from the recent research project Risk and Decision Systems for Critical Infrastructures (DECRIS) funded by the Research Council of Norway, and a case study performed in collaboration with the Emergency Preparedness Group of the City of Oslo and the network company Hafslund Nett. We greatly acknowledge all the partners in DECRIS and all stakeholders involved in the case study.

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