Oddbjørn Gjerde

Risk analysis of critical infrastructures emphasizing electricity supply and interdependencies

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.

Integrated approach for security of electricity supply analysis

The methods and tools needed for security of supply assessment range from long term market models for predicting future generation and load patterns to more detailed network simulation models for analyzing contingencies and reliability of supply for delivery points. This paper describes an integrated approach for security of electricity supply analysis, by the integration of power system reliability analysis with the power market analysis. An integrated methodology will enable the long-term security of supply assessment and provide a more consistent input to the various analyses, as well as a better information exchange and interaction between the different methods and tools involved. The integrated methodology is under development in an ongoing research project focusing the power system reliability analysis, making it possible to take the risk of extraordinary events into account.

Protection System Faults -- a Comparative Review of Fault Statistics

Correct operation of the protection system is of vital importance for the reliability and security of supply. Information of protection system faults is therefore important for different purposes such as quality of supply regulation and as input data to security and reliability analysis. This paper provides a comparative review of the Norwegian and Finnish fault statistics for line and transformer protection systems at the voltage levels 132-400 kV for the period 1999-2004. Unwanted operation is the major fault type, particularly for transformer protection. Human causes dominate by roughly 50%, technical faults count for 20-30% while there are large portions of faults where the cause is not identified. In Norway 20% of the total energy not supplied at these voltage levels are caused by the protection. The corresponding portion in Finland is only 4%

Dependability and security-based failure considerations in protection system reliability studies

The reliability of protection systems has a considerable effect on the reliability of supply, and hence appropriate protection system reliability models must be incorporated in power system reliability studies. These studies assume increasing prominence, especially in the wake of influx of smart grid technologies, making it imperative to handle the accompanying failure dependencies in detail. This paper presents the results of one such related investigation carried out to incorporate the impact of transmission line failure modes on account of various protection system response scenarios on supply reliability indices. In addition to the basic frequency and duration indices, indices such as annual power interrupted, annual energy not supplied, and annual interruption costs are computed, built on a minimal cut set based approach. The approach presented circumvents the need for complex Markov models to include protection system reliability considerations. An illustrative case study is employed to draw attention to the impact of identified comprehensive failure scenarios in protection and control equipment on power system reliability. Comparisons are made with an existing simplified method, and also with a case where protection system reliability is assumed to be perfect. The results bring forward the emphasis to be placed on initiatives to include the study of impact of reliability of protection systems on reliability of supply.

Risk and vulnerability analysis of power systems including extraordinary events

This paper describes a framework and methodology for risk and vulnerability analysis including extraordinary events in power systems. The framework is based on a bow-tie structure and identifies threats, unwanted events, barriers and consequences. Application of the methodology is shown for a real case analyzing extraordinary events in a transmission system. In this case the consequence is unacceptable, but the risk is moderate due to the low probability for the event to occur. The case study and experience so far indicates that one of the most challenging parts of a risk and vulnerability analysis is how to identify the vulnerable operational states and extraordinary events.

Risk-based method for cost-effective reinvestments in distribution networks

In deregulated power markets, electricity utilities are now facing new challenges concerning revenue cap regulations, including quality based incentives. When planning the network every grid investment should contribute to minimising the total costs related to the investment within given safety and environmental requirements. This paper describes a method used for planning the time of replacement for network components. By using the method, the utility considers both qualitative and economic criteria that have to be taken into account when evaluating different solutions. The paper also describes experience from a Norwegian utility using the method when planning renewal of distribution substations.

Modelling of corrective actions in power system reliability analysis

Consequence analysis, including the modelling of corrective actions, is an important component when performing power system reliability analyses. Using an integrated methodology for power system reliability analysis, we investigate the impact of different modelling choices for the consequence analysis on estimates for the energy not supplied. These investigations corroborate the large impact modelling assumptions for corrective actions have on the resulting reliability indices. We have also identified other features of the consequence analysis, such as islanding and distributed slack, that can be important to take into account. The findings and the underlying structured approach contribute to improving the accuracy of power system reliability analyses.

Impact of protection system reliability on power system reliability: A new minimal cutset approach

In order to capture the effect of dependent failures that could arise due to the various transmission protection system response scenarios on power system reliability, complex Markov models or fault trees combined with event trees are typically employed in the predictive reliability studies. A unique approach utilizing minimal cutsets (MC) and the approximate methods of system reliability evaluation, dispensing with the assumption of perfect protection and control, was recently postulated to obtain various power system reliability indices. The approach was basic in that it was applicable to single circuit meshed transmission systems, where only MCs up to a maximum order of two could be handled. However, the parallel structure of transmission lines in multi-circuit meshed transmission systems, in addition to resulting in possible higher order critical transmission line MCs, creates unique topological dependencies among the backup protection system coordination schemes. In this paper, a comprehensive MC approach to capture the impact of protection system reliability on power system reliability is presented, covering all such consequent dependencies. An illustrative sample case study is used to explain the salient features of the proposed methodology.

Impact of substation configuration on protection system failure propagation and its effect on reliability of supply

Relatively fewer studies exist in literature on including the complex effects of transmission protection system related failure dependencies in the reliability prediction models. Usage of extensive Markov models has been usually advocated to capture the impact of protection system reliability on power system reliability. A new analytical method which makes use of approximate methods of system reliability evaluation has been recently proposed by us, which circumvents the need for Markov models. It is a unique minimal cutset-based approach for single circuit meshed transmission systems, where several basic and load/energy oriented reliability indices are obtained. The objective of this paper is to extend the procedure to examine the impact of substation configurations on protection system failure dependency propagation and its effect on bulk load point reliability indices. Preliminary investigations show a marked impact of employing a station configuration with simplified bus representation, especially in multi circuit meshed transmission systems, on the resulting reliability indices. The results of the proposed methodology are demonstrated on a suitably modified four bus illustrative test system, for cases with and without the consideration of protection system failures for a realistic station configuration.

Risk Analysis of Electricity Supply

Society is critically dependent on a reliable electricity supply to maintain its functionality. Electricity supply interruptions lead to direct consequences for the electricity users and will in general have an impact on dependent infrastructures and their services. This chapter describes a quantitative analytical approach for risk analysis of electricity supply. In this approach, the consequences of failures in the electricity system are analysed in terms of electricity supply interruptions to delivery points (DPs) serving for instance societal critical functions or other infrastructures. In a cross-sector risk analysis, this approach can be used in a detailed analysis for instance as input to cascade diagrams in the risk analysis of cascading failures and interdependencies with other infrastructures.