Analysis of extraordinary events in power systems

Society is increasingly dependent on a secure electricity supply to maintain its functionality and cover basic needs. The essential role of electricity is perhaps most evident at the rare occurrences of extensive and long-lasting blackouts. Such blackouts may have a very low probability of occurring, but if they do occur, the societal consequences (impact) can be very high. Therefore, they are often referred to as High-Impact Low-Probability (HILP) events, or extraordinary events. The project has developed methods and tools for analysing risk and vulnerability related to HILP events in power systems. The methods and tools have been developed in cooperation with Transmission System Operators (TSO) and energy regulators. The project was led by SINTEF Energy Research and included project partners Statnett (the Norwegian TSO), Fingrid (the Finnish TSO), NVE (the Norwegian energy regulator), and NTNU. New methods and tools help raise awareness of potential vulnerabilities, identify promising barriers to mitigate the risk of HILP events, and strengthen the ability of TSOs to make the trade-off between security of supply and societal costs in planning and operation of the power system. For the TSOs and for society at large, HILP events are particularly challenging to analyse, understand and identify. Compared to "ordinary" power supply interruptions, the cause and effect relationships are more complex, the uncertainties are greater, and it is psychologically more challenging to deal with events believed to be extremely unlikely or even unimaginable. The challenge is amplified by the increasing complexity and uncertainties of the power system due to integration of distributed renewable power generation, more extreme weather, and stronger integration between the Nordic power system and the power systems in the rest of Europe, among other things. One realization made early in the project was that there is no single holistic, quantitative analysis method that can capture all aspects of HILP events. An appropriate combination of qualitative and quantitative techniques must therefore be employed to capture the characteristic aspects for each case. It is valuable to complement detailed, quantitative analysis with more qualitative initial assessments. A qualitative framework for analysing HILP events has therefore been developed by extending a framework for vulnerability analysis developed in previous projects. The vulnerability analysis methodology was applied in a case study together with Fingrid on vulnerabilities related to the outage of HVDC interconnectors in the Nordic power system. Insufficient system inertia was identified as an important vulnerability influencing factor. The Nordic power system may not be particularly vulnerable to HVDC outages today, but there are reasons to pay attention to how the vulnerability will develop in the future. Especially with regards to decreasing system inertia. Blackouts due to extreme weather is a notable category of HILP events. Weather-related threats can cause simultaneous outage of multiple power system components ("failure bunching"). The project has developed methods for estimating time-dependent probabilities for wind-related failures of overhead transmission lines, and models for how the outage time of transmission lines depend on weather conditions. Some of this research is carried out as part of a PhD study at NTNU on methods for understanding and communicating uncertainties and risk related to extraordinary events. Another main category of HILP events is characterized by often having a single initiating failure event, followed by complex sequences of causally related events and barrier failures, eventually leading to critical consequences. A general modelling framework and quantitative vulnerability analysis methodology has been developed for analysing such sequences of events. Case study results illustrate how considering barrier failures (e.g. failure of corrective rescheduling of generation) is crucial to be able to identify and analyse HILP events. HILP events are intrinsically associated with large uncertainties that are challenging to deal with and interpret for analysts and decision makers. Focusing on system development decisions, we developed a methodology accounting for uncertainties due to HILP events in socio-economic cost-benefit analysis. The methodology provides a more comprehensive description of risk that considers both random variation and lack of knowledge. In a case study in collaboration with Statnett, the methodology is applied to a real grid investment decision. The case study showed how conclusions based on a conventional cost-benefit analysis may not hold when uncertainties are considered more comprehensively. This exemplifies how the project results can help TSOs to make a best possible trade-off between security of supply and societal costs in planning and operation of the power system.

The methodologies developed in the project have been implemented and demonstrated in prototype software tools. Implementations in SINTEF Energy Research's OPAL prototype tool have already been applied to reliability of supply assessment in a separate project. The work led by Statnett on modelling wind-related failures of transmission lines has been implemented in their in-house reliability assessment tool (MONSTER). A methodology for accounting for uncertainties due to extraordinary events has been developed in parallel with Statnett's ongoing system development studies. The methodology supplements the analyses currently used to support grid investment decisions, providing more comprehensive descriptions of uncertainties and risk. The project results can thus help transmission system operators to mitigate the risk of extraordinary events and strengthen their ability of to make the trade-off between security of supply and societal costs in planning and operation of the power system.

An adequate level of security of electricity supply is a crucial requirement for the planning and operation of modern power systems. The complexity and uncertainties of the power system are increasing, a trend that will continue with a stronger integration between the Nordic and European power systems, the introduction of more smart devices, new technologies, renewable and distributed energy resources, and more extreme weather. These developments impose risks and vulnerabilities in the power system needing to be dealt with. In particular, extraordinary events are of special interest as they contribute to substantial consequences for the system itself and for its users. The mechanisms behind these events are not well understood today, and there is a need to increase the ability to identify, understand and assess extraordinary events. By building on today's best methods and theories, methods and tools for analysing extraordinary events in power systems will be developed. The main challenges to be addressed are the identification of causes and underlying mechanisms, quantification of the consequences and quantification of the uncertainties. Applicability will be ensured through the use of case studies of the Nordic power transmission system. The project results will provide decision support to make a best possible trade-off between security of supply and costs in planning and operation of the Nordic power system. New competence will be built and the collaboration with transmission system operators and authorities ensures that the developed methodologies will increase awareness and understanding as well as aid for deciding on remedial actions and preparedness plans for the present and future power systems. Improved understanding of the risks associated with extraordinary events will also be of great value for electric utilities, households and industry in general, as modern society is heavily dependent on a reliable electric supply.