Heavy snow over the Italian Dolomites in December 2013 left tourists enduring an extended blackout in one of Europe’s most exclusive winter resorts. Then, in March 2015, central Italy was hit by a major storm with over a meter of snow combined with strong winds. That left over 200,000 customers without power for hours. The situation was even more critical in mid-January 2017 during three days of ‘the perfect wet-snowstorm’, this time affecting more than 400,000 customers across different provinces of Italy. Power disruptions lasted for days. Nor has extreme snow been the only weather event causing headaches for network operators. Heavy rain in Rome in November 2013 caused flooding of substations while extreme winds and tornados damaged extended sections of lines in Trezzo d’Adda and Tuscany. Later, prolonged heat waves overloaded distribution systems and caused cable joints to fail.
What has been happening in Italy in recent years is being mirrored elsewhere across the globe and raises the question of how best to deal with increasingly frequent extreme weather events that impact energy systems. Michele de Nigris of Ricerca sul Sistema Energetico (RSE) in Milan explores how overhead lines and substations may have to be redesigned, reinforced or made more resilient to take extreme climate into account. He also examines the inverse: namely how power lines themselves are changing local environments. Approaches are proposed to assess impact of energy infrastructure in a lifecycle philosophy while also engaging stakeholders in decisions about technologies and routing of lines.
Climate Change & Power Systems
The Mediterranean has an especially high exposure to the impact of climate change since it is located in the transition zone between arid North Africa and rainy, temperate Europe. This location amplifies natural variations and enhances the impact of extreme weather events that threaten power infrastructure. Estimating climate change and its interaction with power systems has been done using Regional Climate Models (RCMs) that evaluate these phenomena with suitable spatial resolution, relying on meshes that range from 25 km down to 12 km. The ability of RCMs to accurately describe current climatology is verified using different data sets based on observations (e.g. E-OBS or ERA-Interim). For example, RSE carried out an estimate of expected evolution in climate in Italy over the period 2021 to 2050 using numerical models whose accuracy was verified by checking how well these describe climate over the past decades. Projections from these models reveal that the entire Mediterranean region is expected to experience a temperature increase during all seasons of at least 1 to 1.5°C, with the greatest increase being during summer (about 2 to 2.5°C) and concentrated inland and in southern areas. Warming of about 1.5°C is also expected in Alpine regions during winter. The average warming expected to take place during summer will then lead to a significant increase in extremely hot days and tropical nights, particularly in the Po Valley and southern Italy (see Fig. 1). Consequences will be felt not only on electrical line loading through increased use of air conditioners but also as higher thermal stresses placed on cables due to drying terrain that will have reduced heat exchange capability. Projected increases in minimum temperatures will also see reduction in snow cover and substantial melting of glaciers in the Alps.
Winter precipitation combined with relatively high temperature is the classic condition that triggers ‘wet-snow events’: in fact, precipitation rates higher than 10 mm/day and 0°C< Tmax <1.5°C (as in Fig. 2) are expected to increase by 30 to 40% by 2050. The number of dry days, i.e. with precipitation values less than 1 mm/d, is expected to increase as well, mostly during summer, with special reference to southern Italy (see Fig. 3). In terms of heavy precipitation events (i.e. where rainfall is greater than 20 mm/day), central and northern areas of Europe in winter and south-eastern coastal sites during spring will experience increases of up to 30% (see Fig. 4). The situation will become even more severe in the eastern Italian Peninsula during autumn.
Dealing with Climate Change
Projections such as this about climate change indicate that power systems will be exposed to ever-increasing threats that have the potential to jeopardize continuity and quality of supply. This leads to the question: how best to deal with heightened risk? Must entire power systems be reinforced to deal with increased stress levels that were not considered during the design phase? And is the conventional process of system planning and operation based on the concept of reliability still sufficient? Or will there be a need to change approach to the broader concept of resilience? Such a conceptual transition is not trivial and requires elaboration: design and operation of a reliable power system implies availability of adequate infrastructure operating under given security rules. In this context, margins are given to a system that has been dimensioned and designed to operate without failures under given thermal, electrical, environmental and mechanical stresses experienced over its lifetime. In situations where stresses exceed design values or when wear degrades performance, the typical requirement placed on components is that they must fail safely while on the system. The N-1 approach is normally adopted, i.e. the power system must function properly even if a major component fails. But stresses linked with climate change will likely generate situations well beyond N-1 conditions, affecting large portions of a network at one time and causing multiple contingencies that could lead to extended blackouts. The concept of resilience envisages the possibility of a degraded condition and occurrence of multiple failures but considers the capability of the system to rapidly return to a normal, stable operating condition once the severe event that triggered failures has passed.
The mathematical formulation for resilience is highly complex and a separate topic on its own. The focus here will therefore be rather on the basic recipe for resilience. This is comprised of two main elements: the degraded component and the restorative component, related respectively to progress of the disturbance and recovery post-disturbance. The first aspect is linked with threats, vulnerabilities, contingencies and consequences:
• Threats are the menaces to electrical components and systems. These can be external or internal, natural or human-caused, accidental or intentional, etc. Also among these are extreme environmental conditions, actions of hackers and terrorists and also progressive wear and ageing of insulation;
• Vulnerabilities are any weaknesses in the components and systems, thereby impairing their robustness and favouring malfunction and faults. These also represent various limits of operation, e.g. flashover voltage of polluted insulators, mechanical strain on conductors, seismic performance of circuit breakers, etc. Vulnerabilities can be reduced using screens, protections, redundancy, network configuration, etc.;
• Contingencies are the combination of threats and vulnerabilities. They represent the event of malfunction of components and systems and can be single (i.e. compliant with the N-1 approach) or multiple, such as when failure of one component propagates to others. Examples include explosion of a current transformer in an air-insulated substation or cascade tripping of lines under extended pollution events;
• Consequences are the impacts of contingencies. In an electric power system, these are often brought back to the amount of energy not supplied to customers (ENS).
The restorative component of resilience expresses the capability of a system to partially or to fully recover performance. To evaluate this component, there is the need to quantify the actions required to restore power to users, even under unavailability of network infrastructure (reboot), as well as the actions needed to repair damaged infrastructure (recovery). Examples of reboot might be clearing roads blocked by snow to access distribution substations or transitory connection of auxiliary power units to supply critical loads. To increase resilience, each of its various components must be considered: anticipating threats originating from external (forecasting) or internal (monitoring and diagnostics) conditions to adopt proactive corrective measures; reducing vulnerabilities through screening, protection, strengthening; limiting contingencies by means of enhanced network meshing; and limiting consequences through rapid re-closers, etc.