Keeping the Electricity Flowing
By Wolfgang Kröger
Of all the critical infrastructures on which our modern societies depend, the electricity transmission system is arguably the most vital. Industry, communications, transportation – none of these can function without a reliable supply of electric power.
The European high-voltage grid is spread over five synchronous areas, managed by 41 transmission system operators (TSOs) in 34 countries, serving 534 million citizens. Such a highly interconnected multi-component system exhibits complex behaviours and is open to local or spatially distributed impacts. Guaranteeing the normal operation of the electricity network is difficult at the best of times. Current major political and organizational changes in the energy field, namely the targeted increase in the share of renewable energy sources and the move towards an unbundled competitive energy market, are posing new challenges.
Firstly, integrating power from wind or solar energy, which is produced intermittently, often far from consumer areas and at off-peak hours, requires both massive transfers and peak smoothing strategies.
Secondly, co-ordination is complicated by short-term trading entailing the use of close-to-real-time operational data and increasing cross-border power exchanges. In the past, a single entity, which owned and operated the entire supply chain, would typically have the absolute right to provide electricity to consumers. Now, in times of free access and disintegrated monopolies, each constituent follows its own procedures and rules, while the security of supply as a societal good needs to be ensured by a state organization.
In continental Europe, a comprehensive collection of operational principles, technical standards and recommendations helps TSOs to manage their networks and ensure interoperability among them. Interference with market forces is forbidden unless safety is at stake.
Accidents happen
No matter how carefully loads are calculated and monitored, a responsible approach to risk management assumes that accidents will happen. The mishap that split the Western European transmission system into three parts and plunged much of the continent into darkness on 4 November 2006 provides a good illustration of the complex interplay of factors – contextual, technical, human, organizational – that can come together to put a system in danger. The trigger was the outage of two high-voltage lines over the Ems River in northern Germany to let an inland built cruise liner, the Norwegian Pearl, pass on its maiden voyage out to sea. The event was announced months in advance; appropriate calculations were made and provisions taken. But just days before the outage, the shipyard requested an advancement of the time from one o’clock in the morning to late evening. The neighbouring TSOs were not well informed and the congestion forecast not updated. In any case, the load for the earlier time was already sold so that it would have been legally impossible to change short of force majeure.
Nature played its part: when the lines were switched off at 21:39 there were strong winds in northern Germany and the in-feed caused high load flow to the Netherlands. In itself, this would not have been fatal. The load was taken over by remaining lines, in particular between the substations Landesbergen and Wehrendorf, southwest and southeast of the Ems crossing, respectively. But these substations were operated by two different TSOs, and miscommunication ensued. Being unaware of different protection strategies and settings at the other end of the line, they made faulty load flow calculations. The team responsible for Landesbergen decided to couple two bus-bars (conductors for collecting and distributing current), an emergency measure which they expected would reduce the load. It had the opposite effect.
The bus-bars were connected at 22:10:11. Immediately, the line at Wehrendorf tripped out. Within less than 18 seconds – at 22:10:28.7, to be exact – a cascade of automatic line trippings had split the European transmission system into three: two areas of under frequency in the west and south and an area of over frequency in the north-east. While in the north-east the frequency could be brought back down through generator cut-offs, automated load shedding was necessary in the west and the south. Consumers were affected for about half an hour. It took a few hours to re-synchronize the whole grid.
Prevention and mitigation
To guarantee normal operation of electrical networks, protection must be provided against cascade tripping, voltage or frequency collapse and loss of synchronism. The classic approach to preventing sudden disturbances is based on the so-called N-1 principle. According to this principle, when an unexpected failure of a single element of the integrated network, such as a line break, occurs, the remaining active elements must be capable of accommodating the change of flows and avoiding a cascade of trippings or loss of a significant amount of consumption. N-1 security needs to be monitored at all times by TSOs for their system and parts of adjacent systems; after a disturbance each TSO must return to N-1 compliant conditions as soon as possible, usually in about twenty to thirty minutes.
Maintaining N-1 security requires developing accurate lists of contingencies that need to be taken into account. Threats may affect a single critical component or a number of them, directly or indirectly (via failure of another system); the source can be external or internal. To assess the severity of contingencies and identify bottlenecks and critical elements, TSOs use empirical investigations, statistical data and blackout patterns. As all of these are based mainly on experience, however, they potentially lack predictive capabilities.
There is no doubt that N-1 security, if diligently implemented, is a best practice for ensuring the high performance of our power transmission systems. However, advanced, comprehensive analyses as well as surprising situations experienced in the past have taught us that there is a plethora of unprecedented scenarios, involving complex multiple failures, with which it is insufficient to cope. Understanding the behaviour of the electricity network, often part of a system of interdependent systems, is extremely challenging; an all-encompassing approach accounting for all the interwoven issues does not exist. A number of advanced knowledge-based and mathematical modelling techniques – Input-Output Interoperability Modelling, Complex Network Theory and Agent-based Modelling, for instance – are available and widely applied, each with its own strengths and weaknesses.
Natural hazards: a paradigm shift to resilience
Of the roughly twenty major blackouts we have experienced worldwide during the past 15 years, four were caused by bad weather conditions and one by an earthquake/tsunami. This demonstrates the importance of taking account of natural hazards when managing risks to electricity networks. Each of these events was different in terms of power loss (the most extreme being 60 Giga-Watts in the Great Lakes/NewYork City region of the United States in 2003), number of affected people (620 million in India in 2012) and duration (from a few hours to two weeks during the Lothar Cyclone that swept Europe in 1999).
Transmission systems, being large in scale, are subject to many different kinds of natural hazard. Most are multi-type, with one event triggering others. For example, a seaquake may induce a tsunami, followed by a flood and landslides. Economic losses and insurance costs deriving from natural hazards are high and will likely rise as extreme weather conditions increase due to climate change.
Most natural hazards are large-areal by nature. Although some of the most critical components can be identified and protected, it is difficult to fortify transmission systems against them. Therefore, some people suggest a paradigm shift from prevention to resilience, putting emphasis on adapting to and recovering from shocks rather than concentrating on avoiding them.
Reliable, integrated electricity networks are essential for individual states and for regions. Local failures can grow to a global scale. Therefore, raising awareness among states of potential breakdowns, notably those caused by natural disasters, sharing knowledge and facilitating dialogue are of paramount importance. In this regard, organizations like the OSCE have a key role to play.
Wolfgang Kröger is Professor emeritus of Safety Technology at the ETH Zürich and former Executive Director of the ETH Risk Centre.
New OSCE Handbook:
Protecting Electricity Networks from Natural Hazards
In 2013 the OSCE participating States adopted a Ministerial Council decision on protecting energy networks from natural and manmade disasters, tasking the Office of the Co-ordinator of Economic and Environmental Activities (OCEEA) to facilitate knowledge exchange, particularly on protecting electricity networks. The OSCE’s 57 participating States include some of the largest producers and consumers of energy and they often depend on imports to meet local demand. Blackouts in one country can have an impact on an entire region.
The OCEEA organized an expert workshop in June 2014 and the contributions laid the foundation for a practical handbook, now published: “Protecting Electricity Networks from Natural Hazards.” It provides policymakers, operators and regulators a state of the art overview of risk mapping, mitigation and management as well as examples from several countries of how to make electricity networks more resilient to natural hazards.
Download Protecting Electricity Networks from Natural Hazards here: //www.osce.org/secretariat/242651