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Glossary · Energy industry & energy transition

Contingency Analysis

Contingency analysis Outage analysis grid operation N-1 electricity grid Operational safety

In the power grid, contingency analysis refers to the computational check of how a defined outage affects grid operation. The outage may concern an overhead line, a cable, a transformer, a generating unit, a converter, a busbar or another operationally important element. The analysis does not predict which outage will occur. Rather, it asks: if precisely this element failed now, would the grid still remain within the permissible technical limits?

In German, the terms Ausfallanalyse, Ausfallsimulation or Sicherheitsrechnung are also used. The German version of the European guideline on electricity transmission system operation, Regulation (EU) 2017/1485, uses the term Ausfallvarianten-Rechnung (contingency analysis). By this it means the computer-aided simulation of the outages on a contingency list. This list contains the outages with which compliance with the operational security limits is checked. The term is closely linked to the N-1 criterion. According to the Regulation, it means that the elements remaining in operation after an outage within a transmission system operator's control area can adapt to the new situation without exceeding operational security limits. Contingency analysis is the tool used to check this in advance.

What is calculated

The starting point is a grid model. It contains lines, transformers, switching states, infeed, withdrawals, reactive power sources and technical limits. For a given operating state, the initial state is first calculated with a load flow calculation. In the language of the Regulation, this is the N-situation, in which all transmission system elements are available. The calculation shows how power is distributed in the meshed AC grid. What counts here is not trade contracts but the physics of the grid: impedances, voltage levels and topology.

The outages on the list are then simulated one by one. The element is removed in the model and the load flow is recalculated. For each case, it is checked whether lines or transformers exceed their thermal limits, whether voltages leave the permissible band and whether stability limits are reached. The Regulation describes it as follows: in the N-situation, the transmission system operator simulates each contingency on its list and checks whether, in the (N-1) situation, the operational security limits of its control area remain respected. In many applications, this involves steady-state load flow calculations. Where frequency behaviour, voltage dips, protection systems or converter controls are important, dynamic stability studies are added.

A simple example: two parallel lines are each loaded at 60 percent in normal operation. If one of them fails, a large part of its flow shifts to the other line and to further paths. The remaining line can then be well above 100 percent. Whether this is permissible depends on its thermal rating and on whether an overload may be tolerated for a short time. In the (N-1) situation, the Regulation permits temporarily admissible overloads only if remedial actions have been prepared that take effect within the time allowed for them.

Which outages are considered

The selection of outages is itself an important technical step. Regulation (EU) 2017/1485 distinguishes three types:

  • Ordinary contingency: the outage of a single branch or infeed, i.e. the classic N-1 case.

  • Exceptional contingency: several outages with a common cause, for example two circuits on the same pylon or a busbar fault. The German transmission system operators call this a common-mode outage.

  • Out-of-range contingency: several simultaneous outages without a common cause, or a loss of generation greater than the reference incident. Such cases are very rare but can have severe consequences.

The list must contain ordinary and exceptional contingencies. In classifying them, the grid operator also considers how likely an outage is. An exceptional contingency is placed on the list if it becomes significantly more likely because of the operating or weather situation or would have strong effects on its own or neighbouring grids. The calculation itself is deterministic: every case on the list is calculated in full. However, probability influences which cases end up on the list at all. Anyone who fails to include relevant elements obtains a deceptive sense of security.

Distinctions

Contingency analysis clearly differs from short-circuit calculation. Short-circuit calculation determines fault currents in the event of a short circuit and thereby dimensions protection settings and the withstand capability of equipment. Contingency analysis usually considers the state after an element has been disconnected: the element is missing, and the rest of the grid must carry the redistributed flows. Both methods are related but answer different questions.

Contingency analysis should also not be equated with probabilistic reliability assessment. The latter works with outage rates and expected supply interruptions. Contingency analysis, by contrast, is primarily a security-oriented operational calculation. A rare outage can be important if it would be hard to manage. A more frequent event can be less critical if sufficient reserves and countermeasures are available.

Legal framework in the EU and time horizons

In the EU, Regulation (EU) 2017/1485, the System Operation Guideline, applies to transmission system operators. It requires contingency analyses both in real-time operation (Article 34) and in operational planning (Article 72). For operational planning, it prescribes coordinated operational security analyses at least for the following time frames: the year ahead, the week ahead (where applicable), the day ahead and the current day (intraday). In real-time operation, the calculation is repeated continuously with current measurement and switching states. According to the consolidated text of the Regulation, this occurs at least once every 15 minutes. For this purpose, the transmission system operators exchange, among other things, generation, consumption, schedules, planned unavailabilities, substation topologies and forecasts. These obligations apply to the transmission grid. They cannot be automatically transferred to distribution grids.

The results determine the system state. A normal state exists when all security limits are respected in the N-situation and after each outage on the list, with available remedial actions taken into account. The state is in alert if the limits are currently respected but the available remedial actions would be insufficient for at least one outage on the list.

Countermeasures and link to the market

If the analysis shows a critical case, grid operators prepare remedial actions or implement them directly. A distinction is made between preventive measures (taken in advance) and measures for the case that the outage actually occurs (curative). Possible means are grid switching actions that change the topology and thus the load flows, as well as redispatch or countertrading. Further means include transformer tap changers, reactive power, phase-shifting transformers and controllable converters. What is decisive is how quickly a measure takes effect and whether it can actually be activated in legal, market and technical terms. Optimisation methods that include outages directly (security-constrained optimal power flow, SCOPF) combine contingency calculation with the search for the least costly measure.

In this way, contingency analysis links technical grid operation with the market. Trading generates schedules, but the grid must carry the physical flows. If a state resulting from the market were not secure in the N-1 case, grid operators must intervene. The costs of this later appear as costs of congestion management. Security of supply therefore depends not only on sufficient generation capacity but also on whether the grid can transmit the flows and whether operational measures are available.

Typical misunderstandings

The N-1 criterion does not mean absolute security. It requires only that the cases on the list are managed. Out-of-range contingencies, extreme weather conditions, incorrect protection tripping, communication failures or cascades can go beyond this. Contingency analysis reduces risks but does not eliminate them.

Free capacity in normal operation is likewise no proof of security. After the outage of a central element, flows can shift in such a way that impermissible loadings arise elsewhere. It is therefore not sufficient to consider individual line loadings in isolation.

Renewable energies do not make contingency analysis superfluous. However, they change the operating states that must be checked. High wind or solar infeed can direct flows into regions that were rarely critical in the past. At the same time, battery storage, flexible consumers and controllable converters can provide new remedial actions. The prerequisite is that they are measurable and controllable and that market and connection rules permit their use.

Role in the more strongly electrified power system

With heat pumps, electromobility, electrolysers, data centres and electrified industry, it becomes more important when and where electricity is consumed. For contingency analysis, what counts is not the annual amount of energy but the power at particular grid nodes at particular times. An additional load in an already strained region can be inconspicuous in normal operation and trigger a bottleneck only in the event of an outage. A flexible load, by contrast, can help if it can be reliably reduced at the critical moment.

For a long time, contingency analysis was mainly a task of the transmission grid. With more decentralised infeed, controllable consumers and storage, similar calculations are also becoming more important in the distribution grid, especially at high voltage. There, data availability, automation and responsibilities are often regulated differently than in the extra-high-voltage grid. The technical question about an outage quickly turns into organisational questions: who sees the grid state, who may control, who bears the costs, and according to which rules are interventions prioritised?

Contingency analysis is therefore not a single indicator but an operationally necessary method of verification. It shows whether a grid can withstand the next plausible outage, which elements become critical in the process and which measures must be ready in good time. The benchmark is not the fault-free moment but the state of the grid after the outage of a relevant element.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 17:55

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