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Security of Supply and System Adequacy

The peak demand (or peak load) of electricity in Europe is constantly increasing. Over the coming years, the UCTE expects a rise in peak demand of around 1.6 - 1.7 per cent per year (compared with 2 per cent up to 2007) (Figure 6.1). The peak demand is a strategic parameter, since it determines the required generating and transmission capacities. As a matter of convention, for system design purposes, peak load values at specific points during the year are being considered, namely in January and July.

Figure 6.1: Overview of Growth in Electricity Demand in the UCTE Area

 

Note: The above image shows the national annual energy consumption growth rate (2006 with respect to 2005)

Source: UCTE, 2008

The way in which the power system can match the evolution in electricity demand is expressed as ‘system adequacy’. System adequacy measures the ability of a power system to cope with its load in all the steady states it may operate under standard conditions. This adequacy has different components:

• The ability of the generation assets to cover the peak load, taking into account uncertainties in the generation availability and load level; and
  
• The ability of the transmission system to perform, with the flexibility provided by interconnection and import and export flows

System operators are responsible for maintaining system adequacy at a defined high level. In other words, they should ensure that the generation system is able to cover the peak demand, avoiding loss-of-load events, for a given security of supply. The various national regulations regarding this 'security of supply' range from a 99 per cent security level (in 1 out of 100 years the peak load cannot be covered, such as in Germany) to 91 per cent (1 event in 10 years, such as in the UK).

As the whole European system is interconnected, it is logical for national TSOs to harmonise their approaches towards system adequacy. This is addressed mainly by the larger systems, such as UCTE, the Nordic system, the British and Irish systems. The assessment methods of generation adequacy can be deterministic or probabilistic, or a combination of both. Even from a national point of view, the system adequacy assessment involves trans-national issues. This is because, at the moment of peak load, it may be necessary to have access to power produced by a neighbouring country, so the transmission system should be able to carry and direct these trans-national power flows.

The UCTE system’s adequacy is being annually reviewed over a period of ten years. Generation adequacy assessment is based on the estimation of 'remaining capacity', which can be interpreted as:

• The capacity needed by the system to cover the difference between the peak load of each country and the load at the UCTE synchronous reference time ('margin against peak load'); or
• Exceptional demand variation and unplanned outages that the system operators have to cover with additional reserves.

Generation adequacy assessment underscores how each country could satisfy its interior load with the available national capacity. Transmission adequacy assessment then investigates whether the transmission system is large enough to enable the potential imports and exports resulting from various national power balances, thus improving the reliability of the European power system.

In the Nordel zone, TSOs still conduct these reviews, but theoretically the electricity market price signals are considered sufficient to trigger the building of new capacity to fulfil adequacy needs. As long as the results of the reviews are positive, there is no need to keep reserves in the power system. However, in many countries, a number of contracts are drawn up to ensure that there is spare capacity available in extreme loading situations, often with older plants or loads that can be switched on or off in critical situations.

In the adequacy estimation, each power plant is assigned a typical capacity value. This takes into account scheduled and unscheduled outages. There are no plants with a capacity value of 100 per cent, since there is always the possibility that capacity will not be available when required. In its forecast, the UCTE is looking at increasing shares of wind power in the coming years. It is clear from the UCTE system adequacy forecasts that there is not yet a national TSO standard for the determination of wind power’s capacity credit.