Energy planning and assessment of needs for flexible operation of generating units

The main consideration in assessing the feasibility of flexible operation of a nuclear power plant is whether the combined capacity of the nuclear generating units will always be significantly less than the minimum demand (or the minimum residual demand) on the electricity system when the plant is first connected to the grid and subsequently during its operational lifetime, as planned and forecast. If that is the case, it may not be necessary for the plant to operate flexibly. In all other cases, there would be a need for some nuclear power plants to operate flexibly, as explained in Section 3. Hence, energy planning is the main input to the decision making process on the necessity of flexible operation of a plant.

If the energy planning reveals the necessity for nuclear power plants to operate in a mode other than baseload, then it is necessary to consider and implement the capability for the plants to operate flexibly. In this case, the flexibility needs have to be characterized to identify the ranges for frequency control, the minimum stable power

and power ramping, how often the unit will need to do load following and how often there will be significant frequency deviations to control.

For a new nuclear power plant project, this capability is analysed at the feasibility stage because it affects the design, operation and business case of the new plant. For existing nuclear power plants, this capability is analysed considering the needs for flexibility until the end of life of the plant, the existing capability of the plant and the additional capabilities that may be available or may need to be added.

This identification is performed stepwise by analysing a predefined time horizon (e.g. present time and present time+5, +10, +20, +30 years), including the commissioning dates of the plants and other generation sources/

projects implementation.

4.1.1. Analysing the evolution of electricity demand

As explained earlier, electricity demand varies on a daily, weekly and yearly basis, and is influenced by weather factors such as temperature, sunshine and wind. It can also be affected by social events, power management activities, power exchanges with neighbouring countries and electricity demand management. Using the known past variation of electricity demand, combined with scenarios for economic and electricity sector development, the future magnitude and shape of electricity demand can be forecast.

The variation is quite repeatable in most Member States, and the main variations in future demand can be forecast with reasonable accuracy [22]. The additional effect of variations in weather conditions can also be modelled based on past experience and records, to allow good forecasts of electricity demand once a reliable weather forecast is available. The grid system operators in many Member States are able to predict consumer demand typically within 1−2%, including the expected effects of weather conditions, 1 or 2 days in advance.

FIG. 17. Simplified process of a nuclear power plant (NPP) owner/operating organization considering flexible operation.

FIG. 18. Interfaces among flexible operation and plant management activities. NPP — nuclear power plant; R&D — research and development.

It is necessary to consider several different economic scenarios, depending on the energy policy of the Member State, to set the boundaries for future energy use. The scenarios include a range of forecasts of electricity demand for different time periods (year, week, day, etc.) for various times into the future (present time and present time+5, +10, +20, +30 years). The outcome of this analysis of electricity demand evolution is often published in

‘energy forecasts’, ‘power studies’ or ‘power master plans’.

4.1.2. Analysing the evolution of electricity generation

Starting from the installed capacity, electricity generation forecast analysis integrates the evolution of different generation technologies and the main factors affecting their development: the cost of primary energy;

environmental constraints; Member State governmental plans for development of renewable forms of energy, clean energy sources, etc.; and associated economic and technical plans for existing generating units, including future life extension, uprating or retirement/decommissioning. This analysis provides a forecast of installed capacity for each year up to the planned time horizon (present time and present time+5, +10, +20, +30 years).

This yearly installed capacity is then scaled down to include a predicted/proven capacity factor that is specific to each generation technology, to better estimate the overall availability of the generating units.

In the analysis of electricity demand described in Section 4.1.1, the renewable energy generating units connected at distribution levels are generally treated as a decrease in electricity demand accounted for at transmission levels.

The outcome of the analysis of the evolution of the electricity generation is published in ‘generation master plans’.

4.1.3. Modelling power flows and power exchanges

Power flows and power exchanges are analysed to model current and future evolution of power exchanges within an electrical system and, if applicable, the power exchanges with other electrical systems.

If relevant, the power flows and exchanges with the electrical networks in neighbouring countries or regions¹⁰ are included in the energy planning based on the transmission capacity that is available either for commercial exchanges of power or to provide reserves. In accounting of the power flows between electrical networks, power exports are treated as variations of demand and power imports are considered variations of generation.

Grid systems are required to deliver the power from generating units to the centres of demand within acceptable voltage and stability limits while meeting the regulations established to prevent overloading the grid system components. The expected power flows in the electrical network(s) are modelled to ensure the effectiveness of the transmission and distribution system, to assess the electrical losses and to determine any need for reinforcement of the systems and equipment (power lines, power stations, transformers, etc.) in future grid plans.

The results of the grid development studies and power system simulations are published in ‘grid master plans’, which describe the structure of future stable and efficient electrical grid systems.

4.1.4. Assessing the electricity generation–demand balance

For different time horizons (present time and present time+5, +10, +20, +30 years) and different timescales (yearly, weekly, daily), the balance between electricity generation and demand needs to be assessed to identify the risks on security of electricity supply. The power margins are calculated using probabilistic methods. The risks of lack of energy or reserves are also in these probabilistic methods. Sensitivity studies on gross domestic product, primary energy prices and development of renewable energy may complete this step of assessment to provide a better understanding of the ‘system adequacy study’, which is often the name of the document reporting this assessment process and its results.

10 In some cases, the power flow and exchanges may include regional electricity networks (e.g. North America, Scandinavia or the regional electrical systems within the same country, such as in China or the USA).

4.2. ASSESSMENT OF NEEDS FOR FLEXIBLE OPERATION OF NUCLEAR GENERATING UNITS