Assessing optimal implementation of flexible operation of a nuclear

A utility (public or private) that includes nuclear energy sources in its generation portfolio may consider adjusting the output of the nuclear units when optimizing the generation plan because:

14 These may be similar to the emergency load reductions described in a typical agreement between the grid system operator and a baseload operating nuclear power plant.

15 The protocols in some Member States are defined in regulatory documents. For example, the national/regional grid code may require all operational communications go only to the control room of the plant. Nuclear power plant owners/operators that have a 24/7 power trading organization may elect to have the requests for flexible operation communicated through them, although this would not be acceptable in some Member States where communications go directly to the generating plant operators. On the other hand, plant owners/operators without a power trading organization would need to establish a line of communication between the grid control centre and the control room staff.

— Nuclear generation will be balancing during certain periods of time.

— It will be necessary to keep enough non-nuclear generating units on-line to provide the needs for operational reserves and frequency control.

— The actual amounts of power being generated by the non-renewable and renewable energy sources may be higher than expected, thus affecting the initial generation planning.

The planning and scheduling of a generation portfolio, including a nuclear generation share, depend on the requirements by all regulations (nuclear, electrical, financial, commercial, etc.) and the requirements, expectations and specifications of other stakeholders of the electrical system (generation/storage technology utilization, demand response incentives, energy saving plans, development of smart grids, etc.). These requirements may be determined by the various stakeholders in different electrical regimes, and optimization of national and corporate portfolios differs in each case. In addition, in Member States where the electrical grid system is deregulated and privatized, there are incentives — within market rules and regulations — for the generating units, including nuclear units, to provide frequency control and for management of power flows across interconnections. These may include, for example, payments for providing ancillary services and balancing mechanisms.

At the operation level, grid system operators monitor the performance of the generating units (electrical output and frequency, typically in seconds or minutes), taking action to balance overall demand and generation by requesting plants to adjust output, as necessary.

4.2.6.1. Vertically integrated electrical regimes

In vertically integrated electrical regimes, the grid system operator manages the optimization of the generation plan by utilization, in order of merit, of non-renewable energy sources (including run of the river hydroelectric generating units) and renewable energy sources (solar, wind, etc.) as codified in the policy and regulations, and then by identifying and ranking dispatchable generating units from the least to the most expensive according to generation costs provided by plant owners/operators. This optimization of the cost of balancing demand and generation is performed by integrating the costs and constraints for all grid users (unit performances, contracts such as power purchase agreements, redispatch costs due to plant or grid outages or insufficient transmission capacity, loss coverage, etc.).

Again, the obligation to operate a nuclear power plant flexibly will be determined by:

— The probability that the plant will participate in balancing;

— The number of hours in the plant lifetime when it will participate in balancing.

If the probability and the number of hours are not significant, the constraints brought by intermittency of renewable energy sources or by needs for power reserves can be solved using specific regulations imposing curtailment for renewable energy sources and specific capacity payment for reserve capacity, respectively. These regulations can be based on cost–benefit analysis assessing the overall benefit of these measures.

4.2.6.2. Unbundled electrical regimes

In unbundled electrical regimes, the owners/operators of generation, transmission and distribution are generally separate organizations, each with different roles in determining the cost and constraints for their entities. The dispatch of generation may be the responsibility of the transmission system owner, or of a separate organization (e.g. an independent grid system operator). There may also be separate supply companies that trade between generating companies and end consumers. In this arrangement, the different users of the grid system have to be treated in a fair and equable manner, to avoid competition distortion in determining the generation plan.

Optimization of flexibility (i.e. optimization of generation dispatch for grid flexibility) in the unbundled electrical regimes shows wide variety in Member States. Some examples of these practices are:

— In some Member States, optimization of the whole electrical system is performed by the grid system operator(s) by integrating costs and constraints (technical, contracts, outages, etc.) for all users. Each generating company notifies the grid system operator of the availability, prices and technical parameters

such as ramp rates, for each of its generating units, typically for the day ahead. The grid system operator then produces an optimized schedule giving the planned load profile for each generating unit. The individual generating units are then expected to follow the load profile they have been given. The grid system operator will request changes from the planned load profiles in real time where necessary to achieve system balancing.

— In some markets in Member States, individual generating companies attempt to optimize the operation of their generating units to meet their contracts, and submit their planned load profiles to the grid system operator. The individual generating units are expected to follow the load profiles that they have submitted.

In these regimes, however, the grid system operators also try to optimize only their obligations and areas of profit. Therefore, the grid system operator will order changes to the load profiles submitted by the generating units where necessary to achieve system balancing.

— In some Member States, companies hold both generation and demand assets and are responsible for achieving an approximate balance between their own generation and demand, which can include energy transferred across interconnections. Such companies, which are referred to as balancing responsible parties, also provide their demand–generation profiles to the grid (transmission) system operator to ensure that their share in the total portfolio is balanced. The grid system operator collects and integrates all balancing responsible party positions, then adds its specific needs as loss coverage.

4.2.6.3. Fleet consideration at owner/operator optimization

The capability of a fleet of nuclear power plants to be operated flexibly offers more degrees of freedom when optimizing the operating costs of nuclear plants, as follows:

— In developing a schedule for 1–5 years ahead, plant outages for refuelling and for scheduled major maintenance are planned for by considering the best period for electricity prices (when prices are low) and for smoothing the use of industrial resources for repair and maintenance activities (Fig. 25).

— In planning the ‘months to 1 year ahead schedules’, shorter term options, e.g. delaying outages by reducing the thermal power (sequential down power plateaus or coastdowns) within the limitations of effective fuel cycle length and regulatory requirements for shutdowns for maintenance and inspections, are considered.

The concept of ‘usage value’ of generation is practised in French plants to optimize the use of each nuclear generating unit (Box 1).

— In planning the ‘weekly and daily schedules’, the generating units are optimally selected to balance electricity demand and to provide flexibility.

FIG. 25. Weekly break-down of the French nuclear power fleet outages in a ‘1 year ahead’ schedule in 2010 (courtesy of F. Farruggia, Électricité de France, reproduced from Ref. [9]). NPP — nuclear power plant.

Box 1. Usage value for nuclear generation optimization in France.

Usage value management is a way of allocating nuclear fuel savings to a plant in order to keep and/or optimize its outage or its generation schedule [9]. Usage value is an economic criterion used to decide whether the generation is to be used at present or in the future. Treating a nuclear generating unit as a stock of energy, the energy available and rationed throughout the year can be planned.

The usage value can then be used for ranking and classifying the generating units in merit order for supplying electricity. The graph below shows an example of this ranking and classification for every nuclear power plant unit in a fleet.

Courtesy of F. Farruggia, Électricité de France, reproduced from Ref. [9]. NPP — nuclear power plant.