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      The transformation of the energy mix has begun. It will accelerate under the pressure of public policy. The increase in electrical demand, the growth of intermittent renewable energies and their uneven distribution raise new issues for the electricity transmission system operators.

      As solar and wind power are not yet contributing to the flexibility requirement, the electricity system has less and less margin. This is illustrated by the increase in significant system events (SSE) observed in France and the UK over the last five years.  A number of significant incidents has prompted caution: the East-West separation of the European grid in January 2021, then the separation of the Iberian Peninsula in July 2021.

      bilans de surete
      SSEs are on the rise

      Every year, the French grid operator, RTE, measures the operating reliability of the power system by counting significant system events (SSE) classified according to a severity scale starting with 0 (no direct impact on system reliability) and then A to F. These events reflect the occurrence of incidents whose origins may be multiple.

      With 162 SSEs, 2022 is part of an overall trend that has continued to rise since 2017.

      Source: Safety Report 2022 – RTE

      In this context, nuclear power, controllable and adaptable, plays an essential role. Its resilience in the face of these new disruptions must be guaranteed, and the operator must be prepared to deal with a widespread network incident, however rare. The total loss of external power supplies is integrated into the design of the power plants and in the safety cases. Except in the case of islanding (see below), safety is ensured by the diesels and thermosiphon operation before the external sources are restored. Although these situations are foreseen, from a probabilistic point of view they should not be multiplied excessively.

      Separation of the European electricity grid

      On 24 July 2021, the European synchronous power grid split unexpectedly into two zones due to the cascade loss of several lines located on the French-Spanish border.

      The technical analysis of the event carried out afterwards by the European Network Operators Group (ENTSOE) showed that the initiating event was a fire, which caused a short-circuit on one of the two lines of the 400 kV Baixas-Gaudière link. As the reduction in transited power was not rapid enough (initial flow of almost 2,500 MW), a cascade of overloads progressively led to the loss of synchronism between the French and Spanish networks, with the final consequence of completely cutting off the Iberian Peninsula from the rest of the continental European network. Source: ENTSOE

      Nuclear power makes a major contribution to permanent voltage regulation by adjusting the excitation current of the alternator. The alternator produces or consumes reactive power, which modifies the voltage at the point of injection. This effect is subsequently passed on to all neighbouring points. Unlike frequency, maintaining voltage has a local influence. Nuclear power makes a key contribution to this. In spring 2023, RTE asked the Blayais site to keep two of its reactors connected to the grid to support the voltage plan for the South-West region. This resulted in the ten-yearly outage of reactor number 2 being postponed by almost a month.

      In recent years, the trend towards connecting renewable energies directly to the distribution grid and burying power lines has tended to lead to high voltage phenomena. This puts the alternators at nuclear sites closer to their operating limits and closer to the protection thresholds.

      Maintenance sur un transformateur a Penly

      Furthermore, it is crucial that the dynamic characteristics of the grid imposed on future reactors are technologically acceptable. This is why it is important that EDF continue its involvement in drawing up European network codes and reviewing them periodically. The discussions about ROCOF (rate of change of frequency), which will tend to increase with the disappearance of highly inertial controllable production units, are a good example of this involvement. While the development of renewable energies is tending to create a high ROCOF, it is also necessary to preserve the operating margins of controllable machines, which are themselves stabilising.

      Intervention DTG sur un transformateur
      Storm Ciarán

      On 1 and 2 November 2023, the violent winds and heavy rain from storm Ciarán hit north-western France. Lightning struck a very high voltage line, and the wind weakened an electrical bushing on the main transformer at Flamanville reactor No. 2, leading to islanding of both units. The reactors produced the energy required by their auxiliary systems to operate independently in complete safety. The transition took place normally and was correctly managed by the shift teams. Reactor No. 1 was quickly reconnected to the grid, while Reactor No. 2 had to be shut down to replace the damaged bushing. The shift teams’ training in preparation for the event helped to manage these transients.

      Strengthen theoretical and practical training

      EDF’s pressurised water reactors (PWR) have robust designs, with redundancy of external, internal and ultimate electrical supplies, which have been reinforced after the Fukushima Daiichi accident (ultimate emergency back-up diesels).

      The main challenge is the skills required to guarantee a high level of operational control, from 400 kV equipment to the smallest component of the electrical distribution system. From the discussions held on site and with the corporate support groups, I can see that additional efforts are needed in the areas of operations and electrical maintenance.

      Operator training in this area is mainly concentrated during their initial training. I would like to encourage more widespread use of the initiative taken by some sites who organise refresher training. The aim is to ensure that, beyond the application of procedures, the teams maintain an in-depth understanding of the electrical distribution diagram and also produce diagnostic and re-supply strategies. I also think it is important that each operator regularly carry out a scenario involving the loss of external power supplies and the re-construction of the network. Lastly, the skills of the Senior Authorised Persons (SAP) in configuring and isolating 400 kV, 225 kV and 6.6 kV equipment deserve particular attention. The aim is to strengthen their knowledge and give them confidence in their ability to perform such infrequent tasks.

      In maintenance, although initiatives are being taken at national level through guides and communities of practice, the risk is that they will only reach a limited number of local staff. The catalogue of training courses specific to the electrical field is well furnished (e.g., manufacturer training courses). However, everyone needs to improve their knowledge of electrical distribution. In particular, on-call electrical teams, who must work with the operator in the event of an incident, need to have a more advanced set of skills. These must be acquired through training and a development plan that includes carrying out (or observing) infrequent activities such as 400 kV or 225 kV isolations, safety train isolations or switchboard isolations. The training and development of these particular teams need to be monitored more closely.

      gerer equilibre offre demande
      Graphe de variation de puissance un reacteur 1300 MWe

      Nuclear Operations has embarked on a proactive work programme. It includes the creation of periodic interface meetings with the network operator to share information, anticipate developments and define the necessary countermeasures. It also includes an equipment aspect designed to strengthen the resilience of the sites: review of minimum standards for compulsory maintenance, development of lifetime strategies for certain components, targeted investments, and improvement of the network state monitoring methods. While these initiatives are to be welcomed, they have come a little late and need to be implemented quickly. The replacement of the equipment that initiated the event in 2021 will not be completed until 2024 (see 2021 report), hence the need to analyse and deal with the equipment causes of the events in 2023 as soon as possible.

      Europe vue espace empreinte lumineuse

      In conclusion, I encourage Nuclear Operations to take better protection against disruption from the UK grid, and the French to learn from any incidents from across the Channel, which are a precursor to what the massive arrival of renewable energy will cause on the continent.

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