Electricity produced in places far from where it is consumed is fed into  transmission grid and conveyed at extra-high voltage (VHV), so that large amounts of energy can reach the various points in the country without significant losses. Supply is thus guaranteed regardless of the distance from power stations.

As electricity is difficult to store, production must be managed in real time to match consumption. This complex task, which requires highly specialised teams and technology, is called Overall System Management and is carried out by the Transmission System Operator (TSO).

The TSO articulates and monitors the entire chain, from production to consumption, in real time to guarantee that networks (transmission and distribution) have the capacity to deliver energy consumed both during high consumption periods (peak hours) and low consumption periods (off-peak hours).

Such management involves the management of the exit or entry of producers at certain times or on certain days.

To find out more about this, have a look at the pedagogical guide Electricity: how does it work?

Transmission system operator

The transmission system operator (TSO) is REN – Rede Elétrica Nacional, S.A.

The TSO performs the activity of Electricity Transmission, which includes the functioning, planning and development of the National Transmission Grid (NTG), including electricity interconnections with Spain, in order to allow the transmission of electricity from production to consumers fed at extra-high voltage or to delivery points of distribution grids. The planning of the NTG must be carried out in the medium and long term, over a 10-year time horizon, and take into account future development scenarios, both of distribution grids (increased consumption) and of new production capacity, especially of renewable energy, that may be intended to be connected to grids, in compliance with the planning criteria and legally established levels of quality of service.

As Technical Overall System Manager, the TSO must ensure the proper functioning of the electrical system and the coordination of the different infrastructures of the NTG, ensuring the security and continuity of electricity supply. It must also coordinate the management of  operation markets.

European Transmission System Operators provide information on infrastructure through the transparency platform of the European Network of Transmission System Operators for Electricity (ENTSO-E).

Every two years, the TSO draws up a 10-year investment plan: the Ten-Year Indicative Development and Investment Plan for the Electricity Transmission Network (PDIRT-E). This Plan is subject to public consultation and to an opinion from ERSE and the Directorate-General for Energy and Geology, and is approved by the Government, after being discussed in Parliament. This national investment plan must take into account the non-binding EU-wide Ten-Year Network Development Plan prepared by ENTSO-E.

ERSE is responsible for the economic regulation of the electricity transmission activity, setting the tariffs for the use of the transmission network and the operator’s revenues.


Certification of the transmission system operator

The certification of the operator of the National Electricity Transmission Grid (NTG) is an obligation resulting from European directives on the internal energy market (Directive (EU) 2019/944, from the European Parliament and the Council, from 5 of Juin, for the electricity sector) and their transposition into national law (Decree-Law No 15/2022 of 14 of January), which require the unbundling of Transmission System Operators (TSOs) from production and supply activities.

As required by law, ERSE submitted its draft decision to the European Commission in March 2014 and the European Commission published its Opinion in May of the same year (subject to a corrigendum in July 2014).

On 9 September 2014, ERSE issued a decision on certification under a full ownership unbundling scheme, subject to the fulfilment, within 8 months, of a set of certification requirements aimed at ensuring its independence.

The referred company (and REN - Redes Energéticas Nacionais, SGPS, S.A., which controls it) is subject to compliance with the issued certification decision, in order to be approved and appointed as TSO by the member of the Government responsible for the energy sector.

On 31 July 2015, ERSE decided to bring into force the decision on the certification granted  to REN - Rede Eléctrica Nacional, S.A., after having established that the established conditions had been met.

Now that the certification process has been completed, ERSE is exercising its powers to monitor the TSO’s obligations in relation to full legal unbundling, in accordance with national and EU legislation.

Every year, ERSE submits to the European Commission a Report on the Electricity and Natural Gas Markets, assessing compliance with the requirements established in the TSO certification decision.