Transmission ensures the flow of gas through the high-pressure network between entry points and exit points under appropriate conditions of  quality of service.

The National Natural Gas Transmission Network (Rede Nacional de Transporte de Gás - RNTG) integrates high-pressure gas pipelines (between 20 and 85 bar) and connects the gas system infrastructures - underground storage and liquefied natural gas (LNG)  terminal - and international interconnections to high-pressure consumption points (mainly power and cogeneration plants) and exits to the distribution networks. The network includes infrastructure such as pipelines and branches, gas regulating and metering stations (GRMS), custody transfer stations (CTS) and other elements.

Rules governing the use of the transmission network are defined in the Regulation on Access to Networks, Infrastructures and Interconnections (RARII), approved by ERSE, and in European Network Codes - namely the Network Code on Interoperability [Commission Regulation (EU) 2015/703, of 30 April 2015] and the Network Code on Capacity Allocation Mechanisms [Commission Regulation (EU) 2017/459 of 16 March 2017] - the implementation and monitoring of which is assigned to National Regulatory Authorities and to the European Union Agency for the Cooperation of Energy Regulators (ACER).

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

The certification of the operator of the National Natural Gas Transmission Network (RNTGN) is an obligation that stems from the European directives on the internal energy market (Directive 2009/73/EC of the European Parliament and of the Council of 13 July 2009 for the natural gas sector) and their transposition into national legislation (Decree-Law n.º 62/2020, of 28 August), which enforce the unbundling of Transmission Network Operators (TSOs) from production and supply activities.

In accordance with the law, ERSE submitted its draft decision to the European Commission in March 2014 and, in May of that year, the European Commission published its opinion (which was rectified in July 2014).

On 9 September 2014, ERSE issued the decision on certification under full ownership unbundling, subject to compliance, within 8 months, with a set of certification conditions designed to guarantee independence.

REN Gasodutos, S.A. (and REN - Redes Energéticas Nacionais, SGPS, S.A., which controls it) is subject to compliance with the certification decision issued with a view to its approval and designation as a TSO by the member of the Government responsible for the energy area.

On 31 July 2015, ERSE decided to make the REN - Gasodutos, S.A. certification decision effective, after verifying that the conditions it had determined had been met.

Once the certification process has been finalised, ERSE exercises its powers to supervise the TSO's obligations relating to full legal and asset unbundling, under the terms of national and EU legislation.

Every year, ERSE sends the European Commission the Report on the Electricity and Natural Gas Markets, which analyses compliance with the conditions set out in the TSO's certification decision.