Electricity transmission infrastructure has developed differently in many European countries. This is based on a number of national factors including; scale, topography and generation mix. Furthermore, levels of interconnection between countries vary greatly. As such, flexibility in the network code is needed to reflect the different situations which each country finds itself in, and to reflect the different contributions which are needed to ensure a high level of electricity transmission system reliability across Europe.

The Network Code on HVDC Connections and DC-connected Power Park Modules (NC HVDC) recognises these national and regional variations and it is for this reason that many of the requirements in the NC HVDC require further national specification. However, the code provides guidance on the way these requirements will be set and, in many cases, sets ranges within which values must be chosen. This will contribute to the creation of a European level playing field and being a first step in further harmonisation of connection rules.

The existence of HVDC requirements in national grid codes varies. A few countries already have codes in force or are in process of developing codes; but this is not the case for the majority. At present, many countries have no HVDC applications and therefore have had no reason for such rules.

The application of HVDC technology to complement the previously dominant electricity transmission technology of High Voltage Alternating Current is expected to expand greatly in the years and decades ahead. The reasons for the expected rapid expansion in HVDC applications include larger power transfers over longer distances and the connection to shore of very large offshore renewable energy installations. This strengthens ENTSO-E’s and the wider industry view of the need to pursue European-wide requirements, which may be further specified at national level or based on the needs of specific projects.

As the requirements are driven by the need to ensure the necessary contribution of all connections to system security, no distinction is made between different technologies. While it is recognised that different technical solutions may have different inherent technical characteristics, the requirements stated in the NC HVDC are based on system needs and consider the integrity of the power system, development trends in the future, and security of supply.

To ensure the ability to choose the best technical solution for a specific project, there is only one common set of requirements which every connection has to fulfil. The ranges of parameters, where applicable, give sufficient flexibility to apply more stringent requirements where system characteristics warrant doing so, and allow the connection of less advanced and costly technology where no additional capabilities are needed.

The Network Code on High Voltage Direct Current Connections (NC HVDC) will specify connection requirements for new DC connections, links between or within synchronous areas (parts of the European grid which are operated as one system and maintain the same frequency at all times) and new DC-connected Power Park Modules, such as offshore wind farms, which are becoming increasingly prominent in the European electricity system.

As this is a relatively recent energy development, few standards or grid codes exist, making a pan-European approach particularly beneficial. Following on from the Network Code on Requirements for Generators and the Demand Connection Code, the NC HVDC will build on the same foundations, to create a consistent and complete set of connection codes.

The conventional task for HVDC links is the bulk transfer of large volumes of energy over long distances (for example through lines between countries). Additionally, HVDC technology has been used like a firewall in its back-to-back connection between large AC transmission systems. These tasks will remain a major focus and HVDC technology will play a much greater role in connecting power sources located offshore.

As the proportion of electrical power transmitted by HVDC technology to major load centres (areas where large amounts of electricity is required) increases, the characteristics of HVDC technology, including its ability to respond quickly to system changes, will increase in importance and it will play a larger role in ensuring the security of the entire European system.

HVDC technology will be used increasingly in the coming years to develop interconnections between different parts of Europe and it is of the utmost importance that these new facilities are fit for purpose to maintain high levels of power system security and ensure an integration of the European energy market.

DC technology is also increasing in importance, as it is the main way of connecting offshore wind farms. These installations have different technical characteristics to conventional generation. For them to contribute to system security in a comparable way to traditional plants (which is vital as more renewable generation is added to the energy mix and as older plant closes), they need to be designed to provide these services. The NC HVDC will allow more renewable energy to be connected to the electricity transmission system while ensuring that they remain stable.

One of main principles in the development of the NC RfG, DCC and NC HVDC is the goal of a consistent set of connection requirements for new generators, demand and DC links, which take into account local system needs and inherent technical capabilities.

Whereas the NC HVDC details requirements for capabilities, it does not provide answers to operational or market-related issues. These rules can be found within the operational/market network codes, notably NCs Operational Security (NC OS), Load Frequency Control and Reserves (NC LFCR), Electricity Balancing (NC EB), Emergency and Restoration (NC ER) or appropriate national rules.

The NC RfG defines the requirements for generators connected to the main, synchronously connected parts of the European electricity transmission system. This system behaves in a different way to DC-connected (mainly offshore) networks, resulting in different system needs. Furthermore, for certain requirements, it may be possible to exploit the synergy of the PPMs and their connecting links. Such possibility of optimisation is encouraged by the NC HVDC where possible.

However, one objective of the network code is to clearly specify the necessary technical capabilities in order to enable the industry to consider these features for future PPMs and to develop corresponding technical solutions independently from the way of connection, whenever possible and reasonable. This would enable the design of PPM solutions fitting to both markets, i.e. offshore with HVDC and on-/near-shore by AC connection. Therefore, the NC HVDC specifies different requirements from the RfG whenever necessary because of the system specifics and sticks to the known necessities as given by the RfG, if reasonable.

HVDC technology will increasingly be used in the coming years to develop interconnections and it is of the utmost importance for these new facilities to improve power system security. To supplement existing HVAC corridors, extensive development of HVDC systems (either within one or between several control areas) is also planned in order to increase the flexibility and capacity of the entire system.

The overarching principle of network code development is to ensure that HVDC links, as an integral part of the transmission system and similarly to AC transmission lines, fulfil the required reliability standards and are capable of operating across a wide range of system conditions. This ensures that all users contribute to system security in an optimal manner and can connect to and use a robust and reliable system under the changing circumstances.

Furthermore, the requirements set in this network code need to be forward looking: the expected mid- and long-term developments need to be taken into account. Even more, the NC should enable and foster future improvements.

Europe is committed to decarbonising its energy sector and national and European policies are stimulating the development of large volumes of renewable energy sources (RES).

Both the RES technology itself, as well as the power electronics-based DC connections have different technical characteristics to the generators that have traditionally connected to transmission grids. For them to contribute to system security in a similar way to traditional plants (which is vital as more renewable generation is added to the energy mix and as older plant closes), they need to be designed to have certain capabilities and be able to provide certain services.

The requirements in the NC HVDC will allow this to happen. Without these requirements, it would not be possible to integrate very large amounts of renewable energy into electricity grids (for example countries such as Ireland and Denmark produce 50% of electricity from renewables at certain times).  Hence, the NC HVDC (together with NC RfG) is critical for countries to continue to connect large volumes of renewable energy in future.