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Glossary · Energy industry & energy transition

HVDC, High-Voltage Direct Current Transmission

HVDC High-voltage direct current transmission electricity grid transmission offshore wind Interconnector

HVDC stands for High Voltage Direct Current. It refers to the transmission of electrical energy using high-voltage direct current. In German, the abbreviation HGÜ (Hochspannungs-Gleichstrom-Übertragung) is common. The transmission grid is operated predominantly with three-phase alternating current. With HVDC, by contrast, direct current flows between two or more converter stations, which are also called converters. These stations convert alternating current into direct current and, at the other end, back again. A special form is the HVDC back-to-back link (HGÜ-Kurzkupplung). Here, both converters are located at the same site and there is practically no line between them. Such a back-to-back link serves only to connect two alternating current grids that do not run synchronously.

Key parameters: voltage, power, amount of energy

An HVDC system is described primarily by its voltage, current and transmissible power. Most installations operate at several hundred kilovolts. They transmit from a few hundred megawatts up to several gigawatts. Two examples from Germany: the two SuedLink connections operate at 525 kV and each transmit 2 GW, so 4 GW together. For new offshore connections, the Flächenentwicklungsplan (area development plan) sets ±525 kV and 2,000 MW per system as the standard. Older connections in zones of the North Sea closer to the coast use ±320 kV. In China there are ultra-high-voltage installations with significantly higher voltages. These are, however, not typical for Europe.

Power and energy must be kept apart. A 2 GW connection does not automatically deliver a fixed quantity of electricity per year. It first provides a technical transport capacity. How many terawatt-hours actually flow depends on how heavily and for how long the line is utilised. To illustrate: a four-lane motorway does not yet tell you how many cars drive on it in a year.

Distinction from alternating current transmission

Direct current is not fundamentally more modern or better. Alternating current has major advantages in meshed grids, because voltage levels can be changed easily with transformers and many consumers can be supplied well. For this reason, distribution grids and large parts of the transmission grids are built as alternating current grids. HVDC plays to its strengths in three cases: for large power over long distances, for long cables, especially submarine cables, and for coupling grids that do not run synchronously. Long alternating current cables have a high capacitance and constantly require charging current. This demand for reactive power increasingly restricts the usable transmission capacity as length grows. The direct current link itself does not have this problem in continuous operation. How the converters deal with reactive power, however, depends on their design, as the next section shows. Because the power flow is set via the converters, HVDC can also connect different frequencies or one synchronous area with another.

HVDC therefore does not replace the alternating current grid. An HVDC connection is usually a targeted transport axis between two grid points. The alternating current grid remains necessary to distribute electricity across the area, to integrate generators and consumers, and to balance short-term fluctuations. Even an offshore wind farm connected via HVDC ultimately feeds into an alternating current grid.

Converter technology and system design

The converter stations are the central component of an HVDC link. Two basic types are distinguished:

  • Line-commutated converters (LCC) work with thyristors. They are regarded as proven for very high power. However, they themselves consume reactive power, therefore need filters and compensation equipment, and depend on a sufficiently strong alternating current grid. To reverse the direction of flow, the polarity of the voltage is changed.

  • Self-commutated, voltage-source converters (VSC) work with semiconductors that can be switched off, usually IGBTs. They can control active and reactive power largely independently of each other, support the voltage, and can also be connected to weak grids. The German offshore grid connections are built in this self-commutated technology according to the Flächenentwicklungsplan.

The configuration also differs. A monopole has one live conductor. A bipole has two poles of opposite polarity. If one pole of a bipole fails, part of the power can continue to be transmitted. For the 2 GW offshore systems, the Flächenentwicklungsplan prescribes a bipole with a metallic return conductor. The reasons given are higher reliability and better controllability. The effect an installation has on the overall system therefore does not follow from the abbreviation HVDC alone. What matters are the design, control, integration into grid operation, and the grid connection point.

Typical applications

  • Long onshore connections: Large quantities of electricity are brought from generation hubs to consumption centres. In Germany, for example, SuedLink connects Brunsbüttel with Großgartach and Wilster with Bergrheinfeld, predominantly as underground cable. The Bundesnetzagentur (Federal Network Agency) fully approved SuedLink in October 2025; commissioning is planned for 2028 according to project information. The status of this information is the date of the sources cited.

  • Submarine cables and offshore connections: Wind farms far from the coast are usually connected via converter platforms at sea and HVDC submarine cables. Parks close to the coast can also be connected with alternating current.

  • Interconnectors and back-to-back links: An HVDC interconnector links countries or synchronous areas. It can enable electricity trading and allows the power flow to be controlled more precisely than a pure alternating current coupling.

Controllability and integration into the grid

An HVDC line does not by itself transport electricity to where it is needed. The flow is actively specified: according to schedules, market rules, grid security requirements and technical limits. In doing so, grid operators must take into account system stability, bottlenecks in the adjacent alternating current grid, and the outage of individual items of equipment. If the feed-in or withdrawal point in the alternating current grid is connected too weakly, a direct current connection can simply shift a bottleneck to another location. A further point: if a large HVDC link fails, a great deal of power is suddenly missing. Under EU law, HVDC systems must therefore be designed so that the possible loss of active power infeed in a synchronous area remains limited to a value that the responsible transmission system operators set.

Losses and economic viability

Over large distances, HVDC has lower line losses than comparable alternating current solutions, especially for cables. The converter stations, however, have their own grid losses and cause high fixed costs. For short distances, alternating current is therefore often cheaper. The longer the route, the higher the power and the more difficult the corridor, the more likely direct current is to pay off. In the technical literature it is often stated that direct current becomes advantageous for overhead lines from a few hundred kilometres and for cables from just a few tens of kilometres. These are only rough guidelines. In individual cases, length, power, type of route, connection points and the purpose of the connection decide.

A new HVDC link is expensive. Besides the line, it needs converter stations, grid connections, protection and control technology, land, permits and construction work. Depending on the country and installation, this is paid for through grid charges, offshore levies or regulated revenue mechanisms. In return, such connections can reduce costs elsewhere: for example through less curtailment of renewable generation, less redispatch or better balancing across borders. Whether this pays off depends on utilisation and the bottlenecks in the overall grid. What is politically often called an electricity motorway is, in regulatory terms, a concrete grid project with defined costs, risks and responsibilities.

Legal framework and responsibilities

In the EU, the grid connection of HVDC systems and of power generation installations with a direct current connection, such as offshore wind farms, is subject to the requirements of Regulation (EU) 2016/1447. Among other things, it regulates active power control, frequency support, reactive power and voltage. Also covered are HVDC systems that connect synchronous areas or control areas, including back-to-back links. Systems with a grid connection point below 110 kV are excluded, as long as no cross-border impact is demonstrated. For existing installations, only individual articles apply. In Germany, the transmission system operators plan and operate the HVDC connections. For projects that appear in the Bundesbedarfsplan (Federal Requirements Plan) and cross federal state or national borders, the Bundesnetzagentur is the permitting authority under the Netzausbaubeschleunigungsgesetz (NABEG). For cross-border interconnectors, capacity allocation, congestion revenues and cooperation between regulatory authorities come into play. If offshore connections are meshed into direct current grids in future, protection concepts, control and the delimitation of responsibilities will become significantly more demanding.

Common misconceptions

An HVDC link is neither a storage facility nor a power plant. It neither generates electricity nor stores it. It only changes where electricity is available and at which points bottlenecks occur. For security of supply, the installed transmission capacity alone therefore does not count. It is also important whether generation or imports are available on the feed-in side, whether the remote end can take up the electricity, and whether the grids on both sides remain stable. HVDC improves transport and controllability, but does not solve a lack of firm capacity. Local grid reinforcement, flexible demand and market rules for bottlenecks are still needed.

HVDC is important for the energy transition because generation and consumption are located further apart spatially. Much wind power is generated on the coast and at sea. Industry and cities are often far from there. With heat pumps, electric mobility and electrolysers, the need for high-capacity transport routes is growing. The term HVDC only becomes precise when the route, power, converter technology, grid connection point, operating rules and market integration are named.

Sources

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Edited by eCMS on 12.11.2026 05:06

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