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Glossary · Energy industry & energy transition

HVDC, High-Voltage Direct Current Transmission

HVDC High-voltage direct current transmission transmission grid grid expansion Direct current electricity grid grid operation

HVDC stands for high-voltage direct current transmission. Because of the very high voltages, German laws usually speak of extra-high-voltage direct current transmission. It refers to a technology in which electrical energy is transported over long distances as direct current at high voltage. At the ends of an HVDC link there are converter stations. They convert the alternating current from the existing grid into direct current and, at the other end, back into alternating current. Only the link itself therefore carries direct current. The transmission grid around it remains an alternating current system.

The most important parameters are voltage, power and length. The voltage is usually several hundred kilovolts. The German north-south links, for example, use 525 kilovolts. The transmissible power is stated in megawatts or gigawatts. A link rated at 2 gigawatts describes the maximum power it can transmit simultaneously between two specific points in the grid. The energy transported is power multiplied by time. If such a link ran uninterruptedly at full power for an entire year, this would amount to roughly 17.5 terawatt-hours (2 GW times 8,760 hours). This is a theoretical upper limit. How much actually flows depends on whether power can be fed in at one end and taken up at the other. Maintenance periods and outages also count.

A direct current link within an alternating current system

The difference from classic three-phase transmission lies not only in the type of current. Alternating current lines are directly integrated into the synchronous interconnected grid. How much power flows over them follows from the physics of the grid. What matters most are the voltage angles between the nodes, the impedances of the lines (at extra-high voltage mainly their reactance) and the locations where power is fed in and withdrawn. In an alternating current grid, electricity therefore does not take a contractually agreed route. It spreads across all parallel paths.

An HVDC link, by contrast, can be controlled via its converters. Within technical limits, the grid operator specifies how much active power flows in which direction. HVDC is therefore particularly suitable for targeted transport between two grid regions, for submarine cables, for very long point-to-point links and for coupling grids that do not run synchronously.

The converters are not a side issue. They cost a lot of money, need space, cause conversion losses and place their own demands on grid operation. The core of a station is the converter made of power semiconductors. In addition there are transformers and connections for direct and alternating current. Modern installations mostly work with voltage source converters (VSC). They transmit not only active power but can also provide reactive power and thus support the voltage. Even so, an HVDC link does not replace the grid around it. At both ends it needs a sufficiently strong alternating current grid that can absorb the power fed in or withdrawn.

Distinction from grid expansion, power superhighway and storage

HVDC is often equated with grid expansion. This is imprecise. HVDC is one technology within grid expansion, not the whole of grid expansion. The latter also includes new alternating current lines, reinforced existing routes, transformers, switchgear, reactive power compensation equipment, grid automation and better operating procedures. An HVDC link solves a transport problem between defined points. Bottlenecks in distribution grids, at the connections of wind farms or within a heavily loaded region do not disappear automatically as a result.

The word power superhighway (Stromautobahn) is also of limited help. It does show that large quantities of electricity are to be transported over long distances. But it obscures the fact that electricity does not travel from A to B like a single vehicle. The HVDC link itself does transmit in a directed way between its stations. However, it is attached to a meshed alternating current grid, and the loading of that grid continues to follow physical rules. Any change in HVDC power therefore changes the power flows in the surrounding grid.

HVDC must likewise not be confused with a storage facility. A line moves electricity in space, not in time. It can bring wind power from the north to consumption centres, but it does not store it for low-wind hours. Such shifts in time require storage, flexible consumers, power plants or other balancing options. HVDC can make these options more usable because it connects regions. It cannot replace them.

Why HVDC is becoming more important in a renewable power system

The importance of HVDC is growing because generation and consumption are moving further apart spatially. Much wind power is generated on the coasts, at sea or in sparsely populated areas. Large consumers, by contrast, are located in industrial and metropolitan areas. When conventional power plants there run less often or are shut down, more distant generation sites must help to support the supply. This creates transport demand in the transmission grid.

In Germany this is particularly evident between the windy north and the load centres in the south and west. The largest example is SuedLink. The link consists of two direct current projects: from Brunsbüttel to Großgartach/Leingarten and from Wilster to Bergrheinfeld. Each project transmits 2 gigawatts at 525 kilovolts and is built as an underground cable. In October 2025 the Federal Network Agency (Bundesnetzagentur) approved the last section. According to the Federal Network Agency, the lines are to be connected to the grid by 2028. Further HVDC projects are SuedOstLink, A-Nord and Ultranet. Such links are intended to reduce bottlenecks, lower the need for redispatch and bring renewable electricity better into the overall system. In redispatch, grid operators require power plants or other installations to operate differently from what the electricity market has produced, so that grid limits are observed. These interventions cost money. They show that trading transactions and physical grid capacity are not identical.

HVDC is also important for offshore wind energy. Electrically, a cable acts like a long capacitor. With alternating current, it has to recharge in every cycle. This produces a charging current that loads the cable but transports no usable active power. The longer an alternating current cable is, the larger this reactive share becomes. Without compensation, beyond a certain length no active power can be transmitted at all. According to the Federal Network Agency, alternating current underground cables need reactive power compensation after only about 25 to 30 kilometres, and alternating current overhead lines from about 100 kilometres. With direct current, this charging effect does not occur in operation. For large offshore capacities over long sea distances, a converter platform at sea and a station on land are therefore needed. This is costly, but it is what makes transport over such distances possible in the first place.

Losses, costs and economic assessment

HVDC is often regarded as low-loss. This is only true if it is made clear what is included in the calculation. Over long distances, the direct current line itself loses less than a comparable alternating current solution. There are no reactive currents and no dielectric losses, and the skin effect does not apply. On the other hand, additional losses arise in the two converters, regardless of line length. Whether HVDC is more efficient or cheaper overall therefore depends on length, power, voltage, type of route and converter costs.

For overhead lines, direct current usually pays off only over very long distances. A presentation by TransnetBW gives, for an example with 400 kilovolts and 1,200 megawatts, a turning point for losses from about 400 kilometres. In other presentations the values are significantly higher, depending on the assumptions. For underground and submarine cables, HVDC can be advantageous much earlier, because long alternating current cables quickly reach technical limits. None of these thresholds is universally valid. They always apply only to specific assumptions.

In Germany, the choice between underground cable and overhead line is also a political decision. Since 31 December 2015, section 3 of the Federal Requirements Plan Act (Bundesbedarfsplangesetz, BBPlG) has provided that direct current projects marked E are in principle to be built as underground cables. Overhead lines are permitted only on partial sections and under narrow conditions, and minimum distances to residential buildings also apply. This priority for underground cables was meant to increase acceptance, but it makes the lines considerably more expensive. On 29 April 2026, the Federal Cabinet adopted an amendment to the BBPlG. Under it, overhead lines are in principle to apply to newly included direct current projects, expressly named being DC42 and DC42plus. This is intended to reduce costs. For SuedLink and other already approved underground cable projects, the draft changes nothing according to its wording. Whether and in what version the amendment enters into force is decided by the parliamentary procedure. In any case, the costs of grid expansion are passed on to electricity consumers via grid charges.

The economic assessment of an HVDC link must therefore not look only at construction costs. Avoided redispatch costs, less curtailment of renewable installations, more exchange between regions, security of supply and the available alternatives also count. Alternatives can be regional generation, storage, load shifting, other grid reinforcements or changed market rules. A line is worthwhile if its benefit to the system over its entire service life justifies the costs. This benefit does not follow from installed capacity alone but from actual operation in the specific grid.

Controllability and grid operation

A technical advantage of HVDC is controllable active power. Grid operators can set power flows in a targeted way and thus relieve certain bottlenecks. This controllability has limits. An HVDC link only acts where it is connected. If the alternating current grid before or behind the stations is already overloaded, expansion, reinforcement or different operation is needed there too. The grid connection points are therefore not merely the ends of a route but central decisions in grid planning.

Coupling via converters is also important for system stability. Synchronous generators in classic power plants have large rotating masses. These physically slow down rapid frequency changes (inertia) and supply high short-circuit currents in the event of a fault. Converters behave differently. They can emulate or provide grid-supporting functions, but must be suitably designed, controlled and integrated into operational management to do so. At the same time, HVDC decouples grids and nevertheless exchanges power between them. This is useful for international links, because electricity trading becomes possible without all grids having to run synchronously.

The more large converters operate in the grid, the more important control, protection technology and coordination become. An HVDC link is not a passive cable. It is a controllable asset. Its behaviour is connected with frequency and voltage maintenance, short-circuit power, grid protection and congestion management. Incorrectly set parameters or poor coordination can make system operation more difficult. Good planning therefore covers not only routes and capacities but also operating rules.

Institutional context in Germany

HVDC projects do not arise from technical reasons alone. In Germany, the transmission system operators determine the need in the Network Development Plan (Netzentwicklungsplan), and the Federal Network Agency confirms it. The legislator then stipulates in the Federal Requirements Plan Act that a project is necessary and urgent for energy-industry reasons. For projects crossing state or national borders, the Federal Network Agency subsequently carries out federal sectoral planning (Bundesfachplanung) and the plan approval procedure (Planfeststellung). The costs are regulated and refinanced via grid charges. HVDC is thus also an institutional question: who plans, who decides on the need, who bears the costs, who experiences the burdens locally, and who benefits from fewer bottlenecks?

These responsibilities explain why HVDC projects are often controversial in public debates. The benefit of a line arises across regions. Construction sites, routes and interventions in the landscape, by contrast, burden individual places. The alternatives are not free either. If transport capacity is lacking, power plant dispatch is interfered with more often, renewable installations are curtailed, or additional generation and flexibility must be available elsewhere. For an objective assessment, line costs, operating costs, avoided bottlenecks and alternatives therefore all belong on the table together.

HVDC thus denotes a specific technology for large-scale, controllable power transmission. The term becomes precise when it is used neither as a collective word for grid expansion nor as a simple solution to all problems in integrating renewable energies. HVDC is a powerful tool for clearly defined transport tasks. Its benefit depends on the connected grids, market rules, permitting, operational management and the spatial distribution of generation and consumption.

Sources

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Edited by eCMS on 12.11.2026 05:05

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