Direct current · Glossary

Definition and delimitation

Direct current is electric current whose direction does not reverse in the circuit under consideration. The associated direct voltage has a fixed polarity: one terminal remains positive, the other negative. In the International Electrotechnical Vocabulary of the IEC, direct current in the strict sense is a time-independent current. In a broader sense, it also includes a periodic current in which the direct component plays the main role. In practice, direct current is therefore often not perfectly smooth. Behind a rectifier, for example, a residual ripple remains, and switched-mode power supplies or pulse chargers work with clocked currents. Depending on the waveform, technical language then speaks of ripple or pulsating direct current, or of mixed current. What matters is that a direct component with a fixed direction predominates. With alternating current, by contrast, the direction changes regularly, and on average there is no direct component.

Direct current is described using the same basic quantities as any other type of current: current in amperes, voltage in volts, electric power in watts and electric energy in kilowatt-hours. In a simple direct-current circuit, power is the product of voltage and current. A battery at 400 volts delivering 50 amperes therefore supplies 20 kilowatts. If this lasts for one hour, that amounts to 20 kilowatt-hours. Unlike with alternating current, in a steady-state direct-current circuit there is no phase shift between current and voltage, and thus no reactive power in the classical sense. For the power system, the type of current matters because energy flows, conversion losses, protection technology and grid connections depend on which type of current and which voltage level is used.

Where direct current comes from and where it is used

Direct current arises wherever a source with fixed polarity provides charge. Batteries, accumulators, fuel cells and photovoltaic modules supply direct current. Many loads also work internally with direct current: computers, LED lighting, consumer electronics, charging electronics, controllers, servers, telecommunications equipment and a large part of modern industrial electronics. The fact that these devices are usually plugged into a socket of the alternating-current grid does not change this. Between the socket and the device sits a power supply unit. It rectifies the alternating current, adapts the voltage and regulates the current electronically.

Relationship to the alternating-current grid

In the public power grid in Germany and continental Europe, electrical energy is predominantly transmitted and distributed as alternating current at 50 hertz. In transmission and distribution grids, three-phase current is the norm. Three alternating voltages, each phase-shifted by 120 degrees, work together. This technology is closely linked to transformers, rotating synchronous generators, grid protection concepts and synchronous grid operation. Direct current has no grid frequency and no phase angle. As a result, certain alternating-current effects are absent, for example capacitive charging currents in long cables during continuous operation. On the other hand, direct current places its own demands on converters, switches, earthing and protection. And because direct voltage cannot be transformed with a simple transformer, power-electronic converters are needed for this.

Common misconceptions

Direct current is often equated with low voltage or with small devices. This is wrong. A USB charger works with a few volts of direct voltage, whereas high-voltage direct-current transmission works with several hundred kilovolts. In the German SuedLink project, for example, it is 525 kilovolts. Conversely, alternating current is not automatically high voltage. The type of current describes how the current direction behaves over time, but not the voltage level, the power or the danger. The types of current are also treated separately in legal terms. The European Low Voltage Directive 2014/35/EU applies to electrical equipment designed for rated voltages between 50 and 1000 volts alternating current or between 75 and 1500 volts direct current, with certain exceptions. These limits apply to making products available on the market. They are neither a general hazard threshold nor an installation rule for systems. For safety, what counts are voltage, current, duration of contact, the current path through the body, the protective measures and the disconnection behaviour.

A second misconception concerns efficiency. Direct current is not fundamentally low-loss, and alternating current is not fundamentally high-loss. Losses arise in lines through electrical resistance, in transformers, in converters, in switching devices and in the connected loads. In certain applications direct current can bring advantages, for example with long submarine and underground cables, very long point-to-point connections, battery storage, photovoltaic systems or data centres with internal direct-current distribution. In other cases the existing alternating-current grid, with transformers, established protection concepts and widely available equipment, is economically and technically more favourable. Which type of current is suitable depends on the application, distance, voltage level, controllability, protection requirements and integration into the existing grid.

Photovoltaics and inverters

In the power system, direct current shows up above all at the interfaces between generation, storage, consumption and the grid. Solar modules generate direct current. If this electricity is to flow into the public grid, the system needs an inverter. It converts the direct current into grid-compliant alternating current and synchronises itself with the frequency and phase angle of the grid. It can also limit the feed-in power and, depending on its design and the connection conditions, provide reactive power or other grid-supporting functions. The inverter is therefore more than an accessory. It is where the direct-current source and the technical connection rules of the alternating-current grid meet. Many of these rules apply to it directly, such as requirements for grid protection, voltage support or behaviour during grid faults.

Battery storage and electric vehicles

The situation is similar with a battery storage system. A battery stores electrical energy chemically and releases it as direct current. Whether in a home storage unit, a large-scale storage facility or an electric vehicle, the control system and power electronics determine when and at what power it is charged or discharged and how the connection to the alternating-current grid works. In home systems with photovoltaics, a distinction is made, for example, as to whether the storage unit is connected on the direct-current side or, behind the inverter, on the alternating-current side. The number of conversion steps and the losses depend on this. The benefit of a stationary storage unit therefore does not result from battery capacity alone. Also important are the converter power, the grid connection, the measurement and control technology, the market rules and the question of whether the storage unit increases self-consumption, reduces load peaks, provides balancing energy or eases grid bottlenecks.

Electric vehicles follow the same basic principle in a different form. The vehicle battery is a direct-current store. When charging with alternating current, an on-board charger built into the vehicle rectifies the current. Its power limits the possible charging power. With DC fast charging, the charging station takes over the conversion, and the battery is charged directly with regulated direct current. The vehicle's battery management specifies which current and voltage are permissible. AC charging and DC charging therefore differ not only in connectors and charging power. They also distribute power electronics, costs, heat dissipation, grid connection capacity and responsibility differently between the vehicle and the charging infrastructure.

High-voltage direct-current transmission

For the transmission grid, high-voltage direct-current transmission (HVDC) is a field of application in its own right. It is particularly suitable for long cable links, the connection of offshore wind farms and large power flows over long distances, when its technical and economic characteristics fit better than those of alternating current. In Germany, offshore wind farms in the North Sea and Baltic Sea are connected via HVDC. The SuedLink project consists of two HVDC links, from Wilster to Bergrheinfeld/West and from Brunsbüttel to Großgartach/Leingarten. Plastic-insulated underground cables with 525 kilovolts and a total transmission capacity of around 4 gigawatts are planned there. According to the grid operators, the higher voltage can transmit more power than 320 kilovolts and halves the number of cables required.

However, HVDC lines do not simply replace the meshed alternating-current grid. They connect specific points, need converter stations at both ends and must be integrated into grid operation management. An HVDC link can transport large amounts of energy and make load flows specifically controllable. It is not, however, an automatic solution for bottlenecks in the distribution grid, local connection problems or a lack of flexibility.

Switching and protection

Technically, direct current is demanding above all when it comes to switching and protection. With alternating current, the current passes through zero twice in each period, so at 50 hertz one hundred times per second. This helps to extinguish the arc in a switch. With direct current, this natural zero crossing is absent. Fault currents can rise very quickly, especially when capacitors in converters discharge, and arcs often burn more stably. Therefore, protection technology in direct-current systems needs switches and fuses specifically designed for this purpose, fast power electronics, clear earthing concepts and precise fault detection. This also applies to photovoltaic systems: switches and fuses on the direct-current side must be explicitly rated for direct current. Anyone who describes direct current merely as the simpler variant of alternating current overlooks these protection and operational issues.

Growing importance through power electronics

The fact that direct current is becoming more important is linked to electrification and the restructuring of generation. Many new sources and loads are coupled via power electronics: photovoltaics, batteries, electric vehicles, heat pumps with variable-speed drives, data centres and modern industrial processes. As a result, conversion takes place more often. A solar module supplies direct current, an inverter feeds in alternating current, a charger turns this back into direct current for a battery, and a drive converter generates regulated currents for a motor via an internal DC link. Each of these stages can make sense, but it costs money, causes losses and control effort, and creates technical dependencies.

It does not follow from this that the entire power grid should be converted to direct current. Existing alternating-current grids are deeply anchored in equipment, protection technology, standardisation, market processes and responsibilities. Generator connections, balancing, grid operation management, metering and many regulatory requirements build on this infrastructure. In some areas, however, direct current can be very expedient:

within devices and power supply units,
in battery systems and photovoltaic strings,
in fast-charging parks and data centres,
in industrial direct-current grids,
in HVDC links and offshore connections.

The actual system question is then at which point conversion steps can be avoided without compromising protection, standardisation, security of supply or economic viability.

Assessment

Direct current makes visible that modern power supply is increasingly organised via interfaces. Between source, storage, load and grid stand converters, control systems, communication systems and connection conditions. These components determine how quickly power is available, what grid support is possible, what losses occur and who has to disconnect in the event of a fault. The term itself describes only a type of current. Its practical significance, however, lies in the connections between photovoltaics, storage, electromobility, power electronics and grid operation, as well as in the rules under which electrical energy is integrated into the public supply.

Sources

IEC 60050 – International Electrotechnical Vocabulary, IEV 131-11-22: direct current — International Electrotechnical Commission (IEC); unbekannt. IEV-Nummer 131-11-22, Bereich Netzwerktheorie / Allgemeines
Richtlinie 2014/35/EU (Niederspannungsrichtlinie) — Europäisches Parlament und Rat der EU / EUR-Lex; 2014-02-26. Artikel 1 (Gegenstand und Geltungsbereich)
SuedLink: Auftrag für kunststoffisolierte 525-Kilovolt-Erdkabel vergeben — TransnetBW GmbH; 2020-06-29. Pressemitteilung, Haupttext
Technologie und Umwelt – SuedLink — TransnetBW GmbH; unbekannt. Abschnitte zur HGÜ und zu den 525-kV-Kabeln

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Edited by eCMS on 12.11.2026 05:05