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In recent years, the number of electrical devices using direct current in industry, the office and households has increased. This is generally obtained from the alternating current mains network via electronic adaptors. At the same time, more and more energy supply systems that supply direct current are in use, such as photovoltaic systems. In light of this, both companies and private individuals are increasingly becoming interested in direct current supply, at least to complement their existing supply.

Why alternating current?

The reason that power is supplied from the grid with alternating current is not immediately obvious to begin with. In 1882, Thomas Edison was focused on direct current, whilst his competitor George Westinghouse was working on alternating current technology. At one point, both the competing systems ran parallel with each other before the tide turned toward alternating current.

The key advantage of alternating current at the time, was that the alternating voltage can be varied using transformers, which is not possible with direct current. At first the voltage could be increased to a level of a few thousand, and then subsequently to more than 100,000 V, while simultaneously decreasing the current with the same transmission power. This means that smaller cable cross-sections can be selected, making power transmission over longer distances much more cost-effective.


When it comes to the transmission of direct current, however, things have progressed in the last few years. The initial main advantage of alternating current compared to direct current has since been eroded. Converters that can be used to increase or decrease voltage to the required rate have been around for several years now.

This high voltage direct current transmission (HVDC) technology can also be used to transport direct current with an extra-high voltage of up to 1100 kV over long distances. Although these lines are more expensive to build, the higher costs are offset by a significant reduction in energy loss.

Production and use of electricity

More and more electricity is now obtained from renewable energy sources, and it is also more frequently produced where it is used. An example of this is private photovoltaic systems, which produce direct current.

This makes sense as the number of electrical consumers that run with direct current is constantly increasing, from computers to televisions, through to LED lighting.

In practice, however, the types of current used create a paradoxical situation: the direct current produced is converted via an inverter (with losses) before being fed into the grid as alternating current. For certain consumers, the alternating current from the socket is converted back into direct current (again with losses) via suitable adaptors in order to supply computers or lighting with power.

Although the losses in each case are low, in total the countless individual adaptors generate huge amounts of waste heat and considerable costs.





If high-voltage lines can already be operated with direct current, why not also use direct current for low-voltage networks in offices and households? The problem is that the technology and material requirements between both systems are, in part, very different. For example, with alternating current, the switching arc extinguishes itself when the current crosses the zero point.

However, with direct current, depending on the type of load and at high currents, switching arcs no longer extinguish themselves from a voltage of 15–20 V DC after contact opening due to the fact that there is no zero point. Additional measures have to be taken and cables and insulation materials have to be specially adapted for direct current. Various working groups, such as the ‘DC-Schutzsystem’ (DC Protection System) project funded by the Federal Ministry for Economic Affairs, in which Doepke is also involved, are working on the (further) development of suitable components.


There is good reason for the recent increase in interest – using direct current in low voltage ranges, such as homes, office buildings or industrial facilities, offers considerable savings potential. A central transformer replaces the many individual transformers and adaptors, thus saving energy. It can also reduce the price of end devices. There are already some office buildings in which computer centres, air-conditioning systems and lighting are supplied by direct current. There are also industrial pilot projects in which robots in individual production sections are already operated entirely with direct current.

However, little is known about the behaviour of a DC network, in which many different consumers and sources interact, so research is required. The DC-Schutzsystem development project is working on forward-looking network monitoring for DC networks and the components contained therein. Through continuous monitoring. it should be possible to detect deteriorations, such as decreasing insulation resistance in cables. This should make it possible to replace any individual components before any damage or faults occur.


Direct current is due for a comeback. It will certainly not replace alternating current technology for the time being; in some areas, however, the use of direct current makes economic sense. As things stand, the most efficient way is to use both systems in parallel. However, the DC-Schutzsystem development project, amongst others, is looking at exactly what this coexistence might look like, what standards and guidelines are required and how protection systems should be designed.

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