In 1890, George Westinghouse with AC voltage and Thomas Alva Edison with DC voltage competed against each other in the so-called electricity war. At the time, alternating current technology won because of its ability to be transformed to the highest voltages using transformers and the long transmission distances this made possible. Edison's conclusion: "I am not discouraged, because every recognized error is another step forward." Today we can say that direct current was not a mistake, but simply too early for the times.
Compared to those days, energy is now increasingly generated, consumed and transported in the form of direct current. Some applications are shown here.

DC and its advantages

Photovoltaics and fuel cells generate DC voltage directly. Wind power generates DC voltage via full converters with a DC link and then AC voltage again. In some cases, energy is already being transmitted using high-voltage direct current transmission (HVDC). This is made possible by technological advances in power electronic circuits and semiconductors. In addition, the grid voltage is currently rectified in almost all consumers. The DC voltage generated is then used directly (switching network components, storage charging) or converted back into AC voltage, for example with the help of frequency converters.

The basic advantages of direct voltage over alternating voltage are:

• Compared to AC cables and busbars, 50 percent less copper is required due to the elimination of reactive power as well as the higher voltage with DC (≥ peak value of AC voltage with the same insulation requirements). Alternatively, the transmission losses can be significantly reduced. Installation costs are also significantly reduced by eliminating cables.
• The electronics of consumers become smaller and more efficient as the internal mains rectifier is no longer required.
• Renewable energies and storage systems can feed in their energy without further AC voltage conversion.

In view of the energy transition, it is therefore a necessary goal to further expand direct current and utilize its advantages. In the 'DC-Industrie2' research project, 39 partners from industry and research have been working on the question of how industrial direct current grids can be set up and used sensibly in industrial production halls since 2019. The aim is to develop a comprehensive, manufacturer-independent system concept for industrial power supply in production. Ten test facilities and transfer centers are now available to make it easier for users to introduce DC grids. An industrial DC system for a production hall offers additional advantages:

• Energy can be exchanged directly between consumers without converters. Braking energy in particular no longer has to be consumed via braking resistors.
• Photovoltaics from the company roof and storage systems can be integrated and used directly.
• The DC grid can continue to run independently in the event of an AC grid failure if sufficient storage is available.
• Load peaks are drastically reduced when storage systems are used and connected load is reduced. A test system with a peak load of 350 kW was reduced to 50 kW connected load.
• The line losses in the cables are reduced by 40 percent, as only two current-carrying conductors with a lower current are used for DC instead of three current-carrying conductors with a higher current for AC.
• Using the same copper cross-section for DC as for AC pays for itself within months through energy savings (≈64 percent).
• Reduction of energy consumption by five to eight percent in the test systems.
• Stored energy can also be made available across factories to compensate for fluctuations in demand and supply in power generation.

Block Transformatoren-Elektronik is involved in the research project with its own basic research department for the DC power supply units required in a DC grid for the auxiliary voltage supply. The focus is on a wide DC input voltage range, robustness and reliability. Furthermore, parallel connectivity or redundancy modules must enable a supply from the AC mains. Increased efficiency and a reduction in installation space were achieved in the research project through the use of silicon carbide semiconductors and innovative winding materials.

The author: Dr. Dennis Kampen is Head of IT, Research and Knowledge Management at Block Transformatoren-Elektronik.

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A comprehensive and coordinated product portfolio is required to meet the needs of the wide range of industrial applications. DC Industry2 has shown that DC grids make sense. They are particularly worthwhile in companies where many drives with frequency inverters and regular braking energy are installed and photovoltaics and energy storage are planned.