Smart Factory

Direct instead of alternating current?

03. Juni 2020, 15:02 Uhr   |  Tobias Schlichtmeier

Direct instead of alternating current?
© Fraunhofer IPA

In the DC laboratory at the Fraunhofer IPA the DC voltage factory of tomorrow is being tested.

Factories are currently operated with alternating current. However, there can be many advantages to running a factory on direct current. Researchers at two Fraunhofer Institutes are developing which ones these are and what they might look like in practice.

The Fraunhofer Institutes for Production Engineering and Automation IPA and for Integrated Systems and Device Technology IISB have launched the DC Industry 2 project. Together with more than 30 partners, they want to link all the electrical systems in a factory via an intelligent DC network. In this way they want to make the energy supply more efficient, more stable and more flexible. In the predecessor project DC Industry, the researchers already demonstrated the feasibility of such a power supply.

Since the end of the 19th century, alternating current has dominated the supply of electrical energy. However, this has changed with the advent of renewable energies. Producers such as photovoltaic systems generate direct current. In the same way, things of daily use such as smartphones or LEDs work with direct current. It has to be expensively converted into alternating current and the other way round – with high losses. In the factory of the future, which will be operated with direct current, such losses are a thing of the past.

Avoiding losses

Five to ten percent increase in efficiency – that is the result of the DC Industry project. The savings are achieved in particular by using surplus braking energy from speed-controlled drives. Four test systems were equipped with DC components from different manufacturers and put into operation.

Now the next step follows: In the DC Industry 2 project, the researchers want to transfer the concept to the production hall. »In the follow-up project, we want to achieve even greater energy efficiency, reduce CO2 emissions and be able to react flexibly to the introduction of climate-neutral technologies. With a local direct current network in the factory, it is easier to compensate energy fluctuations, such as those caused by weather-related renewable energies or increasing fluctuations in the public supply network,« says Timm Kuhlmann, scientist at the Fraunhofer IPA in Stuttgart.

In addition, the majority of the drives used in production are speed-controlled using frequency converters. Converters are also direct current consumers. So in order to provide an electric motor with a variable voltage and frequency, the mains AC voltage must first be rectified. If the frequency inverter is supplied directly with DC voltage, energy conversion losses can be prevented and the recovery of braking energy is easily possible. A further advantage: Rectifier stages in particular lead to a high harmonic load on the AC grid, which makes complex and cost-intensive filtering measures necessary. With a DC grid, this effort can be dispensed with.

Intelligent energy supply

Another advantage of the DC factory is that all electrical systems in the factory are linked to form an intelligent DC grid. It enables the availability and quality of the electrical supply to be improved on site, thus increasing the reliability of production. »In concrete terms, we are establishing micro-grid topologies, i.e. control clusters, which allow us to balance and coordinate energy storage, generation and consumption on site. They can be operated autonomously,« says Kuhlmann.

The new grid structure is based on one or more interfaces to the AC distribution grid, which provides the DC voltage supply for the production plants via active or passive rectifiers. All installed consumers, such as frequency-controlled electric motors or lighting, are supplied directly with direct current and are connected via a common DC network that operates in a voltage band of ± ten percent around a nominal value of 650 volts. It allows direct energy exchange between all drives that regularly accelerate and brake, for example in robots or tool spindles. Components that normally consume excess energy, such as braking resistors, are no longer required. Such infrastructures have only become possible with the further development of power semiconductors: the high costs for components, for switching direct currents, have fallen considerably with the use of modern, power electronic components.

Further tests have already been started in test halls and in Factory 56 in Stuttgart-Sindelfingen. In the production facility of the project partner Daimler there are active, bidirectional rectifiers, so-called Active Infeed Converters, which are directly connected to the mains and supply the first systems with DC power. The trick: With a bidirectional supply, energy is offered to the external AC grid as a service if large generation capacities are available. In this way the consumer benefits from the energy turnaround in Industry 4.0.

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Fraunhofer IPA (Institut für Produktionstechnik und Automatisierung)