When electrical energy comes from renewable sources such as photovoltaics or wind power, the use of direct current is particularly efficient. These sources produce direct current (DC), which must first be converted into alternating current (AC) via inverters. However, if the end consumer is also a digital device such as a laptop, cell phone, LED light, charging point for electric vehicles or the intelligent production unit in a factory, it has to be converted twice because these end consumers only work with direct current. This results in large conversion losses. Analyses assume 10 to 15 %. If, hypothetically, the entire electricity grid in Germany were to switch completely to direct current, we could save around 35% of the total energy requirement.

"The long-term phase-out of fossil fuels can only be achieved efficiently if we consistently switch more and more to direct current and avoid conversion losses. In short, we need a transition without change," says Guido Ege, Head of Product Management and Product Development at Lapp.

The use of direct current is still in its infancy. In industry, there are only niche applications so far. Large car manufacturers, for example, have installed test cells. There is talk of efficiency gains of up to 30 %. However, the big picture is still missing. There is still a need for action on many levels. Lapp got involved with the topic of direct current at a very early stage and is actively developing cables. The company already has a portfolio of cables for various applications. These include the Ölflex DC 100 with new color coding of the cores according to the 2018 updated standard DIN EN 60445 (VDE 0197):2018-02 for direct current cables: red, white and green-yellow. Other cables include the Ölflex DC Servo 700 for stationary applications and the Ölflex DC Chain 800 made of TPE for moving applications. Or the first DC robot cable Ölflex DC Robot 900 with TPE core insulation and a PUR sheath. This makes the company a pioneer in the development of cables for industrial low-voltage direct current networks.

Research project on DC supply

But DC cables alone are not enough. There are other areas of work in standardization, and DC-compatible components still need to be made available, for example for plugs and switches. Further research is needed here, because with direct current, for example, an arc fault does not go out on its own. This can be life-threatening. Lapp is therefore a funded partner in the DC-Industrie2 research project. In this project, researchers from Fraunhofer IPA and Fraunhofer IISB, in cooperation with more than 30 partners, have set themselves the task of developing a concept for an intelligent DC supply system and testing whether it can supply a production hall with direct current at low cost. The Federal Ministry for Economic Affairs and Energy (BMWi) is funding the project. Lapp is researching the long-term stability of insulation materials for cables and wires.

Much earlier, Lapp and the TU Ilmenau had already discovered in experiments that the insulation materials exhibit a different ageing behavior in a DC voltage field than in an AC voltage field. Over a period of 2,590 hours, researchers at TU Ilmenau exposed individual conductors with different insulation materials to 1 kV DC voltage in a water bath at 80 °C in order to understand the effects in fast motion. The results: Cables with PVC or polyolefin insulation failed significantly faster than all test specimens with TPE insulation. Two thirds of the defects resulted in a breakdown of the insulation. Further research is needed to make more precise statements. For example, it is not yet clear what happens chemically and physically in the plastic. The degradation of the polymer or swelling in water as well as the release of additives or the formation of "water trees" could be possible causes.

Lower losses, fewer components

Nevertheless, there is no reason not to use cables with PVC insulation in DC voltage applications. However, the prerequisite is that these cables are laid firmly, i.e. without movement, and without mechanical stress, for example due to excessively tight bending radii. In addition, the environment should always be dry. If these conditions are not met, for example in moving applications in energy chains, users can switch to other insulation materials such as TPE.

Guido Ege: "We see great economic opportunities in direct current. Not only for the automotive and process industries. Many consumers are already using direct current today. By reducing conversion losses, we are increasing efficiency. By eliminating converters, we need fewer components and therefore less space. Regenerative and decentralized energy sources can be integrated more easily. Kinetic energy is also fed back via DC. The electric motor becomes a generator."

As mentioned, a direct current supply would be the ideal prerequisite for drives to be able to feed energy back into the DC grid when braking. As with electric or hybrid cars, this energy would be stored in batteries until the drive accelerates again. This energy could also be used to supply consumers with high power requirements, for example during welding. This would allow companies to cut peak loads and avoid having to draw large amounts of energy from the grid for short periods, which would reduce costs and also relieve the burden on the energy supplier and the power grid.

Lapp has had an extensive DC-compatible portfolio for photovoltaic and wind power plants for years. DC is used, for example, to avoid conversion losses in wave power plants. For example, a wave power plant from Munich-based Sinn Power is in operation in the port of the Greek city of Heraklion. In future, the DC cable Ölflex DC 100 for the 800 V DC bus will be used there for the decentralized mini-grids. The cable will bridge a distance of around 700 m to the feed-in point. The Ölflex DC 100 is permanently laid in installation pipes on the harbor wall and runs to the grid inverter for feeding into the public power grid.

Charging stations on the DC grid

Lapp has developed a customized solution for the Sortimo Innovation Park in Zusmarshausen, which was opened last summer. Here, Lapp developed the complete DC bus for connecting the charging stations, including hybrid DC cables for control and online monitoring. The cable structure is sophisticated: the cable specially designed for the charging park consists of an aluminum conductor with a 30 ^mm2 cross-section. The core insulation is radiation cross-linked and made of polyethylene. The shielding is achieved using copper wires spirally applied over the core. The special feature: The structure is supplemented by two stainless steel tubes, each fitted with six fiber optic cables. These are used to measure the temperature and raise the alarm if the temperature at the charging points becomes too high. The bus is then used to control where alternative free charging capacities are available. The outer sheath is made of PVC in accordance with IEC 60502. "DC technology will decisively change industrial production and the energy supply of cities and urban districts. It therefore represents an important element of the energy transition," emphasizes Guido Ege.