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Research & Development Direct current DC networks: What the DC cable must be able to do for industry

| Author / Editor: Bernd Müller * / Jochen Schwab

Energy supply with direct current offers enormous savings potential, especially in industry. But can AC cables also be used without problems? We take a look at current research projects.

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DC industry2: DC networks with central conversion could be established as an energy-saving alternative, especially for drives in production..
DC industry2: DC networks with central conversion could be established as an energy-saving alternative, especially for drives in production..
(Bild: Lapp)

Digitization and artificial intelligence were the major topics at the Hannover Messe 2019. Another topic that is attracting increasing attention was the power supply with direct current (DC). The ZVEI had even set up a pavilion, especially for this purpose. Lapp also dedicated its Futurelab to the topic of DC and showed its first products there.

The world market leader for integrated cable and connection systems is one of the pioneers of the DC renaissance and was also an associated partner in the DC industry research project, which was recently completed. This autumn, the follow-up project DC-Industrie2 will be launched.

The projects deal with the question of how to establish DC grids with central conversion as an energy-saving alternative, especially for drives in production, and how to better integrate regenerative energies. In this ambitious project, the connection technology specialist from Stuttgart is dealing, among other things, with the question of the suitability of certain cable types about the long-term stability of insulation materials for cables and wires.

First DC line worldwide comes from Stuttgart

The company has already developed cables that are suitable for DC networking. The first cable explicitly developed for DC applications is the Ölflex DC 100. The color-coding of its cores follows the standard DIN EN 60445 (VDE 0197):2018-02, updated in February 2018, for DC cables: red, white, and green-yellow. The insulation of the cores is made of special PVC, and the jacket is made of PVC.

Further cables in the manufacturer's portfolio are the Ölflex DC Servo 700, a DC connection cable with special PVC insulation, and the Ölflex DC CHAIN 800 for continuous movement in energy guiding chains. The latter has an insulation made of TPE, a thermoplastic elastomer.

The Ölflex DC 100 Hybrid has also been developed as a cable for the DC industry consortium. The DC hybrid cable contains two cores for power transmission plus a protective conductor, a Cat. 6A data cable with four shielded core pairs, two cores for safe torque off (emergency stop), and a 24 V control pair for the brake.Further cables in the manufacturer's portfolio are the Ölflex DC Servo 700, a DC connection cable with special PVC insulation, and the Ölflex DC CHAIN 800 for continuous movement in energy guiding chains. The latter has an insulation made of TPE, a thermoplastic elastomer.

The Ölflex DC 100 Hybrid has also been developed as a cable for the DC industry consortium. The DC hybrid cable contains two cores for power transmission plus a protective conductor, a Cat. 6A data cable with four shielded core pairs, two cores for safe torque off (emergency stop), and a 24 V control pair for the brake.

Energy turnaround: Energy supply with direct current is necessary

If the energy turnaround is to be successful, energy supply with DC voltage is almost inevitable. It is not just a matter of generating the most significant possible quantities of renewable energies such as solar and wind power.

Often overlooked potentials for the conversion to a sustainable energy supply also lie in saving energy. Direct current is a great opportunity here because it eliminates losses when converting between AC and DC. Pilot projects are already underway in the automotive industry to supply direct current only to production cells and later entire factories.

Figure 1: The DC cable for moving applications ÖLFLEX DC CHAIN 800 from Lapp is insulated with TPE.
Figure 1: The DC cable for moving applications ÖLFLEX DC CHAIN 800 from Lapp is insulated with TPE.
(Bild: Lapp)

Drive technology: Energy buffer with a direct current for high outputs

A supply of direct current is a prerequisite for drives to be able to regenerate energy during braking quickly. As with electric or hybrid cars, this energy would be stored in batteries until the drive accelerates again. Both drives and batteries consume direct current. This energy buffer can be used to supply consumers with high power requirements, for example, during welding.

Companies could use it to cut peak loads and would not have to draw large amounts of energy from the grid for a short time. This reduces costs and also pleases the energy supplier, who does not have to worry that his power grid will be overloaded.

Direct current is by no means only of interest to industry, and households could also benefit. Many electrical consumers, from LED lights to electric cars, work with direct current, which until now has had to be converted from alternating current from the socket. Also, more and more systems that generate direct current, above all photovoltaics, are feeding electricity into the increasingly decentralized power grid.

Are AC cables suitable for direct current?

A question Lapp is particularly interested in: Are low-voltage AC cables also suitable for the direct current? There have been no research results on this so far, probably because most experts were of the opinion that this was easily possible. Long-term tests conducted by the research group in the field of "Electrical Devices and Systems" at the Technical University of Ilmenau under the direction of Prof. Frank Berger have now proven for the first time that this is an error.

Over 2590 hours, Berger's team loaded individual conductors with various insulation materials in a water bath at temperatures of 80°C with a DC voltage of 1 kV to understand the effects in fast motion. The Stuttgart team provided the test equipment and the lines. The research results show that a DC voltage field has a different effect on the aging behavior of insulation materials than an AC voltage field. Many experts had disputed this so far.

Prof. Berger is now planning aging tests that do not require a water bath but will take longer. On the other hand, he wants to understand what happens chemically and physically in plastics. The degradation of the polymer or swelling in the water as well as the dissolution of additives or the formation of "water trees" could be possible causes.

Direct current: PVC for stable, TPE for moving applications

Until reliable data are available, there is no reason to do without cables with PVC insulation in DC voltage applications, for example. However, users should ensure that these cables are laid firmly, i.e., without movement, and without mechanical stress, e.g., due to bending radii that are too narrow. Besides, 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, for example, to the elastomer TPE, which performed excellently in the water bath tests.

This article was first published in German by Elektronikpraxis.

* Bernd Müller works as a freelance author in Wiesbaden.

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