DC-Industry is the acronym for a project that has its strategic roots in the ZVEI's electrical drives division and - with funding from the German Federal Ministry for Economic Affairs and Energy - has quickly developed into a beacon. The aim is to make industrial production more energy-efficient and flexible across all sectors. Since mid-2016, a total of 15 partners (Siemens as consortium leader, Bauer Gear Motor, Baumüller, Bosch Rexroth, Daimler, Danfoss, Eaton, KHS, Lenze, LTI Motion, Weidmüller, Fraunhofer IISB, Fraunhofer IPA, Ostwestfalen-Lippe University of Applied Sciences, University of Stuttgart) together with the associated partners (ABB Stotz-Kontakt, E-T-A Elektronische Apparate, Harting, Homag Group, Jean Müller Elektrotechnische Fabrik, Leoni Special Cables, Phoenix Contact, SEW-PowerSystems, U.I. Lapp, Yaskawa) and the ZVEI.

For drives, however, the three-phase motor is more efficient, more durable and more cost-effective than a DC motor. However, three-phase current is only required for mains motors that do not need to be speed-controlled. All others can be operated via frequency converters, which first use a rectifier to generate a DC link voltage before using an inverter to generate the variable voltage for the three-phase motor. In other words: Basically, frequency inverters already have an internal DC network!

Depending on the type of feed-in, DC-Industry defines two voltage bands: - 650 V: Suitable for regulated feed-in and unregulated feed-in on the 480 V (AC) grid (left band) - 540 V: Suitable for unregulated feed-in on the 400 V (AC) grid (right band).

© DC industry

The idea of DC Industry is now to extend the DC network to all consumers in a control cabinet, a machine, a production cell or an entire production hall. If the DC voltage is based on the largest loads - i.e. the 3x400/480 V inverters that dominate in production facilities - the following advantages arise:

• Energy can be exchanged directly between generators and consumers without intermediate converters. This is usually not possible with AC devices. In addition, load flow control is simpler.
• Installation costs are reduced, as only two current-carrying conductors are required in addition to the protective conductor instead of the three current-carrying conductors required for three-phase current. Furthermore, the inductive voltage drop and reactive power are eliminated. If the insulation capacity of previous AC components with 3x400/480 V is utilized for DC, a saving of 40 % or more copper is possible.
• The integration of storage systems and solar energy generation is simpler. All you need is a DC/DC converter instead of a much more complex inverter. This not only saves costs, but the dynamics and efficiency are also higher.
• If the AC grid fails, the DC grid can continue to run independently, provided that sufficient storage and other generators are available. This increases availability.
• The converters are simpler, smaller and more efficient. They only consist of the motor-side inverter, which is already present, and associated filters for the switching function and EMC. The inverter-operated three-phase motor does not change. A DC/AC inverter is much easier and cheaper to integrate on or in motors, which is expected to give a boost to decentralized drive technology.

The goals of the DC industry

In order to be able to implement the benefits of the project quickly, it is particularly important that many manufacturers can establish components for this DC grid on the market as soon as possible. Therefore, the subtitle of the project "Intelligent open DC grid in industry for highly efficient system solutions with electric drives" expresses the main objectives:

Intelligent: the above-mentioned characteristics are predestined to realize a DC-based 'smart grid' that can flexibly adapt to demand and grid situations. The basic functions for smart DC grids are to be provided in the project.

Openness: The system boundaries are shifting from the AC grid to the DC grid. This means that open but clear rules must apply to the design, development and operation of the devices, components and control systems of the DC grid. The usability of the rules must be proven in the project. It was therefore particularly important to integrate many suppliers of components and devices into the project.

Industry: The project is about industrial applications, not residential areas or data centers.

Highly efficient: Energy saving is a key objective. Depending on the application and expansion of the above properties, the aim is to achieve more than 15 %.

Electric drives: They are the focus - therefore the rules must be selected in such a way that current AC converters can be adapted with minor changes in order to use as much of what is available as possible.

There were also some serious challenges that the project partners had to face:

• The DC grid is capacitive and very dynamic, especially in the event of faults. Therefore, one focus was on the construction and testing of fast DC switches that enable selectivity.
• There are no defined DC limit values for EMC that are standardized and therefore resilient. The project makes sensible suggestions here, which are based on the current standards for AC and photovoltaics.
• There are no preferred grid structures for AC grids. In practice, grown structures are the rule, with all the negative consequences. Against this background, a grid structure has been created for the DC industry that simplifies planning and development and promotes harmonized structural components.

The consortium has already agreed on a system specification that describes the technical rules for DC industry-compliant grids and defines the key properties of the devices and structural components used there (drive inverters, rectifiers, active front-end converters, switching elements, connectors, cables, auxiliary power supplies, DC/DC converters, battery storage). In addition, a management specification for corresponding grids has been developed, which covers stability and controllability in four expansion stages. DC-Industry thus provides a comprehensive manufacturer-independent system for industrial power supply in production, which is designed to be future-proof and can be provided on the basis of the latest technology.

What happens next?

Following the system specification, the component manufacturers and research institutes have now developed and built more than 100 different devices and structural components according to a master plan and supplied them for the model applications that have been implemented so far. The latter represent four use cases - from a single machine to logistics processes to a production cell - and were designed, built and provided for the evaluation of the system specification by users from the automotive manufacturing, mechanical engineering and beverage technology sectors.

The final phase of the evaluation measurements has been underway since the beginning of the year. The evaluations will be available by the end of the project in summer 2019. The joint research project DC-Industrie (funding code 03ET7558A to N) is funded by the German Federal Ministry for Economic Affairs and Energy (BMWI) and managed by Project Management Jülich (PTJ). A follow-up project is already in preparation, which will further advance and expand the topics of DC-Industry.

Author:
Prof. Dr.-Ing. Holger Borcherding is a professor at the Ostwestfalen-Lippe University of Applied Sciences in Lemgo, scientific director of DC-Industrie and one of the initiators of the project idea.

As part of a multi-part series of articles, Computer&AUTOMATION will be taking a closer look at individual technologies that are essential for the implementation of DC-Industrie in the coming issues:

• Drive technology/power distribution
• Protection and switching technology/load zone concept
• Planning and design tools/energy management