The decision to generally introduce alternating current was made in 1893 during the World's Fair in Chicago. This was 126 years ago. It was preceded by an electricity war between Edison and Westinghouse to secure market share. However, as already explained in Part 1 of the series of articles on the 'DC industry' project, much has changed after more than 100 years and we are now facing a new technical discussion during the energy transition: What should the production grid of the future be based on? Should it be based on AC technology as before or on direct current technology, for which there are many good reasons?

The search for an answer to this question leads directly to the question of the biggest hurdles and challenges, which are as follows:

1. Engineering and technology
2. Standardization and legal framework
3. Implementation strategy

Engineering and technology

It is important for users to be clear about the level of maturity that the existing prototype solutions have reached to date. The key questions are therefore:

• What are the special features and degrees of novelty of direct current technology?
• Where do we stand technologically today?

The concept of DC grids relies on several active principles to increase energy efficiency. In addition to the possibilities for reducing conversion losses, there is great potential in the use of energy from braking processes, with the aim of buffering the energy within the DC grid or the factory where possible or distributing it as required. The key components in terms of energy efficiency are therefore the energy storage system and the grid or energy management system.

The characteristic-based power control presented in part 2 of the article series enables a stable grid in which several different suppliers such as PV systems, grid feed-in and energy storage systems provide the energy proportionally. The advantage of this characteristic-based control is the self-regulating, robust grid operation without additional communication infrastructure or control systems, which improves availability.

The use of extended grid control can open up additional optimization potential in energy management. This enables the control characteristics of the active participants to be changed via a communication network, allowing the operating behaviour to be adapted and system flexibility to be achieved. In this way, it is possible to react to changing electricity prices or energy purchases by consumers by using appropriate storage operating modes or switching off auxiliary consumers, such as ventilation systems. These options are easier to implement in the presented approach than with the previous AC supply, as the energy status of the grid is in principle known to every participant via the voltage. The grid management module for energy storage systems relies on existing battery management systems for status monitoring and ensures process-optimized charging and discharging behaviour.

Energy storage systems are very diverse due to the product portfolio available on the market, the basic principle and the performance. A rough classification can be seen in Figure 1. Of the electrochemical and hybrid energy storage systems, lithium-ion batteries, lithium-ion capacitors and supercapacitors are the most interesting for electrical power distribution.

The overall concept determines whether centralized and/or decentralized energy storage makes sense, which application is involved and which grid level is to be covered. The most important applications are

• Uninterruptible power supply (UPS)
• Covering peak power (peak shaving) or uniform power consumption on the grid side
• Ensuring optimized, i.e. minimized energy consumption by absorbing and releasing braking energy as required

The last two applications often allow a considerable downsizing of the supply units.

Figure 2: Homag has implemented a central battery storage concept for its Centateq P-310 woodworking center as part of DC-Industrie.

© Homag

In the model system of a CNC woodworking center (see Figure 2) implemented as part of the 'DC Industry' project, these use cases were realized with a machine-related central battery storage system and successfully tested with two possible AC grid failure scenarios. In normal operation, the battery storage system is used to absorb the braking energy, which - as things stand today - is converted into heat in the system by means of braking resistors. The operating strategy of the battery storage system is to absorb as much energy during a machine cycle as is released during the cycle. Consequently, the state of charge of the battery storage at the start of the cycle is the same as the state of charge at the end of the cycle. As the systems are usually operated with variable cycles, this is not guaranteed with a fixed characteristic curve and is adapted by a corresponding automatic characteristic curve adjustment while ensuring stability. For the additional function of an uninterruptible power supply in the event of an AC power failure, the lower threshold of the state of charge must be set so that the required residual energy for an AC power failure is available at all times.

Furthermore, a decentralized storage concept was implemented in a production cell in body-in-white construction, in which a flywheel mass storage unit, a battery storage unit and a capacitor were integrated into the network. While the flywheel mass storage system supplies the short-term, extreme load peaks of over 200 kW of power during welding, the battery storage system bridges power failures and ensures uninterrupted production or a controlled shutdown of the system. The capacitor smoothes out the energy drawn from the grid and enables the regenerative power supply to be dimensioned according to output. This storage and control concept makes it possible to reduce the dimensions of the supply unit by a factor of 10 compared to the first DC conversion of the production cell.

Standardization and legal framework conditions

Cross-manufacturer harmonization is the most important prerequisite for the spread of DC grids. For the time being, this means standardizing the DC voltage level as globally as possible and defining the earthing and protection concepts so that all components within a DC grid work together without interference. In the DC industry project, the interoperability of various components in a DC network has been successfully demonstrated across manufacturers for the first time. This has created a very valuable basis for the implementation of these goals, which can also be used for a market launch. To date, there are no comparable initiatives at international level.

However, as far as efforts to increase energy efficiency standards and norms are concerned, there has been no change in thinking towards holistic, ecologically effective regulations. As things stand at present, legal regulations in this respect are only being implemented for the mains operation of three-phase motors. The definition of legal efficiencies is based on rated speed and rated torque. In many applications - especially with variable-speed motors - the energy-saving effect is meaningless due to operating conditions that deviate from the nominal value. At the same time, such optimized motors require a significantly higher use of materials - copper, for example. However, an ecological approach requires a holistic, systemic view of the energy, material and economic balance. The system approach of the DC grid is intended to meet this requirement and contribute to reversing the trend towards an isolated approach. This means that the electric motors must be optimized for operation with frequency converters, which, together with other intelligent system components, results in a very high degree of freedom for increasing energy efficiency and reducing system costs.

While standardization is a prerequisite for the further development of DC technology, changes to regulations represent an uncertainty factor for users. The standards for electrical safety (VDE 0100) are equally applicable here, as the effects of direct electrical currents on the human body are very similar to those of alternating electrical currents. Nevertheless, it should be noted that DC systems must also comply with the important basis for safe working on electrical systems (the five safety rules), which is why they must not cause any hazards due to touchable voltages in the event of a fault (VDE AR E2510-2). In addition, there are the hazards caused by battery storage systems (e.g. chemical hazards) and the issues of fire protection, storage and transportation.

Implementation strategy

In general, the following aspects are relevant for the introduction of DC grids in an industrial environment:

• Availability of a consistent, standardized and manufacturer-independent product portfolio
• Quantification of the return on investment (ROI)
• Integrative effects of technological and socio-economic trends such as the energy transition, digitalization, decentralization, modularization and flexibilization
• Production availability during the technological transition

Together with the trend towards decentralization and compactness, DC technology will initially bring about product changes for frequency inverters. As the frequency inverter supplied with DC voltage contains fewer components due to the elimination of the rectifier and mains filter, it is more compact and can be easily integrated into the motor. Already today, around 35% of all newly sold asynchronous motors are operated with an additional frequency inverter in order to be able to continuously adjust the motor speed to the current demand. A DC network together with a holistic approach and project planning approach makes it possible to further increase the proportion of variable-speed drives, which is urgently needed both for the implementation of Industry 4.0 applications and to increase energy efficiency.

The conditions in the factory and the energy-saving potential that can be realized using DC technology vary greatly from application to application and are very complex due to the increased range of functions and the number of active participants in DC networks. Quantifying technological advantages and answering investment questions can therefore only be done with the help of comprehensive modern design and planning tools. These tools must be able to answer the following questions in the planning phase: Structure/architecture issues of the network, type and dimensioning issues for all components, energy and efficiency issues as well as configuration and cost issues. In addition, it should ideally be possible to directly compare the system with a comparable AC technology solution and define suitable control characteristics. For this purpose, an adapted planning process and an initial design tool have been developed as part of the DC industry project.

Once the planning objectives have been achieved, implementation can begin. As the modernization periods in most industrial sectors are often in the region of 30 years, especially for technical building equipment, the introduction of DC technology in factory automation will largely take place gradually or incrementally. An incremental approach is also advantageous in exceptional cases where there is widespread coverage. This approach is in line with current trends towards flexibilization and modularization. The key requirements for DC technology are therefore modularization, grid expandability and grid flexibility.

Authors:
Dr. Mirjana Ristic is Innovation Manager Technology and Innovation at Bosch Rexroth;
Philip Crowe is Marketing Manager at Bauer Gear Motor;
Timm Kuhlmann is Project Manager at the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA).

Interview: Karl-Peter Simon, Managing Director Bauer Gear Motors

"The legal requirements for regulating components are leading to a dead end," says Karl-Peter Simon.

© Bauer Gear Motors

Mr. Simon, what ultimately prompted the industry to launch the 'DC industry' project?

Karl-Peter Simon: Let me first answer the question from the perspective of drive technology. Electric motors account for 70% of electricity consumption in industry. Since January 1, 2017, all new three-phase AC motors sold in Europe in the power range from 0.75 to 375 kW must meet the requirements of energy efficiency class IE3 or IE2 when operated with a frequency converter. From 2021, this exception will also no longer apply and only IE3 motors may be used. These efficiency classes are defined for three-phase asynchronous motors at the operating point of rated speed and rated torque - i.e. for an operating point that is not relevant in practice. This increases the costs for electric motors and the expected reduction in energy consumption is not achieved. In other words, the legal requirements for the regulation of components lead to a dead end, as they further increase costs and at the same time reduce both energy efficiency and the competitiveness of the manufacturing industry. This was the real reason for Bauer Gear Motor to initiate the 'DC Industry' project together with the ZVEI in 2014, in order to focus on the system approach.

With what specific goal in mind?

Karl-Peter Simon: Frequency inverters usually have an integrated rectifier with a mains filter, which always requires a lossy conversion of electrical energy from AC to DC voltage. By supplying the frequency inverter directly with DC voltage, all decentralized energy conversions can be omitted. As centralized energy conversion - from AC to DC - is much more efficient, conversion losses are significantly reduced. In addition, the direct supply of all electric motors via a frequency inverter with DC voltage means that all installed motors are connected via a common network. This enables direct energy balancing of all driving and braking drives. The electric motors will then increasingly no longer be optimized for a 50 Hz network, but for optimum use in operation with frequency inverters. This will enable significantly better utilization of the asynchronous machine, which in turn will reduce costs and increase energy efficiency.

Ultimately, the 'DC Industry' project supports both the energy transition in Germany and Industry 4.0. A central DC grid makes it easier to integrate a photovoltaic system as it generates DC voltage. By integrating all active participants, additional information about the energy status is made available via the grid management system. This enables energy-cost-optimized operational management, which can support services in the cloud, as information from different participants converges there with production data and future energy requirements. Network management can intervene and optimize, taking all relevant information into account. In addition, it is possible to analyze energy consumption and thus identify preventive measures to avoid possible outages - for example, the timely charging of storage systems to buffer critical load states.

On which products or components will DC technology have the greatest impact?

Karl-Peter Simon: To implement the protection concepts, many components for switching and monitoring the grids will have to be redeveloped. The frequency converters, on the other hand, will become more compact and also more cost-effective due to the elimination of the input rectifier.

What future scenarios for DC technology do you see as realistic?

Karl-Peter Simon: We assume that DC grids will initially be introduced in the industrial environment. Today, around 35% of all newly sold asynchronous motors are operated with an additional frequency converter in order to be able to continuously adjust the motor speed to the current demand. A DC network makes it possible to significantly increase this proportion due to the expected lower system costs.

Energy-saving LED lights also require DC voltage, as do all electronic control units and computers. There are therefore many good arguments for equipping factory networks with a central DC voltage supply in the future.

Interview: Frank Maier, Member of the Management Board at Lenze

"DC technology will play an important role in the context of the climate debate," says Frank Maier.

© Lenze

Mr. Maier, what challenges does 'DC industry' as a holistic approach at factory level pose specifically for drive technology and automation technology manufacturers such as Lenze?

Frank Maier: Strictly speaking, as a manufacturer of inverter systems, the topic is not so new to us. The classic supply via the rectifier is no longer necessary, the system interface is shifting to the DC link. For Lenze, the DC link has always been an intensively used element for optimizing drive solutions. Now there are some additional rules that allow devices from different manufacturers to work together. We consider the resulting challenges such as component design, robustness against mains interference or EMC to be basically solved by the 'DC industry' project. The motor control itself does not change, so we can react and offer solutions based on the series technology of AC inverters. In practice, however, it can of course be assumed that some compatibility problems will arise when components from different manufacturers interact along a new standard. The challenge therefore lies more in the design and troubleshooting of the overall system and less in the individual components.

In your opinion, what are the most important priorities for the continuation of 'DC industry' activities?

Frank Maier: For us, the focus is on optimizing the drive solution - especially the decentralized technology. By eliminating rectifiers and filters and using wide-bandgap semiconductors, we can drastically reduce the size and installation costs of the DC network. Fast, but also cost-efficient protection and switching technology is an issue for the overall solution. New engineering and troubleshooting tools that work across manufacturers are needed to manage the overall system. Furthermore, we should not forget that the 'DC industry' has so far been a purely German issue. International standardization must follow in any case and further ideas and concepts will certainly have to be integrated.

What future scenarios for DC technology do you see as realistic?

Frank Maier: I believe that DC technology will play an important role in the context of the climate debate. It will be easier to replace mains motors with inverter-operated motors with a high energy efficiency class. In addition, the elimination of the many feed-in points of an AC grid will lead to a significant reduction in filters and chokes. We expect up to 40 % less copper, which in combination with the smaller designs of highly efficient motors also saves a lot of energy in the production of these components. The controllability of energy consumption also plays an important role - the 'DC industry' provides the smart grid for this. Especially in 'greenfield' situations, such as newly built logistics centers or automotive plants, the approach naturally works particularly well and we are already seeing great customer interest here.

Doesn't the introduction of DC grids mean enormous additional costs or effort for operators if they now have to operate different technologies in parallel in the factory?

Frank Maier: In fact, I don't believe that every existing plant will be completely converted to DC grids quickly. This is more likely to happen gradually. First it will be about machines, then about islands of networked machines, and finally about networking the islands up to the complete hall. The 'DC industry' supports this approach perfectly with its requirement that there must be no mutual interference in the event of a fault. But of course, we're not just talking about the machines here, but also about installations and protective equipment, possibly even the medium-voltage transformer - that's a lot of work. However, if I want to use renewable energies in my factory or if grid availability is important, then a DC grid - even if it only supplies part of it - will save costs. Robustness against grid failures is an important argument in many countries, especially in Asia. And the social and legislative pressure on energy-efficient systems will continue to increase over the next few years - drastically!

Conference on 'DC industry'

On 10 and 11 September 2019, the "DC 2019 - Industrial DC Grids" conference organized by the Ostwestfalen-Lippe University of Applied Sciences will take place at the Phoenix Contact Arena in Lemgo. The transfer event with accompanying exhibition aims to contribute to the exchange between companies and research institutes and to present the state of development of industrial DC technology.

Further information and registration are available online.