DC/DC converters in the industry

Reading time – approx. 5 minutes – DC/DC converters in the DC voltage network of the industry offer a number of advantages. In this article, we would like to show you the reasons in favour of switching to direct current. Specifically, in an industrial environment.

At the end of the 19th century, Tesla, Westinghouse and Edison engaged in a so-called electricity war. Due to several decisive advantages, Tesla and Westinghouse were able to prevail in this conflict with alternating current technology. As a result, a long-lasting era of this type of energy supply began. Now, almost 125 years later, the German Federal Ministry for Economic Affairs and Energy has been funding a cross-sector research project called “DC Industry” since July 1, 2016. The aim of this project is to convert energy supply in industrial production to direct current using DC-DC voltage converters. This conversion affects both supply networks and the power electronics in industry to the same extent. The advantages and disadvantages of both types of supply are analyzed below. Additionally, the resulting consequences for consumers are considered.

Reasons for switching to DC-DC converters

When choosing a suitable power supply, a distinction must be made between two separate requirements: the energy transmission from the generator to the consumer and the functioning of the consumers. High voltages are required to transport electricity over long distances with as little loss as possible. In the past, it was only possible to adjust the voltage economically with alternating voltage.

However, transporting energy using alternating voltage leads to losses depending on the distance and the necessary insulation. These losses are due to the capacitive and inductive properties of the line. These properties lead to a constantly occurring reactive current depending on the AC frequency and the length of the line. Furthermore, as a result of the so-called skin effect, in which the charge carriers move more on the surface of the conductor due to high frequency, thicker cables are required.

This is needed than for a comparable DC transmission, in order to provide the current with a larger conductor surface.

The development of high-power thyristors and IGBTs (insulated gate bipolar transistors) has made it possible to convert high AC voltages to high DC voltages and back on a larger scale. This enabled more efficient energy transmission via HVDC lines (high-voltage direct current transmission). A few years ago, the majority of consumers such as light bulbs, vacuum cleaners and various tools and machines drew their energy from alternating current. While resistive loads like light bulbs or heaters can be operated with both types of current, motor types were developed early on that can generate mechanical energy directly from alternating current. These types, like the asynchronous motor and the reluctance motor, are mostly maintenance-free and without brushes. Other motor types, such as the universal motor or single-phase series-wound motor, require brushes but work with both direct and alternating current.

This has changed and will change drastically with the current industrial revolution and advances in battery development. Whereas the overall efficiency of the supply of electrical energy used to be around 65%, current estimates suggest that it is now only around 56%. This drop is due to the increased demand for DC voltage. Also, the resulting need for conversion from AC voltage contributes to this drop. If there is no rapid switch to direct current in the course of energy supply decentralization, the energy balance is likely to become significantly worse. This is particularly true in combination with the changed consumer requirements.

DC/DC converters in the direct current supply

A high-voltage direct current supply offers the economically viable option of a redundantly secured decentralized energy supply in industry. The use of battery storage to buffer load peaks and to bridge grid failures reduces energy costs. Additionally, it reduces the dimensioning of supply lines. This approach requires various DCDC converters for implementation. Powerful regulated DC/DC converters with an integrated charge maintenance program are needed. These are required to charge the batteries of the energy storage systems from the DC grid. By networking the power electronics involved, the charging process can be carried out with low-cost night-time electricity. Additionally, it can be done at times of energy oversupply, which reduces the load on the energy infrastructure.

12 VDC is required to supply computers and servers. It is also necessary for the low-voltage electrical systems of vehicles in automotive plants. These 12 VDC can be provided with high stability and accuracy by DC-DC converters with a wide-range input. This remains true despite possible fluctuating HV battery voltages. Around 70% of industrial power consumption is used to supply electric motors. These drives are increasingly using frequency converters with their own DC link for speed control. By using a central DC-DC converter, which supplies several frequency inverters with the necessary DC link voltage, the design of the frequency inverters can be significantly simplified. This not only reduces the acquisition costs, but also increases the efficiency of the system.

Conclusion

Whether direct current will completely replace alternating current in the future, or whether it makes sense for the two variants to coexist, can only be clarified once a number of technical and economic issues have been resolved. However, according to Gunther Koschnick, Managing Director of the ZVEI Automation Association, eliminating many current transformers in drives, electronics and charging infrastructure could lead to energy savings of around 10%. At the same time, the supply networks can be made more stable. Additionally, the devices can become more compact and fail-safe.