DC/AC and back: What will drive the factory in the future?
Episode 3
Trend towards more DC voltage in the industry
For decades, the vast majority of people have lived with alternating current (AC). But for some years now, the industry has increasingly been moving towards direct current (DC). The first companies are converting their production lines or robotic cells to DC. Why? Because DC voltage promises economic advantages, saves energy and makes the factory a prosumer, i.e. an energy producer and consumer.
Jochen Clemens from Danfoss Drives knows the DC world well, because many ships are already completely supplied with DC voltage today. He still struggles in everyday life with contamination of the networks and implements filter technology in industrial plants. But Prof. Dr. Holger Borcherding from the OWL University of Applied Sciences signals that a solution to this problem already exists: Grid stability also gains with the use of DC voltage in industry. Why? You can hear the answers in Episode 3 of Torque – The Drive Podcast.
The principle 'DC network
Whenever direct current (DC) networks in industry are discussed on podiums and in specialist circles, Jochen Clemens from Danfoss Drives always has to smile a little. "Shipbuilders are already a few steps ahead of the industry. DC grids are widely used in the marine sector," explains the engineer. In some cases, however, DC networks are already being used in production: "There are already robot and servo applications that are supplied with DC voltage," says Clemens.
"In what are called common DC networks, all devices use a common DC network. The classic alternating current (AC) grid, generators or photovoltaic systems supply it with electrical energy. AC/DC converters convert dhe AC voltage is converted into DC voltage for this purpose. This creates a stable DC grid with a constant voltage level," explains Clemens. The number of connected consumers is arbitrary. DC/AC converters supply the electric machines. In addition, so-called micro-grids are generated from the common DC grid, which then in turn form different on-board networks, for example in marine systems.
Battery systems can also be charged and discharged via DC/DC converters on the DC grid. They store electrical energy and can replace emergency generators in an emergency. "This brings significant savings in installation space – in other words, more space – which is a very powerful argument on a ship," Clemens knows from experience. In this way, continuous energy exchange and energy recovery is ensured via the DC network, so that braking resistors or additional regenerative units can be dispensed with.
Step by step to more DC voltage in the industry
Step by step, the industry is currently discovering these advantages of DC voltage for itself. And the arguments sound convincing: with DC voltage, harmonic filters are eliminated, power quality increases, classic rectifiers are a thing of the past, conversion losses could be reduced, and drive systems become more compact. There is also enormous flexibility in the use of DC converters. "In the future, we will experience a simple energy exchange to storage and the factory will become a prosumer," emphasizes Prof. Dr. Holger Borcherding from TH OWL. His vision: The factory draws energy from the external AC grid, which is converted once. Internally, the machines, motors and storage units use a DC network. This leads to fewer power outages and saves energy. Borcherding has been researching the topic for years.
The principle 'frequency converter
Direct voltage in industry, so-called DC networks, are being used more and more frequently. However, much of the power supply continues to run on traditional AC grids that use transformers to supply different voltage levels.
Motors that require a constant speed can be connected directly to the AC mains. In very few cases, however, does it make sense to run a motor constantly "at full power"; most industrial applications require a speed adapted to the situation. Frequency converters are used for this purpose. Your advantage: You continuously adjust the motor speed to the current demand, which ultimately leads to greater efficiency and energy savings. The frequency inverter is supplied by AC voltage. It first converts this into a DC voltage via a rectifier, which is then in turn converted into an AC voltage with variable frequency and voltage via a downstream inverter in order to electronically change the speed of a three-phase motor.
If the three-phase motor operates in braking mode, for example in lowering mode on a crane, the energy flow changes. "However, a frequency converter with a classic rectifier cannot feed this energy back into the grid," says Jochen Clemens, "because the diode rectifier only allows energy to flow in one direction." That could change in the future: When a motor brakes, the resulting energy could be used directly in the shared DC grid to accelerate another motor. In addition, when the rectifier is no longer needed in the frequency converter, designers save space and the power electronics move closer to the motor. Mains perturbations caused by converters and additional harmonic filters could also become a thing of the past.
Right at the top of the agenda: Compensating harmonics
Clemens is particularly familiar with electromagnetic compatibility (EMC) and filter technology. "Motor control with inverters is leading to ever-improving efficiency and performance of systems in the classic AC grid. However, more and more devices also mean more disturbances. Keyword: harmonics. The quality of the grid voltage degrades if this is not taken into account." Clemens and his colleagues therefore rely on filter technology to minimize grid feedback. "We manage to make the rectifier’s grid current run as sinusoidal as possible again. We don’t manage 100 percent, which doesn’t make economic sense, but it’s enough for good power quality."
In addition to the frequency converters, filter technologies are used to reduce the mains feedback to a "healthy level". Passive and active filter technologies are used for this purpose. It is also possible to use extended frequency inverters which, due to their electrical design, emit significantly lower network feedback effects into the network from the outset. These include inverters with active front-end technology, low harmonic drives or inverters, with higher pulse feed.
Borcherding predicts that if more DC applications come on stream, then grid feedback will also decline. He is certain: DC voltage will become established – in e-cars and also in factories. "I will live to see it in my professional life," the scientist is convinced. "It’s not a century thing. In 20 years, DC power will be in the industrial sector at a higher double-digit percentage."
This brings us full circle to Episode 7 of Drehmoment – Der Antriebspodcast. This episode is about green energy for industry, on-grid and off-grid projects, balancing peak loads with storage, and information around the capacity of today’s battery storage. Frequency converters form the link between battery storage and the power grid. This makes it the most important component in the hybridization project, along with the storage itself. Listen in!
