A transformer to drive the transition from AC to DC

EPFL researchers have developed a compact and efficient medium-frequency transformer. Their device is poised to enhance the flexibility and efficiency of tomorrow’s smart grids and DC power distribution networks.


Alternating current or AC electricity, which is how most of our power grid works, is radically different and much more complicated. The principle behind AC electricity is rooted in the linkages between electricity and magnetism, which you may have learned about in a high school physics class.

However, direct current (DC) is now back in fashion. It may be due to advances in power electronics – is already becoming the new norm.

While high-voltage DC is an efficient and proven method for transporting power over large distances, the interconnection still requires legacy AC grids. Even if achieving DC grids to facilitate smart grid concept, further technological progress in this area is still needed. This transition will require flexible, efficient and high-performance power electronic conversion devices – commonly referred to as solid-state transformers (SSTs).

SSTs can perform any desired electrical energy conversion (i.e., AC-AC, AC-DC, DC-DC, DC-AC), depending on the needs of the application. They are mostly similar to a multi-purpose Swiss Army knife.

EPFL scientists now came up with a way of optimally designing and producing medium-frequency transformers (MFT)- rated for 100kW and operated at 10kHz- to enable technologies for SSTs. After careful testing, scientists found that the MFT serves as the basis for technical tutorials, some of which have already been given to various specialists from the academic and industrial worlds.

Marko Mogorovic, one of the device’s designers said, “Full controllability can be achieved. We can be highly flexible and quickly alter the power flow – and we can do that very efficiently. This will be very important when it comes to integrating the intermittent energy generation from renewable sources into tomorrow’s smart grids.”

Drazen Dujic, director of PEL said, “Another plus is the small size of the device: In an AC system, the frequency at which transformers operate depends on that of the surrounding grid. In Europe, that frequency is fixed at 50Hz.”

“In a DC system, however, transformers operate within converters at very high frequencies of up to several tens of kilohertz, thanks to power electronics. And the higher the frequency, the more compact the device.”

Mogorovic said, “The reduced size of these transformers will be particularly useful in traction systems, in terms of both efficiency and integration: A lighter locomotive would consume much less energy.

In traction systems, the device would change the AC from the railroad lines into DC for the traction/propulsion chain. The railroad framework in Switzerland works at 16.7Hz, which up to this point has converted into rather massive transformers inside the trains.

In the meantime, be that as it may, miniaturization speaks to a real challenge for specialists, who need to manage many cross-disciplinary constraints including thermal, dielectric and magnetic issues.

EPFL researchers developed a set of sophisticated and very fast models that can quickly generate several million designs. It makes it possible to then select the best design, depending on the performance they want to achieve.

Dujic said, “The fact that we’ve made this type of transformer inside a lab is a major step, given the safety and function-related problems that usually arise. We managed to get it to work perfectly. That’s what’s important for experts in this field.”

A gathering of such experts is already scheduled. The European Center for Power Electronics (ECPE) will hold a workshop called “New Technologies for Medium Frequency Solid State Transformers” on 14–15 February 2019 at EPFL. The workshop, which will be chaired by Drazen Dujic (EPFL) and Johann Kolar (ETHZ), has attracted a record number of participants from industry and academia.


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