While high-voltage DC is an efficient and proven method for transporting power over large distances, the interconnection still requires legacy AC grids. SSTs can perform any desired electrical energy conversion, whether AC-AC, AC-DC, DC-DC or DC-AC), depending on the needs of the application.
The team from EPFL’s School of Engineering’s Power Electronics Laboratory (PEL) developed a way of optimally designing and producing a medium-frequency transformer (MFT) as a key enabling technology for SSTs, producing a protoype rated for 100kW and operated at 10kHz.
“We can be highly flexible and quickly alter the power flow – and we can do that very efficiently,” said Marko Mogorovic, one of the device’s designers. “This will be very important when it comes to integrating the intermittent energy generation from renewable sources into tomorrow’s smart grids.”
The development represented a real challenge for engineers, who have to deal with 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.
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,” said Drazen Dujic, director of PEL. Since the frequency cannot be changed, miniaturization is impossible. “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,” he 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,” said Mogorovic. In traction systems, the device would transform the AC from the railway lines into DC for the traction/propulsion chain. The railway grid in Switzerland operates at 16.7Hz, which until now has translated into rather bulky transformers inside the locomotives.
“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,” said Dujic. “We managed to get it to work perfectly. That’s what’s important for experts in this field.”