ETH Zurich electrical engineers have developed a smart electronic transformer that works extremely efficiently to transform medium-voltage into low-voltage. Smart transformers of this kind are also much smaller than standard transformers. This makes them particularly suitable for use wherever space is limited or weight must be kept to a minimum, as is the case for instance in rail locomotives.
Most power grids for rail applications supply alternating current at medium-voltage level. The locomotives then step the voltage down to a lower value. “For technical reasons, the lower the frequency of the alternating current, the bigger the transformer required to step the voltage down. And at 16.7 Hertz, the frequency used in rail transport in Switzerland and several other European countries is relatively low,” explains Daniel Rothmund, one of the two doctoral students in ETH Professor Johann Kolar’s group who built the new transformer.
To get around this size problem, smart transformers have a trick up their sleeve: First, a front-end converter greatly increases the frequency of the alternating current, meaning the transformer itself can be much smaller. Then a subsequent converter produces alternating current at the desired frequency.
Switching at extremely high frequencies
Rothmund and his colleague Thomas Guillod had to develop many of the components for their transformer themselves, as there are few components available off the shelf for the medium-voltage of 10,000 volts they work with. Components made from silicon carbide that enable extremely fast switching – prototypes manufactured by a U.S. firm – were especially important. Using these, the ETH doctoral students were able to convert medium-voltage to a very high frequency of 75,000 hertz; as a result, the transformer system they built is just one-third the size of previous smart transformers with similar power ratings. And while earlier systems achieved an energy efficiency of around 96 percent, Rothmund and Guillod managed 98 percent – in other words, they were able to halve energy losses from 4 percent to just 2 percent.
Their research work was conducted as part of the Swiss National Research Programme “Energy Turnaround” (NRP 70), which is focused on researching the technologies needed to implement the country’s Energy Strategy 2050.
