The electricity grids in Germany are being expanded as more and more renewable energy plants are networked in. Power electronics play a decisive role in the transition as they connect the green energy systems to the grid, controlling feed-in and feed-back, and increasingly they must perform other grid-supporting tasks.

Therefore, the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg has been working on a compact inverter for direct connection to the medium voltage grid. As part of the project "SiC-MSBat - medium-voltage inverters with high-voltage SiC power modules for large-scale storage and system-serving distribution grids” – which was funded by the Federal Ministry for Economic Affairs and Energy (BMWi) under the 6th Energy Research Programme – a 250-kW inverter stack has now been developed and tested for feeding into 3-kV AC grids.

The inverters currently in use (designed to feed into low-voltage grids) require large 50 Hz transformers in order to be coupled to a medium-voltage grid. The new super compact transistors developed at ISE are made of silicon carbide (SiC) with very high blocking voltages, making it possible to connect the inverters directly to a higher voltage network.

SiC inverters achieve higher power densities and switching frequencies and have intelligent controls, so they can act as active power filters to compensate for harmonics in the medium-voltage grid, for example. Furthermore, they could be used in urban areas where space is limited and retrofitted into existing plants, bringing down grid infrastructure costs.

The ISE’s SiC inverter has so far achieved a very high efficiency rate of 98.4 percent at rated power. The design also allows modular interconnection of multiple inverter stacks to achieve system outputs of several megawatts. The commercial partners were Semikron Elektronik GmbH & Co. KG and STS Spezial-Transformatoren Stockach GmbH.