Highly compact inverter for direct feeding into the medium-voltage grid

Researchers at the Fraunhofer Institute for Solar Energy Systems ISE have developed an inverter based on new types of silicon carbide (SiC) transistors with high blocking voltages so the inverters can be connected directly to the medium-voltage grid. The high control dynamics of SiC inverters can take on grid-stabilizing tasks and act as active power filters to compensate for harmonics in the medium-voltage grid. SiC inverters can achieve high power densities, resulting in a compact design. 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, a 250kW inverter stack was developed for feeding into 3kVAC grids. Here, 3.3kV SiC transistors are used with their low power losses, so the inverter stack operates with a switching frequency of 16kHz. The high switching frequency provides savings on the passive components, as these can be dimensioned in a smaller format. The inverter features its active liquid cooling with a synthetic ester as cooling medium. This medium is pumped through the inverter and cools both the transistors via a liquid heat sink and the filter chokes, which are housed in a closed tank. At the same time, the cooling medium for the filter chokes serves as an electrical insulation medium, allowing the filter chokes to be made even more compact. The inverter achieves an efficiency rate of 98.4% at rated power. The design of the inverter stack allows modular interconnection of multiple stacks to achieve system outputs of several megawatts.

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Volume: 2021 March/April