Silicon-carbide inverter for medium-voltage grids


Germany's Fraunhofer Institute for Solar Energy Systems (ISE) has developed a 250-kW silicon-carbide (SiC) inverter that can be used in utility-scale PV projects connected to a medium-voltage grid. Compared to a standard PV inverter with silicon transistors, the creators of this SiC device claim it eliminates the need for a 50 Hz transformer when PV installations are linked to medium-voltage networks.

The inverter was built with 3.3-kV SiC transistors that have lower power losses than standard silicon transistors. “This makes it possible to operate the inverter stack with a switching frequency of 16 kHz,” the Fraunhofer scientists specified. “With state-of-the-art silicon transistors, only about 10 times lower switching frequencies are possible in this voltage class.” According to the scientists, the device has a higher switching frequency, which results in a smaller format of the passive components.

Fraunhofer ISE's inverter was also equipped with an active liquid cooling technique, where a liquid synthetic ester is pumped into the device to cool the transistors. “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 researchers further explained.

The inverter runs at 98.4% efficiency and can be installed in a modular interconnection of multiple inverter stacks, which makes it ideal, according to its creators, for the deployment of systems at megawatt scale. “Taking into account additional installation space for switchgear and cooling unit, a volume saving of the inverter system of up to 40 percent can be achieved compared to commercial inverter systems of this voltage class,” the research institute stated.

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These inverters, which the scientists define as “high-blocking SiC devices,” can have a wide range of applications in the medium-voltage range. “Especially for large photovoltaic power plants, the trend is towards higher and higher voltages,” affirmed Andreas Hensel, Head of Team Medium Voltage Power Electronics at Fraunhofer ISE.

Silicon carbide-based inverters are known for providing higher power density and having less need for cooling, which results in lower overall system costs than traditional inverters based on silicon transistors. However, defects at the interface between the silicon carbide and the insulating silicon dioxide material are still a major barrier to bringing the technology to mass production.

German inverter manufacturer Kaco new energy, a subsidiary of German industrial conglomerate Siemens, unveiled two SiC string inverters for applications in large-scale PV projects in December. Other inverter makers have thus far developed silicon carbide devices only as prototypes or in small-scale production runs.

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