The "MS-LeiKra" project team at the Freiburg-based Fraunhofer Institute for Solar Energy Systems ISE recently celebrated a world premiere with a successful feed-in to the medium-voltage grid and at the same time provided proof that a higher voltage level is technically possible for photovoltaic inverters. For photovoltaics, this can mean enormous cost and resource savings for passive components and cables, among other things. According to Fraunhofer ISE, the device establishes a new system concept for the next generation of large-scale PV power plants, which can also be used for applications in wind power plants, electromobility or industry.

Current PV string inverters work with output voltages between 400 volts and 800 volts. There are two reasons why the voltage has not been increased further to date despite further increases in power plant output: Firstly, the challenge of building a highly efficient and compact inverter based on silicon semiconductors. Secondly, the current PV-specific standards, which only cover the low-voltage range. In the "MS-LeiKra" project funded by the German Federal Ministry for Economic Affairs and Climate Protection (BMWK), Fraunhofer ISE, in cooperation with project partners Siemens and Sumida, therefore developed an inverter that allows the output voltage to be increased to the medium voltage range (1,500 volts) with a power output of 250 kilovolt amperes. This was made possible by the use of highly blocking silicon carbide semiconductors and a special cooling concept with heat pipes.

And why all this? A typical photovoltaic power plant often has several dozen kilometers of copper cables. There is considerable potential for savings here by increasing the voltage: for a string inverter with an output of 250 kilovolt-amperes, a minimum cable cross-section of 120 square millimetres is required at today's possible output voltage of 800 volts. If the voltage is increased to 1,500 volts, however, cables with a cross-section of just 35 square millimeters are sufficient. This reduces copper consumption per kilometer of cable by around 700 kilograms. "Our resource analyses show that copper will become a scarce raw material in the medium term due to the electrification of the energy system. Increasing the voltage makes it possible to use these valuable resources sparingly," says Prof. Dr. Andreas Bett, Director of Fraunhofer ISE.

In addition to its use in photovoltaics, the step beyond the limits of low voltage is also interesting for other applications such as wind turbines, where large cable cross-sections are also required due to the increasing turbine outputs. However, a medium-voltage inverter also offers potential savings in the charging infrastructure for larger electric vehicles or fleets or industrial networks by reducing cable cross-sections.

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