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More flexibility through modular multilevel inverters

Why is flexQgrid also investigating power electronic converters as a link between generation units, storage and the grid?

In the power grid, many households are no longer just energy consumers, but also energy suppliers. The expansion of photovoltaics means that more and more energy is being generated decentrally. However, the time at which the generated energy is fed into the grid has so far been directly linked to environmental influences, such as the momentary solar radiation. Energy storage systems can, on the one hand, optimize power consumption in households and, on the other hand, support secure grid operation within the framework of the quota-based grid traffic light system. In this context, power electronic converters also play an important role in the power grids of the future as a link between generation units, storage systems and the grid.

Integration of the inverters into flexQgrid

As part of the "flexQgrid" project, the Electrotechnical Institute (ETI) at the Karlsruhe Institute of Technology (KIT) is developing a "Modular Multilevel Converter" (MMC) with integrated battery modules for use in the low-voltage distribution grid.

The MMC consists of a large number of cells of the same type, which work with semiconductors of lower reverse voltage than in conventional low-voltage inverters. On the one hand, this reduces losses, and on the other, the smaller voltage steps result in a smoother voltage curve and thus higher voltage quality. In contrast to previous AC- or DC-coupled storage systems, with the MMC the battery storage can be integrated directly into the inverter cells. For this purpose, part of the cells is equipped with battery modules, as indicated in the diagram.

The inverter is built as a prototype and controlled with the signal processing system developed at ETI. The self-developed signal processing system makes it possible to integrate an interface with little effort, which allows the inverter to be operated with maximum flexibility with regard to the power to be supplied. The Controllable Local Systems (CLS) interface provided for this purpose by the Smart Meter Gateway (SMGW) is integrated directly in the inverter by this concept.

However, in the event of critical grid conditions and network faults, the latency of this TCP/IP-based communication system is too high in many cases. In the case of such dynamic processes in the grid, the inverter must derive a grid-stabilizing behavior from the electrical variables of the grid.

The grid codes specify how the converter must behave in the event of voltage and frequency deviations. Measures for rapid grid identification and grid support are the subject of research at the Electrotechnical Institute.

In the last phase of the project, such fast and critical network states are therefore to be measured with the set-up converter on the basis of real grid data from the measurement campaign (as reported here ) can be simulated in the laboratory and the real-time capability of the control and the converter topology can be tested.

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