In this work a 200 kVA battery inverter including a grid-forming control was developed for integrating PV energies into an island grid. The inverter has a three-phase ANPC topology with two interleaved legs per phase and includes a DCDC converter with flying capacitors with four independently interleaved phases. Usually, renewable energy sources such as PV systems turn off when they detect an island grid. However, the integration of renewable energies will become more important for future grids and island grids. To ensure that an island grid is maintained as long as possible without using very large, distributed battery storage systems, these PV systems may be integrated and controlled using an energy management system. Maintaining an island grid can be very useful for increasing grid reliability or in disaster areas.

The aim of the energy management system is to integrate renewable energies, in particular PV systems, in case of an island grid operation. Especially if there is too much PV power in an island grid, it must be ensured that the battery is not fully charged so that it can no longer operate in a grid-serving operation. Due to the VDE 4105, power generation system must reduce its active power fed into the grid during over frequency. This guideline can be used to control the PV systems by controlling the grid frequency. For this purpose, a function was developed that determines the reference active power ????0 of the grid-forming control as a function of the battery SoC and grid frequency. At a high SoC, the reference active power ????0 of the grid-forming control is increased. That leads to an increasing grid frequency in an island grid so that the PV systems are reducing its active power. Hence, in steady state the complete active power needed for the microgrid is provided by the PV. Further, at low SoC the battery needs to be charged. During normal operation the SoC should not be very high, since it increases its calendar aging. The SoC should also not be too low so that in case of an island grid the battery inverter can provide an uninterruptible power supply. An optimum SoC is therefore achieved when connected to the distribution grid or at a grid frequency of 50 Hz.

This concept is shown in laboratory tests using the developed inverter connected to a power-hardware-in-the-loop (P-HiL) setup. Therefore, a real-time model is used, which is simulated on an Opal OP5707XG and includes an ideal distribution grid with a constant grid frequency of 50 Hz connected to a microgrid. A circuit breaker can connect or disconnect this distribution grid. The microgrid model itself is based on a real existing grid in Neusorg – Bavaria – Germany and includes a PV system modeled according to the WECC guideline. The developed grid-forming inverter (GFI) is connected via air coils to an Egston CSU200GAMP6 power amplifier. Using the damping impedance method at the P-HiL setup the GFI is integrated in the model and connected to the microgrid. For the verification test, a test case with more generated PV power than load in the microgrid is evaluated. In normal operation, the circuit breaker is opened to activate island grid operation. It is shown that the grid voltage remains uninterrupted and the GFI adjusts the grid frequency so that the simulated PV model reduces its generated power until the entire load of the microgrid is supplied by the PV in steady state.