Backup power supply concepts for low-voltage microgrids with directly coupled or inverter-interfaced grid-forming generators

Due to the increasing number of distributed generation in distribution grids and because of the decreasing number of conventional power plants in the transmission grid, the European electrical power system loses inertia and, thus, stability margins decrease while the system’s complexity rises. Therefore, new security concepts in case of a system blackout need to be developed to maintain the reliability of the electrical system in the future. This work investigates two backup concepts in a variable microgrid test environment: a black start concept and a voltage maintaining (“intercepting”) concept. Based on an analysis of existing approaches, two types of grid-forming generators are analyzed: a directly coupled synchronous generator emulating a combined heat and power plant and an inverter-interfaced generator acting like a battery storage system. For the first time, this research work provides a comparison between both concepts for grid-forming generators. The results show that an inverter-interfaced generator has clear advantages in comparison to a rotating mass with respect to its control speeds, maximum overshoots and settling times. For a resilient backup, grid-forming inverters need to be short circuit proof to withstand the transformers’ inrush currents. The intercepting concept requires a robust island detection method and a basic communication infrastructure in the microgrid.