Experimental Validation of the Parallel Operation of Grid-Forming Converters and Synchronous Generators in Temporary Islanded Microgrids

Due to the rising amount of distributed power generation, suitable medium voltage grids can temporarily be operated in islanded mode ensuring the power supply e.g. during blackout situations. In these temporary islanded microgrids (TIMs), the dynamic behavior is determined by the dimensioning and control of converter coupled generation units and synchronous generators (SGs) as well as their electromechanical behavior. To ensure the controllability of sudden changes in power balance (e.g. caused by switching measures in the framework of TIM energy management), criteria for the selection and control of grid-forming units have to be found. This paper presents an experimental and simulative study in a low voltage laboratory microgrid, which is fed by a grid-forming converter (GFC) and a SG. By applying load steps, the results are assessed by evaluating the voltage and frequency deviation and the active power sharing among the generation units. Moreover, the experimental investigations are compared to dynamic time-domain simulation results. It is demonstrated that choosing the droop control of the GFC appropriately can avoid transient overloading of the GFC as well as unacceptable frequency deviations. Furthermore, implementing a droop control instead of an isochronous control at the SG does not necessarily impact the assessment criteria.