Design and implementation of static voltage support via Q-V characteristics on grid-forming inverters in combined operation with grid-following inverters

This paper presents investigations focussing on the parallel operation of grid-forming and grid-following inverters with a focus on static voltage support with Q(V) characteristics. The focus here lies on the stable operation of the various inverter controls, the effectiveness of Q(V) characteristics in grid-forming inverters and the distribution of the reactive power provided in parallel operation of both inverters. In order to investigate these aspects, a laboratory test was set up in which the parallel operation of the two inverter types was tested and compliance with the Q(V) characteristic curves was verified for both inverters. In addition, the dynamic behaviour of the reactive power supply per Q(V) characteristic curve of the grid-forming inverter was recorded in the laboratory and compared with the requirements of the grid code VDE-AR-N 4105. In the next step, the laboratory tests were used to verify simulation models, which were then used to investigate the behaviour of both inverters in a low-voltage grid. The simulations show the effectiveness of Q(V) characteristics in grid-forming inverters in terms of supporting the local voltage. Furthermore, it is shown that the provision of reactive power in parallel operation is not dominated by one of the two inverter control types.