The increasing penetration of inverter-based generation in distribution grids, coupled with the replacement of synchronous generators in transmission grids, has a profound impact on the stability of the electricity grid. To address this, grid-forming controls for inverters that emulate the behaviour of traditional synchronous generators are essential. However, multiple renewable power plants with multiple inverters inside a photovoltaic or wind park are required to replace a single conventional synchronous generator in the power grid. Therefore, power oscillations, as a result of mutual interference of the control algorithms, must be prevented both in the power grid and within the park.

Thus, the control interaction between two inverters operated in parallel to each other are investigated. By systematically varying the control parameters of one inverter for the implemented virtual inertia as well as the active and reactive power control, we investigate the dynamic power sharing between the two inverters in the event of a repetitive load step. Different inertia constants will not only affect the frequency’s nadir and rate-of-change-of-frequency (ROCOF), but are expected to show unbalanced loading of the inverters. This unbalance is also anticipated for distinctive power control parameters. Therefore, it is investigated whether this unbalance is of short duration or will be maintained after the load step event. To overcome the limitations of simulations, which may not account for unforeseen hardware behaviour, a Power Hardware-in-the-Loop (PHiL) testbed is set up. It consists of two low-voltage grid-forming rapid control prototyping inverters, both implemented with identical control algorithms, line emulators and a variable resistive load.

This PHiL testbed enables the analysis of the dynamic power distributions of the two inverters resulting from the differing control parameters. This enables us to determine suitable parameter values that react quickly to errors but are at the same time slow enough in order not to influence each other. The experimental results demonstrate that mutual interferences between the controllers destabilises the overall system. Furthermore, by integrating various emulated electricity grids, the PHiL testbed is able to validate these parameters for far more complex network topologies in the future.