The starting of decarbonization of electric power generation lead to an increase of renewable energy sources and a resulting reduction of conventional high-power plants. Thus, the inertia / stability of the grid is decreasing in many countries. In order to counteract this change grid forming control structures with virtual inertia for inverter based renewable energy sources have been developed in academia. In case of frequency and phase transients the frequency inertia of grid forming control will respond with grid supporting active power transients like synchronous generators would do. This is problematic for frequency and phase drops, if the regenerative power plant is already operating at its maximum active power available from the primary source. This paper describes effective measures to droop based grid forming control to avoid overloads at frequency or phase drops if there is no reserve power available. At the same time the presented measures do allow grid forming behavior to the extend active power reserve is available. Within this work an analytical solution for the power transient due to phase jumps is presented. Simulations and laboratory results with real world parameters confirm the effectiveness of the proposed measures and the coincidence with the theoretical results.

This paper describes an enhanced droop-based grid forming control method for inverter based distributed energy resources. The important advantage of inverter control over traditional synchronous machine behavior in terms of independent control of frequency and phase is highlighted. This degree of freedom is used to give the inverter control an inherent damped reaction to grid events, like voltage jumps, frequency or phase changes. The control is analyzed in the state space for three essential applications, i.e. grid parallel operation, two weak coupled power resources and two area system with two resources per area. It is shown, that the suitable damping coefficient mainly depends on the parameters of the power resource itself, like short circuit impedance and filter time constants, and is fairly independent on the residual system parameters. Simulation results confirm the analytical design of the damping coefficient. Experimental results gained in a complex power system laboratory validate the behavior for typical grid events.