Design of Damping Coefficient in an Enhanced Droop-Based Grid Forming Control

Abstract

This article presents an enhanced droop-based grid-forming control method for inverter-based energy resources connected to the transmission grid. The main advantage of inverter control—its ability to control frequency and phase independently—over conventional synchronous machine behavior is emphasized. This additional degree of freedom is used to implement an intrinsically damped response to grid disturbances such as voltage steps, frequency deviations, and phase jumps. The proposed control is analyzed in the state-space for three representative operating scenarios: grid parallel operation, two weakly coupled power resources, and a two-area system with two resources per area. A design goal for the control gain is formulated and applied to the operating scenarios under consideration, using only characteristic per-unit quantities and time constants. It is shown that the damping coefficient at the boundary case of all real-valued poles mainly depends on the intrinsic parameters of the power resource, such as short-circuit impedance and filter time constants, and is fairly independent of other system parameters. Simulation results confirm the analytical design of the damping coefficient, using a two-area system as a benchmark case. In comparison to conventional power plants, the proposed grid-forming control with an optimized damping coefficient demonstrates a significantly improved damped response to grid events, such as load or generation changes or partial tie line tripping. Experimental results gained in a complex power system laboratory validate the well-damped behavior for typical grid events.