Autonomous grid-forming inverter exponential droop control for improved frequency stability

This paper introduces a novel control strategy for grid-forming inverter-based resources, Droop-e, which establishes a non-linear, active power–frequency droop relationship based on an exponential function of the power output. A primary advantage of Droop-e as compared to traditional linear droop is an increased utilization of available power headroom that directly mitigates system frequency excursions and reduces the rate of change of frequency. First, the motivation for Droop-e as compared to a linear grid-forming control is established, and then the full control system is described, including the mirrored inversion at the origin, the linearization at a parameterized limit, and the auxiliary autonomous power sharing controller. Subsequently, the analytic stability of the controller is assessed, including a continuity and smoothness proof, adherence to synchronization criteria, and a small-signal stability analysis. This is followed by electromagnetic transient simulations of this controller with full order inverter models and accompanying DC-side dynamics, connected in parallel with synchronous generators for an exhaustive range of dispatches on a simple 3-bus system. Finally, IEEE 39-bus system simulations highlight the improved frequency stability of the system with multiple, Droop-e controlled grid-forming inverters.