Control of Low-Inertia Power Grids: A Model Reduction Approach

A major transition in the operation of electric power grids is the replacement of conventional power generation using synchronous machines by distributed generation based on renewable sources interfaced by power electronics. In contrast to synchronous machines, which stabilize the power system through a combination of their inherent physical properties and their controls, power converters do not inherently stabilize the power system. Moreover, the models used for grid-level stability analysis also crucially depend on the properties of synchronous machines. As a first step toward addressing these challenges, we propose a novel reduced-order model for analysis and control design of low-inertia power systems. Starting from a detailed nonlinear first-principle model of a low-inertia power system, including detailed power converter models and their interactions with the power grid, we use arguments from singular perturbation theory to obtain a tractable model for control design. We use insights gained from the reduced model to bridge the gap between grid-level objectives and device-level control by introducing an internal model and matching controller that exploits structural similarities between power converters and synchronous generators. Moreover, we propose a nonlinear droop control that stabilizes the power system.