Distributed Optimization Approach for Frequency Control with Emulated Virtual Inertia in Islanded Microgrids

Microgrids (MGs), whether entirely islanded or operated as part of a larger electrical power system, are emerging as a useful strategy not only for integration of distributed energy resources (DERs) but also to improve resilience and efficiency in power systems. When a MG is islanded from the main grid, maintaining frequency control is challenging since fast-acting DERs are connected via power-electronic inverters and thus lack rotational inertia. In this paper, we adopt the virtual inertia concept and propose a distributed frequency control design that ensures stable and synchronized operation at a nominal frequency while guaranteeing proportional active power sharing among DER interface converters (DICs). The novelty of this design lies in incorporating DICs to emulate virtual inertia while providing stability and performance guarantees from an optimization view. With modest assumptions on the MG, the distributed frequency control is cast as a consensus optimization problem and solve by the partial primal-dual (PPD) algorithm. Interestingly, the control updates based on PPD are well suited for the cyber-physical coupling of the MG network. Specifically, parts of the updates mimic the network dynamics, which is seamlessly implemented by the physical system itself. We provide initial numerical results based on real-time simulation of an abrupt system load change, demonstrating that the proposed frequency control quickly restores frequency to a nominal point and enhances the transient stability by inertia emulation.