Abstract
The introduction of Renewable Energy Sources (RES) has resulted in increased integration of power electronics into the grid. RES integrated with power electronics differ in principle from conventional Synchronous Machines (SMs), because they lack rotational inertia and utilize a driving circuit to synthesize the frequency. Works in the literature proposed SM emulation control schemes for Voltage Source Converters (VSCs) to introduce Virtual Inertia (VI). However, frequency stability issues may still be prevalent as systems without rotational inertia are more sensitive to disturbances, and detailed simulations of grid-forming VSCs alongside grid-following Current-Controlled VSCs (CC-VSCs) such as PV interfaces are lacking. To that end, this paper presents an optimal frequency-oriented scheme to control VSCs, using Linear Quadratic Regulator (LQR)-based Proportional-Integral (PI) control. The swing equation is used to define the frequency of a VSC as the rate of change of its modulation angle. A Microgrid (MG) consisting of a grid-forming VSC with energy storage and a CC-VSC with a Photovoltaic (PV) module has been modeled and simulated, and the proposed method has been compared with conventional droop and Virtual Synchronous Machine (VSM) control schemes, providing better results in terms of MG frequency control under disturbances.