energies Article Grid-Following Mode Operation of Small-Scale Distributed Battery Energy Storages for Fast Frequency Regulation in a Mixed-Source Microgrid Amir Hussain and Wajiha Shireen *   Citation: Hussain, A.; Shireen, W. Grid-Following Mode Operation of Small-Scale Distributed Battery Energy Storages for Fast Frequency Regulation in a Mixed-Source Microgrid. Energies 2021, 14, 7710. https://doi.org/10.3390/en14227710 Academic Editors: Wajiha Shireen and Adel Merabet Received: 27 September 2021 Accepted: 15 November 2021 Published: 17 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Electrical and Computer Engineering, College of Technology, University of Houston, Houston, TX 77004, USA; exploreamir@gmail.com * Correspondence: tech139@central.uh.edu Abstract: As the share of power converter-based renewable energy sources (RESs) is high, a microgrid, in islanded mode, is more vulnerable to frequency instability due to (1) sudden power imbalance and (2) low inertia. One of the most common approaches to address this issue is to provide virtual inertia to the system by appropriately controlling the grid-side converter of the RESs. However, the primary frequency controller (PFC) presented in this paper focuses on the fast compensation of power imbalance without adding inertia to the system. The proposed method is based on estimating the real-time power imbalance caused by a disturbance and compensating it using multiple small-scale distributed battery energy storage systems (BESSs). The power imbalance is estimated by observing the initial rate of change of frequency (RoCoF) following a disturbance. Based on the estimated power imbalance and the rating of the BESSs, the reference power for the BESSs is determined. The BESSs are controlled in grid-following mode to compensate for the power imbalance. The performance of the proposed PFC is verified using a Typhoon real-time simulator for various scenarios and is compared with the conventional virtual synchronous generator (VSG) controller. Keywords: islanded microgrid; frequency control; battery energy storage; model-based control; droop and VSG control 1. Introduction In the past few years, the penetration of renewable energy sources (RESs), particularly wind- and PV-based sources, has been continuously growing in the present power system. These RESs are often integrated with interconnected loads and energy storage units and act as a single entity, called microgrid. A microgrid can be integrated with the main grid or it can also be operated autonomously [1,2]. When connected to the main grid, the voltage and the frequency of the microgrid are controlled by the main grid and the microgrid is controlled only to exchange a certain amount of active and reactive power with the main grid. However, an islanded microgrid must be responsible for its voltage and frequency control. Unlike the conventional power system where the power sources are dominated by synchronous generators (SGs), most microgrids are comprised of only inverter-based sources that possess low or negligible inertia. Therefore, islanded microgrids are more prone to frequency instability in the event of a sudden power imbalance [3]. Some microgrids [4,5] which are more likely to be operated in islanded mode, also comprise SGs to have some amount of mechanical inertia. Still, the issue of frequency instability continues to be a major challenge in the operation of islanded microgrids. In the conventional SG-dominated grid, frequency control is achieved in three stages, known as primary, secondary and tertiary frequency controls [6–8]. The primary frequency controller (PFC) acts immediately after the occurrence of a disturbance and lasts for a few seconds. The purpose of PFC is to achieve the balance between power generation and demand after a disturbance. The droop-based controller is a widely accepted PFC in Energies 2021, 14, 7710. https://doi.org/10.3390/en14227710 https://www.mdpi.com/journal/energies