Modeling of Inverter-Based Microgrid for Small-Signal and Large-Signal Stability Analysis

Abstract

Integration of inverter-based resources (IBRs) such as solar photovoltaic, wind, and battery storage, is both a boon and a bane for electric power systems. On one hand, IBRs have helped in making electrical energy a clean (carbon-free) source of energy. On other hand, the dynamics of IBRs have changed the way power system studies have been carried out. With the advantages IBRs offer over conventional resources, it is assumed that a small distribution power system (microgrid) will have a 100% IBRs penetration in the future. Such a microgrid can operate in grid-connected mode or in an islanded mode. Both modes of operation bring challenges to system stability due to faster control dynamics of IBRs and can pose problems such as blackouts if the dynamics are not studied and mitigated properly. IBRs in a microgrid can operate in a grid-following (GFL) mode or grid-forming mode (GFM). We studied the effect of GFM inverters, GFL inverters, and elements of microgrid (such as lines, and transformers) on microgrid stability. We studied how solar radiation affects the bus voltage and how the inverter gains and line impedance can make a system unstable. Based on the study stable limits for a single inverter connected to a strong point of common coupling (PCC) were established. Since the study of a microgrid is still an evolving topic it has been observed that some elements such as a transformer are not modeled in detail for the stability study of the system. It was shown how detailed modeling of different transformer models will help in understanding the cause of high frequencies in the system following a disturbance. Droop type GFM inverters mimic the droop characteristics of the synchronous generator to control the frequency and voltage. Droop control is best suited for a power system where lines are highly inductive which is not the case for lines of a microgrid that are highly resistive. This results in a weak microgrid system. A modified primary control technique was developed for improving reactive power sharing between the inverters and thus making the microgrid system strong. Virtual Synchronous Generator (VSG) type of control of GFM inverters is another widely used control technique for GFM type of inverters. In an islanded microgrid GFM and GFL inverters work in parallel and interaction between these different types of inverter controls plays a huge role in system stability. Therefore, it is necessary to study the interactions of