Virtual Inertia-Based Inverters for Mitigating Frequency Instability in Grid-Connected Renewable Energy System: A Review  
[PERSON]  
1 Electrical and Computer Systems Engineering (ECSE), School of Engineering, Monash University Malaysia, 47500 Subang Jaya, Selangor Darul Elhsan, Malaysia; [EMAIL_ADDRESS] (K.Y.Y.);  
[EMAIL_ADDRESS] (J.M.-Y.L.)1  
[PERSON]  
1 Electrical and Computer Systems Engineering (ECSE), School of Engineering, Monash University Malaysia, 47500 Subang Jaya, Selangor Darul Elhsan, Malaysia; [EMAIL_ADDRESS] (K.Y.Y.);  
[EMAIL_ADDRESS] (J.M.-Y.L.)1  
[PERSON]  
1 Electrical and Computer Systems Engineering (ECSE), School of Engineering, Monash University Malaysia, 47500 Subang Jaya, Selangor Darul Elhsan, Malaysia; [EMAIL_ADDRESS] (K.Y.Y.);  
[EMAIL_ADDRESS] (J.M.-Y.L.)1  
Footnote 1: email: {dun-yee-lim-mounash-mounash-mounash-mounash-mounash-mounash-moun  
By interfacing RES using a conventional inverter, it degrades the frequency stability due to lack of rotating masses [11]. The RES is intermittent and thus by interfacing RES to the grid via a fast-responding inverter, the RES is presented to grid as a fast-dynamic system [12]. This quick interaction causes frequency, phase angle, and voltage amplitude instability [13]. It causes more significant frequency deviations and transient power exchanges in the event of a power fault. To solve the problems introduced by grid-connected RES, virtual inertia (VI), also known as artificial inertia or synthetic inertia control strategy, has been proposed and researched extensively in conventional inverters. VI emulates the inertia response of a traditional synchronous machine (SM) mathematically via pulse width modulation (PWM). Figure 1 presents the general connection of VI-based inverter in grid-connected solar and wind energy.  
VI-based inverters and their control strategy can be implemented in various applications such as grid-connected wind power [14; 15] and solar power plant [16], high voltage direct current (HVDC) transmission [17; 18], energy storage system (ESS), microgrid [19], electric vehicle (EV) chargers [20], static synchronous compensator (STATCOM), virtual inertia machine (VIM), modular multilevel converter (MMC)-based direct current (DC) system [21], electronic appliances, and flexible loads to support frequency stability. Figure 2 presents the application and implementation of VI in the modern power system.  
Figure 1: The general connection of grid-connected solar and wind energy.  
Figure 2: The implementations of VI emulation technique.  
As the amount of solar and wind energy generation increases, grid-connected RES requires VI emulation, and thus VI-related publication and research are increasing [22]. There are limited studies that compare VI-based inverters, namely virtual synchronous generator (VSG), virtual synchronous machine (VSM) and synchronverter, concerning their technical implementations, merits, demerits, and applications. Given the rising demand for inertia to improve frequency regulation [23] as the RES penetration increases, this paper explores and studies the potential of VI for every use in the power grid. The primary objective of this paper is to provide a useful insight into the state-of-the-art VI applicability in RES-dominated power grid quality control and stabilization. This paper presents a comparative review of the various topologies used to implement VI in an inverter. The paper begins with an introductory overview of the RES and VI in Section 1. Following this, VI-based inverters are discussed extensively in Section 2. Section 3 discusses the applications of VI. Section 4 presents the evaluation, comparison, and technical analysis of each VI-based inverter techniques, including VSM, VSG, and synchronverter. The assessment of VI emulation techniques, including the critical analysis, current challenges, and future research on virtual emulation, are presented in Section 5. Section 6 summarizes the recent development of VI emulation in inverters and concludes the overall comparative review.