applied sciences Review Virtual Inertia-Based Inverters for Mitigating Frequency Instability in Grid-Connected Renewable Energy System: A Review Kah Yung Yap , Charles R. Sarimuthu * and Joanne Mun-Yee Lim Electrical and Computer Systems Engineering (ECSE), School of Engineering, Monash University Malaysia, 47500 Subang Jaya, Selangor Darul Ehsan, Malaysia; [email protected] (K.Y.Y.); [email protected] (J.M.-Y.L.) * Correspondence: [email protected]; Tel.: +60-3-5515-9732 Received: 15 October 2019; Accepted: 22 November 2019; Published: 5 December 2019 Abstract: This study paper presents a comprehensive review of virtual inertia (VI)-based inverters in modern power systems. The transition from the synchronous generator (SG)-based conventional power generation to converter-based renewable energy sources (RES) deteriorates the frequency stability of the power system due to the intermittency of wind and photovoltaic (PV) generation. Unlike conventional power generation, the lack of rotational inertia becomes the main challenge to interface RES with the electrical grid via power electronic converters. In the past several years, researchers have addressed this issue by emulating the behavior of SG mathematically via pulse width modulation (PWM) controller linked to conventional inverter systems. These systems are technically known as VI-based inverters, which consist of virtual synchronous machine (VSM), virtual synchronous generator (VSG), and synchronverter. This paper provides an extensive insight into the latest development, application, challenges, and prospect of VI application, which is crucial for the transition to low-carbon power system. Keywords: virtual inertia; renewable energy system; inverter; droop control; frequency stability; swing equation; grid angular frequency 1. Introduction Over the last decades, global electricity demand has increased significantly [1]. To cater to these demands, many countries are utilizing environmentally friendly power generation such as solar photovoltaic (PV) energy and wind energy. Renewable energy sources (RES) is much preferred because of its merit factors, including cost-effectiveness, energy efficiency, and sustainability [2]. The tremendous growth of RES gravitates modern power grid towards the inverter dominated power system [3], from the conventional synchronous generator (SG) rotational generator dominated power system [4]. Grid-connected or grid-tied RES is beneficial in improving the voltage profile, increasing the reliability of the power system, and controlling the active and reactive power flow [5]. Despite the numerous advantages, studies have shown that high penetration of grid-connected RES introduces critical frequency stability issues [6] and power grid security issues [7], which include the following: 1. RES exhibits low or non-existent inertial responses which lead to frequency instability [8]. 2. In the event of a fault and sudden load change, the system frequency deviates from the nominal frequency [9]. 3. The frequency nadir and rate of change of frequency (RoCoF) are expected to be higher in the event of a fault or sudden load change, which activates the load-shedding controller to trip frequency relay [10]. Appl. Sci. 2019, 9, 5300; doi:10.3390/app9245300 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 30 Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 30 3. The frequency nadir and rate of change of frequency (RoCoF) are expected to be higher in the Appl.3. Sci.eventThe2019 offrequency, 9 ,a 5300 fault nadiror sudden and rate load of change,change of which freque acncytivates (RoCoF) the areload-shedding expected to becontroller higher in to the2 oftrip 29 frequencyevent of relaya fault [10]. or sudden load change, which activates the load-shedding controller to trip Byfrequency interfacing interfacing relay RES RES [10]. using using a aconventional conventional inverter, inverter, it degrades it degrades the thefrequency frequency stability stability due dueto lack to oflack rotating ofBy rotating interfacing masses masses [11]. RES The [ 11using]. RES The a conventional is RES intermittent is intermittent inverter, and thus it and degrades by thus interfacing bythe interfacingfrequency RES to stability RESthe grid to due the via to grid lacka fast- via respondinga fast-respondingof rotating inverter, masses inverter, the[11]. RES The the is RES presented RES is isintermittent presented to grid asan to ad gridfast-dynamic thus as by a interfacing fast-dynamic system RES [12]. system to This the quickgrid [12]. via interaction This a fast- quick causesinteractionresponding frequency, causes inverter, phase frequency, the angle,RES is phase presentedand angle,voltage to andgrid amplitude as voltage a fast-dynamic amplitudeinstability system [13]. instability [12].It causes This [13 quick]. more It interaction causes significant more frequencysignificantcauses frequency, frequencydeviations phase deviations and transientangle, and and transientpower voltage exchange poweramplitude exchangess in instability the event in the [13]. of event aIt power causes of a power fault.more fault. Tosignificant solve To solve the problemsthefrequency problems introduced deviations introduced by and grid-connected by transient grid-connected power RES, exchange RES, virtual virtual sinertia in inertiathe (VI),event (VI), also of also aknown power known asfault. artificial as artificialTo solve inertia inertia the or syntheticorproblems synthetic inertia introduced inertia control control by strategy, grid-connected strategy, has has been been RES, proposed proposedvirtual inertia and and researched(VI), researched also known extensively extensively as artificial in conventional inertia or inverters.synthetic VI VI inertia emulates emulates control the the inertia inertiastrategy, response response has been of of a aproposed traditional traditional and synchronous synchronous researched machine machineextensively (SM) (SM) in mathematically mathematicallyconventional viainverters. pulse width VI emulates modulation the inertia (PWM). response Figure of1 1 apresents presents traditional thethe synchronous generalgeneral connectionconnection machine of of(SM) VI-basedVI-based mathematically inverterinverter inin via pulse width modulation (PWM). Figure 1 presents the general connection of VI-based inverter in grid-connected solar and wind energy. grid-connected solar and wind energy. FigureFigure 1. 1. TheThe The general general general connection connectionconnection ofof grid-co grid-connectednnected solar solar solar and and wind wind energy. energy. VI-based inverters and their control strategy can be implemented in various applications such as VI-basedVI-based inverters inverters and and their their control control strategystrategy can be implementedimplemented in in various various applications applications such such grid-connectedas grid-connected wind wind power power [14 [14,15],15] and and solar solar power power plant plant [[16],16], high voltage voltage direct direct current current (HVDC) (HVDC) as grid-connected wind power [14,15] and solar power plant [16], high voltage direct current (HVDC) transmissiontransmission [17 [17,18],,18], energy energy storage storage systemsystem (ESS), microgrid [19], [19], elec electrictric vehicle vehicle (EV) (EV) chargers chargers [20], [20 ], transmission [17,18], energy storage system (ESS), microgrid [19], electric vehicle (EV) chargers [20], staticstatic synchronous synchronous compensator compensator (STATCOM),(STATCOM), virtualvirtual inertia machine machine (VIM), (VIM), modular modular multilevel multilevel static synchronous compensator (STATCOM), virtual inertia machine (VIM), modular multilevel converterconverter (MMC)-based (MMC)-based direct direct current current (DC)(DC) systemsystem [2 [211],], electronic electronic appliances, appliances, and and flexible flexible loads loads to to converter (MMC)-based direct current (DC) system [21], electronic appliances, and flexible loads to supportsupport frequency frequency stability. stability. Figure Figure2 presents 2 presents the applicationthe application and implementationand implementation of VI of in theVI in modern the support frequency stability. Figure 2 presents the application and implementation of VI in the powermodern system. power system. modern power system. VI emulation VItechniques emulation techniques Energy VI-based StorageEnergy Units InvertersVI-based Storage Units Inverters DC-link Capacitors Batteries VSM Capacitors DC-link Capacitors Batteries VSM Capacitors Supercapacitors VSG Supercapacitors VSG Ultracapacitors Synchronverter Ultracapacitors Synchronverter Figure 2. The implementations of VI emulation technique. Figure 2. The implementations of VI emulation technique. Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 30 Appl. Sci. 2019, 9, 5300 3 of 29 As the amount of solar and wind energy generation increases, grid-connected RES requires VI emulation,As the and amount thus ofVI-related solar and publication wind energy and generation research are increases, increasing grid-connected [22]. There are RES limited requires studies VI emulation,that compare and VI-based thus VI-related inverters, publication namely virtual and research synchronous are increasing generator [22 (VSG),]. There virtual are limited synchronous studies thatmachine compare (VSM) VI-based and synchronverter, inverters, namely concerning virtual synchronoustheir technical generator implementati (VSG),ons, virtual merits, synchronous demerits,