Dynamic Frequency Support for Low Inertia Power Systems by Renewable Energy Hubs with Fast Active Power Regulation

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Article Dynamic Frequency Support for Low Inertia Power Systems by Renewable Energy Hubs with Fast Active Power Regulation

Jose Rueda Torres 1,* , Nidarshan Veera Kumar 1, Elyas Rakhshani 1 , Zameer Ahmad 1 , Ebrahim Adabi 1, Peter Palensky 1 and Mart van der Meijden 1,2

1 Department of Electrical Sustainable Energy, Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands; [email protected] (N.V.K.); [email protected] (E.R.); [email protected] (Z.A.); [email protected] (E.A.); [email protected] (P.P.); [email protected] (M.v.d.M.) 2 TenneT TSO B.V., 6812 AR Arnhem, The Netherlands * Correspondence: [email protected]

Abstract: This paper concerns the feasibility of Fast Active Power Regulation (FAPR) in renewable energy hubs. Selected state-of-the-art FAPR strategies are applied to various controllable devices within a hub, such as a solar photovoltaic (PV) farm and an electrolyzer acting as a responsive load. Among the selected strategies are droop-based FAPR, droop derivative-based FAPR, and virtual synchronous power (VSP)-based FAPR. The FAPR-supported hub is interconnected with a test transmission network, modeled and simulated in a real-time simulation electromagnetic transient (EMT) environment to study a futuristic operating condition of the high-voltage infrastructure covering the north of the Netherlands. The real-time EMT simulations show that the FAPR strategies (especially the VSP-based FAPR) can successfully help to significantly and promptly limit undesirable Citation: Rueda Torres, J.; Kumar, N.V.; Rakhshani, E.; Ahmad, Z.; large instantaneous frequency deviations. Adabi, E.; Palensky, P.; van der Meijden, M. Dynamic Frequency Keywords: fast active power regulation; fast frequency control; renewable power generation; elec- Support for Low Inertia Power trolyzers; renewable energy hubs Systems by Renewable Energy Hubs with Fast Active Power Regulation. Electronics 2021, 10, 1651. https:// doi.org/10.3390/electronics10141651 1. Introduction Frequency is a systemic parameter, which ideally stays at a constant value, e.g., Academic Editor: Ali Mehrizi-Sani 50/60 Hz, at all voltage levels. However, when a power system is subjected to sudden perturbations, such as the loss of components, increase/decrease in load, or other events Received: 26 April 2021 that cause load-generation imbalances, dynamic frequency excursions are excited [1]. Accepted: 9 July 2021 The time evolution of the frequency of conventional power systems is usually bounded Published: 11 July 2021 within well-defined technical limits. Acceptable Rate of Change of Frequencies (RoCoF) and maximum frequency deviations (a.k.a. frequency Nadir/Zenith) have been largely Publisher’s Note: MDPI stays neutral ensured due to the predominance of conventional power plants with heavy rotating syn- with regard to jurisdictional claims in published maps and institutional affil- chronous generators [2]. On one hand, such heavy machinery has traditionally entailed an iations. inherent opposition (i.e., high inertia within a time frame of several milliseconds) against steep RoCoF, whenever a sudden perturbation of the active power balance occurs [3]. On the other hand, conventional plants performing primary frequency control have traditionally offered enough headroom for proportional adjustment (within a time frame of several seconds) of their active power output to prevent the violation of the allowed Copyright: © 2021 by the authors. Nadir/Zenith [4]. Licensee MDPI, Basel, Switzerland. Due to environmental concerns, conventional power plants, especially fossil-fuel-fired This article is an open access article plants, are being progressively phased out. This is leading to a significant reduction in distributed under the terms and conditions of the Creative Commons the level of inertia and the absence of the major players of primary frequency control. In Attribution (CC BY) license (https:// this context, the violation of the limits of RoCoF and Nadir/Zenith is a major cause of creativecommons.org/licenses/by/ concern [1]. Currently, significant research efforts are devoted to the development of new 4.0/). active power–frequency control concepts, i.e., Fast Active Power Regulation (FAPR), in

Electronics 2021, 10, 1651. https://doi.org/10.3390/electronics10141651 https://www.mdpi.com/journal/electronics Electronics 2021, 10, 1651 2 of 10

an attempt to improve the dynamic frequency support that could be offered by power electronic interfaced devices (e.g., voltage source converters—VSCs—applied in renewable plants, responsive demand, and controllable storage and compensation elements) [5,6]. To date, most of the established power electronic interfaced devices perform system- integration-oriented control actions under a grid-following approach, i.e., the network-side VSC unit adjusts the current injection according to the reference signal defined at the point of common coupling (PCC). In principle, there is no mechanical power used to directly support the system’s frequency control. Solar photovoltaic (PV) generation is a classic example of a fully decoupled (Type-4) renewable plant, in which the priority is to generate the highest possible amount of active power based on a so-called maximum power point tracking (MPPT) characteristic [7]. Since VSC-interfaced solar PV generation does not involve any rotating element on the DC side of the network-side VSC, the main option for fast active power–frequency control actions at the AC side of the network-side VSC is by modifying the MPPT, which entails a reduced margin for active adjustment [7]. Type-4 wind turbines are becoming a preferred option for new wind power plants. Due to decoupling from the power system by a back-to-back converter, wind turbines do not directly react to perturbations occurring in a power system. In this way, the natural inertia that could have been extracted from the wind turbine cannot be fully and promptly deployed [8]. Furthermore, the VSC units belonging to HVDC systems [9], or responsive demand (e.g., large-size electrolyzers [10]), could be involved in fast active power–frequency control. Nevertheless, the active power set points required to fulfil the required power transfer may restrict the headroom and speed of active power adjustment. Alternatively, hybrid resource-based solutions are receiving more attention for future developments, e.g., flywheels combined with tidal and biogas-based generation [11], and hybrid energy storage systems placed at renewable power plants [12]. Obtaining enough levels of headroom for effective fast active-power control, under highly variable operating conditions in systems with very high shares of VSC-interfaced devices, remains an open research challenge. In view of the above review, this paper concerns an investigation of the feasibility and effectiveness of FAPR, when implemented and deployed in a renewable energy hub comprising a solar PV farm and a proton exchange membrane (PEM) electrolyzer. Unlike the current state of the art, the paper provides insight on the degree of improvement of the frequency response of a power system, as affected by individual and collective fast active power adjustment of the controllable elements belonging to the hub. Among the selected strategies are droop-based FAPR, droop derivative-based FAPR, and virtual synchronous power (VSP)-based FAPR. The FAPR-supported hub is interconnected with a test transmission network, modeled and simulated in a real-time simulation electromagnetic transient (EMT) environment to study a futuristic operating condition of the high-voltage infrastructure covering the north of the Netherlands. The reminder of the paper is structured as follows: the characteristics of the test system are presented in Section2, whereas the implemented energy hub is described in Section3. The rationale behind the applied FAPR strategies is overviewed in Section4. Numerical results are shown in Section5. Finally, Section6 summarizes the concluding remarks.

2. Description of Test Model of the North of the Netherlands Network (N3) The test model was built by using RSCAD software and inspired future operational scenarios and expansion plans presented for the future Dutch power system [13]. In a hypothetical future scenario (e.g., the year 2030), the installation of a large-scale electrolyzer plant is assumed to happen in the north of Netherlands. This part of the network includes a large-scale generation center, the connection of large-scale offshore wind plants, and submarine HVDC interconnections with Norway (NorNed) and Denmark (COBRAcable) at Eemshaven and Eemshaven Oudeschip substations. Eemshaven Oudeschip is also a suitable location for a future 300 MW electrolyzer plant, as abundant renewable energy generated by the offshore wind farm can be converted into hydrogen gas. The electrolyzer Electronics 2021, 10, x FOR PEER REVIEW 3 of 11

Denmark (COBRAcable) at Eemshaven and Eemshaven Oudeschip substations. Electronics 2021, 10, 1651 Eemshaven Oudeschip is also a suitable location for a future 300 MW electrolyzer 3plant, of 10 as abundant renewable energy generated by the offshore wind farm can be converted into hydrogen gas. The electrolyzer plant can also support the power system stability by plantparticipating can also in support ancillary the services. power system The modeled stability systemby participating covers the in extra ancillary-high services. voltage The(EHV) modeled infrastructure system coversof voltage the extra-highlevels 380 kV voltage and 220 (EHV) kV, infrastructureand the year 2030 of voltage is taken levels into 380account kV and to create 220 kV, synthetic and the yearprofiles 2030 of isgeneration taken into and account demand. to create The syntheticsystem also proﬁles features of generationtwo 2250 andMVA demand. thermal The power system plants, also features each one two equipped 2250 MVA with thermal two power synchronous plants, eachgeneration one equipped units. withAdditionally, two synchronous renewable generation energy units. is exchanged Additionally, via renewablethe HVDC energy inter- isconnectors exchanged with via theDenmark HVDC (COBRAcable) inter-connectors and with Norway Denmark (NorNed), (COBRAcable) both operated and Norway at the (NorNed),rated power both transfer operated capacity at the of rated 700 power MW. Apart transfer from capacity this, ofthe 700 test MW. N3 Apartnetwork from houses this, the3058 test MW N3 onshore network wind houses energy 3058 and MW 600 onshore MW of wind offshore energy wind and energy 600 MW (Gemini of offshore wind windpark) energydistributed (Gemini around wind the park) area, distributed which further around aggregates the area, into which their further corresponding aggregates 380 into kV theirsubstations. corresponding 380 kV substations. TheThe synchronous synchronous generators generators models models illustrate illustrate dynamic dynamic behavior behavior due to due detailed to detailed steam turbinesteam governorsturbine governors enabled with enabled droop with control, droop AVR (Automaticcontrol, AVR Voltage (Automatic Regulators) Voltage and PSSRegulators) (Power and System PSS Stabilizers).(Power System This Stabilizers). supports dynamicThis supports control dynamic and the control provision and the of ancillaryprovision services of ancillary during services disturbance during and dist post-disturbanceurbance and post periods.-disturbance Wind periods. turbines Wind were representedturbines were by represented generic models, by generic which models, are available which are in the available software. in the For software. this study, For it this is assumedstudy, it thatis assumed the HVDC that inter-connectors the HVDC inter did-connectors not participate did not in participate the regulation in the of the regulation system. Additionally,of the system. the Additionally, local demands the werelocal clustereddemands andwere modeled clustered as and constant modeled loads. as Figureconstant1 representsloads. Figure the 1 load represents ﬂow results the load of an flow N3 results network of ofan scenario N3 network 2030 of [13 scenario]. 2030 [13].

to Denmark to Norway HVDC COBRAcable NorNed 380 kV  220 kV MW 600 MW MW VSC station LCC station ≤ 110 kV MW MW (GEMINI) 450 450 690 690

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Figure 1. Example power ﬂow proﬁle of the N3 test system for year 2030 [13].

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Electronics 2021, 10, 1651 Figure 1. Example power flow profile of the N3 test system for year 2030 [13]. 4 of 10 Figure 1. Example power flow profile of the N3 test system for year 2030 [13]. 3. Modifications Performed in the N3 Network to Integrate Elements Equipped with FAPR3. Mo difications Performed in the N3 Network to Integrate Elements Equipped with 3. Modiﬁcations Performed in the N3 Network to Integrate Elements Equipped withFAPRThree FAPR main modifications were undertaken in the N3 test system for the sake of the analysis of its frequency stability. Option 1: a 300 MW PEM electrolyzer with FAPR placed ThreeThree mainmain modiﬁcationsmodifications werewere undertakenundertaken ininthe theN3 N3 test test system system for for the the sake sake of of the the at EOS (cf. Figure 1). Option 2: a solar farm of 300 MW with FAPR. The loads in the N3 analysisanalysis of of its its frequency frequency stability. stability. Option Option 1: 1: a a 300 300 MWMW PEMPEM electrolyzerelectrolyzer with with FAPR FAPR placed placed test system were increased according to the projected growth by year 2030. In this atat EOSEOS (cf.(cf. Figure 11).). Option 2: 2: a a solar solar farm farm of of 300 300 MW MW with with FAPR. FAPR. The The loads loads in inthe the N3 condition, the N3 test system was prone to congestion, and the occurrence of an active N3test test system system were were increased increased according according to to the the projected projected growth growth by year year 2030. 2030. In In this this power imbalance excited dynamic frequency excursions. condition,condition, thethe N3N3 testtest systemsystem waswas proneprone toto congestion, congestion, andand thethe occurrenceoccurrence ofof an an active active powerpower imbalance imbalance excited excited dynamic dynamic frequency frequency excursions. excursions. 3.1. Representation of a 300 MW Electrolyzer 3.1.3.1. RepresentationFigureRepresentation 2 illustrates ofof aa 30030 0 the MW M W interconnection ElectrolyzerElectrolyzer of the 300 MW PEM electrolyzer, which constitutedFigureFigure 2 an illustrates2 aggregatedillustrates the the interconnection model interconnection representing of the of a 300 farm the MW 30 of0 PEM300 MW electrolyzer electrolyzer, PEM electrolyzers which, with , consti-1 which MW electrolyzertutedconstituted an aggregated model an aggregated scaled model up representing model to a model representing a farmof a 300 of 300 a MW farm electrolyzers, electrolyzer of 300 electrolyzer with farm. 1 MW Thes, electrolyzer withderivation 1 MW andmodelelectrolyzer testing scaled of model upthe tomodel ascaled model can up ofbe to a found 300 a model MW in [13]. electrolyzer of a 300 MW farm. electrolyzer The derivation farm. The and derivation testing of theand model testing can of bethe found model in can [13 ].be found in [13]. VSC DC/DC Converter 3-Phase Transformer VSC DC/DC Converter 3-Phase AC DC PEM Transformer Grid DC Electrolyser AC DCDC PEM Electrolyser Grid DC DC

Power Conversion Syetem BoP Power Conversion Syetem FigureBoP 2 . Connection of the inverter interfaced 300 MW PEM electrolyzer [14]. FigureFigure 2. 2.Connection Connection ofof thethe inverterinverter interfaced interfaced 300 300 MW MW PEM PEM electrolyzer electrolyzer [ 14[14].]. 3.2. Representation of a Type 4 Wind Turbine and Type-4 Solar PV Farm 3.2.3.2. RepresentationFigureRepresentation 3 represents ofof aa TypeType 4the 4 Wind Wind connection Turbine Turbine and and diagram Type-4 Type- 4Solar ofSolar the PV PV FarmT Farmype-4 renewable power generationFigureFigure (i.e.3 represents3, windrepresents or thesolar connection the PV) connection plants diagram indicated diagram of the in Figure Type-4 of the 4 renewable. NoteType that-4 power renewablethe generators generation power of these(i.e.,generation windplants or are(i.e. solar equipped, wind PV) or plants solarwith PV)a indicated battery plants energy indicated in Figure storage 4in. Figure Note system that 4 .(BESS), Note the generatorsthat which the isgenerators connected of these of plants are equipped with a battery energy storage system (BESS), which is connected to the tothese the correspondingplants are equipped back- withto-back a battery link by energy using st aorage DC-DC system converter. (BESS), The which FAPR is connected strategy corresponding back-to-back link by using a DC-DC converter. The FAPR strategy adjusts adjuststo the correspondingthe BESS to provide back-to fast-back active link power by using–frequency a DC-DC support converter. through The FAPRthe grid strategy side the BESS to provide fast active power–frequency support through the grid side converter. converter.adjusts the BESS to provide fast active power–frequency support through the grid side converter.

FigureFigure 3 3.. LayoutLayout of of the the T Type-4ype-4 renewable generators belonging to the renewable energy hub [[15].15]. Figure 3. Layout of the Type-4 renewable generators belonging to the renewable energy hub [15].

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Figure 4. Screenshot of the RSCAD implementation of a renewable energy hub, including a solar farmFigureFigure and 4. a4. ScreenshotPEM Screenshot electrolyzer, ofof the the RSCAD withRSCAD a theoretical implementation implementation expansion ofof a ato renewable renewable include a energy Typeenergy-4 hub, windhub, including turbineincluding [13]. a a solar solar farmfarm and and a a PEM PEM electrolyzer, electrolyzer, with with a a theoretical theoretical expansion expansion to to include include a a Type-4 Type- wind4 wind turbine turbine [13 [13].]. 4. Selected FAPR Strategies 4. Selected FAPR Strategies 4. SelectedFAPR constitutes FAPR Strategies a supplementary control acting on power electronic converters that FAPR constitutes a supplementary control acting on power electronic converters that link renewableFAPR constitutes generation, a supplementary or storage, or controlcontrollable acting demand on power with electronic an electrical converters power that link renewable generation, or storage, or controllable demand with an electrical power link renewable generation, or storage, or controllable demand with an electrical power system.system. The The input input signal signal can can be be the measuredmeasured systemsystem frequency frequency and/or and/or the the deviation deviation of of ac- system. The input signal can be the measured system frequency and/or the deviation of activetive power. power. Next, Next, a deﬁned a control defined structure control dynamically structure adjusts dynamically the injection/absorption adjusts the injection/absorptionofactive active powerpower. in an Next, of attempt active a powerto defined quickly in controlboundan attempt the structure frequency to quickly dynamically deviation bound resulting the adjusts frequency from the a deviationsuddeninjection/absorption activeresulting power from imbalance. of a sudden active power active power in an imbalance attempt to. quickly bound the frequency deviation resulting from a sudden active power imbalance. 4.1.4.1. Droop Droop-Based-Based FAPR FAPR 4.1.Frequency FrequencyDroop-Based droop droop FAPR control control adjusts adjusts the the active active power power following following a alinear linear characteristic. characteristic. FigureFigure Frequency 55 illustrates droop the the droop-based droopcontrol-based adjusts FAPR. FAPR. the active Here, Here, powerf frepresents represents following the the measureda measuredlinear characteristic. system system fre- frequencyquencyFigure at 5at theillustrates the PCC. PCC.fref fref theis is the the droop frequency frequency-based set-point. FAPR.set-point. Here, f represents the measured system frequency at the PCC. fref is the frequency set-point.

Vbus f ∆f ∆P Vbus ref branch PMU Deadband Kp I f ∆f ∆Pref Ibranch PMU Deadband Kp

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Electronics 2021, 10, 1651 6 of 10 worksasas describeddescribed as described inin thethe in previousprevious the previous section.section. section. TheThe actuationactuationThe actuation ofof thethe of droopdroop the droop controllercontroller controller isis activeactive is active forfor thethe forentireentire the entire frequencyfrequency frequency containmentcontainment containment period.period. period. TheThe secondsecond The loop secondloop isis aa loop derivativederivative is a derivative control,control, whosewhose control, out-out- works as described in the previous section. The actuation of the droop controller is active whoseputput is isoutput aa derivativederivative is a derivative gaingain ofof gainthethe frequencyfrequencyof the frequency erroerrorr signal. signal.error signal. TheThe derivativederivative The derivative controllercontroller controller isis onlyonly forentire the frequency entire frequency containment containment period. The period. second The loop second is a derivative loop is control, a derivative whose control, output is onlyactiveactive active forfor thethe for initial initialthe initial fewfew few millisecondsmilliseconds milliseconds toto a ato fewfew a few seconds,seconds, seconds, andand and lastslasts lasts untiluntil until thethe the frequencyfrequency frequency re-re- whoseis a derivative output is gain a derivative of the frequency gain of the error frequency signal. The error derivative signal. The controller derivative is only controller active responsesponsesponse stabilizesstabilizesstabilizes atatat the thethe maximum maximummaximum allowed allowedallowed frequency frfrequencyequency deviation. deviation.deviation. The TheThe combined combinedcombined effect effecteffect of ofof isfor only the active initial for few the milliseconds initial few milliseconds to a few seconds, to a few and seconds, lasts until and lasts the frequency until the frequencyref response thethethe outputs outputsoutputs of ofof the thethe droop droop-baseddroop-based-based loop looploop and andand derivative derivative-basedderivative-based-based loop looploop produces producesproduces Δ PΔΔrefPP, ref which,, whichwhich responsestabilizes stabilizes at the maximum at the maximum allowed frequency allowed frequency deviation. deviation. The combined The effectcombined of the effect outputs of modulatesmodulatesmodulates the thethe active activeactive power powerpower response response ofof of thethe the device(s)device(s) device(s) toto to improveimprove improve bothboth both RoCoFRoCoF RoCoF andand andmaxi-maxi- theof the outputs droop-based of the droop loop and-based derivative-based loop and derivative loop produces-based loop∆P produces, which modulates ΔPref, which the maximummummum frequencyfrequency frequency deviationdeviation deviation (e.g.,(e.g., (e.g. Nadir).Nadir)., Nadir). ref modulatesactive power the response active power of the device(s) response to of improve the device(s) both RoCoF to improve and maximum both RoCoF frequency and maximumdeviation (e.g.,frequency Nadir). deviation (e.g., Nadir). DroopDroopDroop Control ControlControl

Droop Control DeadbandDeadbandDeadband DroopDroopDroop Function FunctionFunction KpKpKp

VbusVbusVbus Deadband Droop Function Kp ff ∆∆PPref f ∆f ∆∆ff ∆Pref ref IbranchIbranch PMUPMUPMU IbranchVbus f ∆f ∆Pref PMU Ibranch RoCoFRoCoF freffref DeadbandDeadband WashWash-outWash-out-out RoCoF fref Deadband LPFLPFLPF KdKdKd FilterFilterFilter Wash-out RoCoF fref Deadband LPF Kd DerivativeDerivativeDerivative Control ControlControlFilter

FigureFigure 6.6. SchematicSchematic ofof thethe droopdroop derivative-basedderivative-based FAPR.FAPR.Derivative Control Figure 6. Schematic of the droop derivative-based FAPR. FigureFigure 6. 6. SchematicSchematic of of the the droop droop deriv derivative-basedative-based FAPR FAPR.. TheTheThe droop droopdroop derivative derivativederivative FAPR FAPRFAPR strategy ststrategyrategy should shouldshould be be betuned tunedtuned (taking (taking(taking into intointo account accountaccount system system-system-- dependentdependentdependent dynamic dynamicdynamic properties) properproperties)ties) to totocause causecause a prominent aa prominentprominent ramping rampingramping of ofofthe thethe active activeactive power powerpower output outputoutput TheThe droop droop derivative derivative FAPR FAPR strategy strategy should should be be tuned tuned (taking (taking into into account account system system-- at atatthe thethe AC ACAC side sideside of ofofa gridaa grid-sidegrid-side-side converter, converter,converter, whenever whwheneverenever an anan over/under over/under-frequencyover/under-frequency-frequency event eventevent occurs occursoccurs dependentdependent dynamic properties) toto causecause aa prominentprominent ramping ramping of of the the active active power power output output at [5][5].[5].. atthe the AC AC side side of of a grid-sidea grid-side converter, converter, whenever whenever an over/under-frequencyan over/under-frequency event event occurs occurs [5]. [5]. 4.3.4.3.4.3. VSP VSP-BasedVSP-Based -Based FAPR FAPR FAPRFAPR Figure 7 depicts the VSP-based FAPR, which measures the power required at the bus 4.3.Figure VSPFigure-Based 7 depicts7 7 depicts depicts FAPR the thethe VSP VSP-basedVSP-based-based FAPR, FAPR,FAPR, which which which measures measures measures the the the power power power required required required at at theat the the bus bus bus at atatthe the the PCC, PCC, PCC, and and and compares compares compares it withit it with with the the the the the the reference reference reference power. power. power. In InInthis this this control control control scheme, scheme, scheme, is 𝜁𝜁 istheis the the Figure 7 depicts the VSP-based FAPR, which measures the power required at the bus dampingdampingdamping factor factor factor and andand n 𝜔𝜔 isnn istheis thethe natural naturalnatural frequency frequencyfrequency associated associatassociat associatededed with withwith a second a a second-ordersecond-order -order function. function.function. at the PCC, and compares it with the the reference power. In this control scheme, is the BothBothBoth parameters parameters parameters are areare calibrated calibrated calibrated on onon a systema a system-depedent system-depedent-depedent basis basis basis to to toachieve achieve achieve a desireda a desired desired rate rate rate of ofof damping factor and n is the natural frequency associated with a second-order function. activeactiveactive power power power injection injection injection and and and oscillation oscillation oscillation daming daming daming ratio ratio ratio [5]. [5]. [[5].5 ]. Both parameters are calibrated on a system-depedent basis to achieve a desired rate of active power injection and oscillation daming ratio [5].

FigureFigure 7.7. SchematicSchematic ofof thethe VSP-basedVSP-based FAPR.FAPR. FigureFigure 7. Schematic 7. Schematic of the of the VSP VSP-based-based FAPR. FAPR. Figure 7. Schematic of the VSP-based FAPR. 5. 5.Simulation5. SimulationSimulation Results ResultsResults TableTable1 1 showsshows thethe activeactive power share in the considered futuristic operating condition. 5. SimulationTableTable 1 shows 1 shows Results the the active active power power share share in inthe the considered considered futuristic futuristic operating operating condition. condition. TheTheThe performance performance performance of of ofthe the the FAPR FAPR FAPR is isexaminedis examinedexamined by by by taking takingtaking into intointo account accountaccount a perturbationa a perturbationperturbation in ininthe thethe Table 1 shows the active power share in the considered futuristic operating condition. formformform of ofof of 200 200200 200 MW MWMW MW sudden suddensudden sudden losslossloss loss of of of GeminiGemini Gemini windwind wind wind ge ge generation.neration.neration. generation. InIn In thethe In the firstfirst the first firstcase,case, case, case, onlyonly only onlythethe the elec-elec- the The performance of the FAPR is examined by taking into account a perturbation in the electrolyzertrolyzerelectrolyzertrolyzer performsperforms performs performs underunder under under FAPR.FAPR. FAPR. FAPR. InIn In thethe In the the secondsecond second second case,case, case, case, aa solarsolara asolar solar PVPV PV PV farmfarm farm farm ofof of 300300 of 300 300 MWMW MW MW kicks-inkicks-in kicks kicks-in- toto form of 200 MW sudden loss of Gemini wind generation. In the first case, only the in performtoperformto performperform FAPR.FAPR. FAPR.FAPR. electrolyzer performs under FAPR. In the second case, a solar PV farm of 300 MW kicks- in5.1. to Frequencyperform FAPR. Support through FAPR Implemented in Electrolyzer TableTableTable 1. Active1.1. ActiveActive power powerpower share shareshare in inthein thethe N3 N3N3 test testtest system systemsystem for forfor year yearyear 2030. 2030.2030. The electrolyzer present in the N3 network was modified with the inclusion of droop, Generator/HVDCGenerator/HVDC LinkLink YearYear 20302030 ScenarioScenario Tablecombined 1. Active droopGenerator/HVDC power derivative, share in the and Link N3 VSP-based test system FAPR. for year Therefore, 2030. Year during 2030 theScenario active imbalance GEMINIGEMINI WindWind farmfarm (EOS)(EOS) 450 450 MWMW event, the FAPRGEMINIGenerator/HVDC controllers Wind farm are expected(EOS) Link to reduce the active powerYear450 2030 absorption MW Scenario accordingly. NorNedNorNed ConnectionConnection (EEM)(EEM) 700 700 MWMW Figure8 representsNorNedGEMINI Connection the Wind active farm power (EEM) (EOS) variation of the electrolyzer700 and450 MW Figure MW 9 depicts the resulting frequencyCOBRAcableCOBRAcable improvement ConnectionConnection due (EOS)(EOS) to the action of each FAPR−−700700 strategy. MWMW As observed COBRAcableNorNed Connection Connection (EEM) (EOS) −700700 MW MW from the base plot in both figures (cf. black curves), the electrolyzer on its own does not reg- COBRAcable Connection (EOS) −700 MW ulate its power demand to mitigate the resulting under-frequency. However, due to action of the droop-based FAPR, taking into account the resulting frequency deviation, the power

Electronics 2021, 10, x FOR PEER REVIEW 7 of 11

GEN1 (EOS) 3 × 430 MW GEN2 (EOS) 2 × 430 MW Electronics 2021, 10, 1651 GEN3 (EOS) 233 MW 7 of 10 Total 3490 MW

demand5.1. Frequency of the Support electrolyzer Through was FAPR proportionally Implemented reduced, in Electrolyzer resulting in an improvement of the frequencyThe electrolyzer Nadir. present On the otherin the hand,N3 network the droop was derivativemodified with FAPR the helps inclusion in achieving of droop, a fastercombined active droop power derivative, reduction. and This VSP improves-based FAPR. RoCoF Therefore, signiﬁcantly, during and also,the active as a by-product, imbalance evenevent, an the improvement FAPR controllers of the frequency are expected Nadir is to observed. reduce The the last active controller power implemented absorption wasaccordingly. the VSP-based Figure FAPR8 represents controller. the active Here, power the VSP-based variation FPR of the is notelectrolyzer integrated and with Figure the BESS,9 depicts since the the resulting electrolyzer frequency is a load, improvement and the power due regulation to the action is exclusively of each FAPR performed strategy. by reducingAs observed the electrolyzer’sfrom the base activeplot in power both figures demand. (cf. The black output curves), of VSP the iselectrolyzer directly given on its to theown active does powernot regulate reference its input, power as demand in the case to of mitigate droop and the combined resulting underdroop- derivativefrequency. controller.However, Additionally, due to action please of the note droop that-based the droop FAPR, controller taking is into always account active the in combina- resulting tionfrequency with the deviation, derivative the and power VSP demand for the entire of the time. electrolyzer The time was duration proportionally and active reduced, power reductionresulting inapplied an improvement by the derivative of the and frequency VSP controller Nadir. are On different. the other hand, the droop derivative FAPR helps in achieving a faster active power reduction. This improves RoCoF Table 1. significantly,Active and power also, share as ina theby- N3product, test system even for an year improvement 2030. of the frequency Nadir is observed. TheGenerator/HVDC last controller Linkimplemented was the VSP-based Year 2030FAPR Scenario controller. Here, the VSP-based FPR is not integrated with the BESS, since the electrolyzer is a load, and the GEMINI Wind farm (EOS) 450 MW power regulationNorNed Connectionis exclusively (EEM) performed by reducing the electrolyzer 700 MW ’s active power demand.COBRAcable The output Connection of VSP is directly (EOS) given to the active power−700 reference MW input, as in the case of droop andGEN1 combined (EOS) droop derivative controller. Additionally, 3 × 430 MW please note that the droop controllerGEN2 is (EOS)always active in combination with the 2 derivative× 430 MW and VSP for the entire time. The GEN3time duration (EOS) and active power reduction applied 233 MW by the derivative and VSP controller are different.Total 3490 MW

Electronics 2021, 10, x FOR PEER REVIEW 8 of 11

Figure 8. Shape of the power output of the PEM electrolyzer duedue toto thethe actionaction ofof FAPR.FAPR.

FigureFigure 9.9. Shape of the system frequency response duedue toto the action of FAPR onon the PEM electrolyzer.electrolyzer.

5.2. Frequency Support Through VSP-Based FAPR Implemented in the Solar PV Farm A 300 MW solar PV farm integrated with BESS and FAPR is taken into account. In this case, with a 200 MW sudden loss of Gemini wind generation, the solar PV farm provides support of a 10% increase in active power, i.e., 30 MW during 10 s. More support, in terms of higher active power injection and time of operation, is limited by the ratings of the BESS and grid-side converter. Figure 10 depicts the improvement in frequency due support of the solar PV farm.

Figure 10. Frequency response due to the support of a solar PV farm with VSP-based FAPR.

5.3. Deployment of FAPR by the Different Controllable Sources within the Hub Figure 11 shows that a higher degree of improvement in RoCoF and Nadir is achieved when FAPR acts in all controllable active power sources within the hub. Note also in Figure 11 that the frequency stabilizes at a relatively lower value when there is an uneven share of the fast active power adjustment. As shown in [12], this issue can be solved by performing an optimal and coordinated tuning of the FAPR strategies acting on the controllable devices. This aspect is also relevant for the design of an optimal transition between primary and secondary frequency control.

Electronics 2021, 10, x FOR PEER REVIEW 8 of 11

Electronics 2021, 10, 1651 8 of 10 Figure 9. Shape of the system frequency response due to the action of FAPR on the PEM electrolyzer.

5.2. Frequency Support Through VSP-Based FAPR Implemented in the Solar PV Farm 5.2. Frequency Support through VSP-Based FAPR Implemented in the Solar PV Farm A 300 MW solar PV farm integrated with BESS and FAPR is taken into account. In A 300 MW solar PV farm integrated with BESS and FAPR is taken into account. In this this case, with a 200 MW sudden loss of Gemini wind generation, the solar PV farm case, with a 200 MW sudden loss of Gemini wind generation, the solar PV farm provides provides support of a 10% increase in active power, i.e., 30 MW during 10 s. More support, support of a 10% increase in active power, i.e., 30 MW during 10 s. More support, in terms in terms of higher active power injection and time of operation, is limited by the ratings of higher active power injection and time of operation, is limited by the ratings of the BESS of the BESS and grid-side converter. Figure 10 depicts the improvement in frequency due and grid-side converter. Figure 10 depicts the improvement in frequency due support of support of the solar PV farm. the solar PV farm.

Figure 10. Frequency response duedue toto thethe supportsupport ofof aa solarsolar PVPV farmfarm withwith VSP-basedVSP-based FAPR.FAPR.

5.3. Deployment of FAPR byby thethe DifferentDifferent ControllableControllable SourcesSources withinwithin thethe HubHub Figure 11 11 shows shows that that a higher a higher degree degree of improvement of improvement in RoCoF in andRoCoF Nadir and is achieved Nadir is whenachieved FAPR when acts FAPR in all acts controllable in all controllable active power active sources power withinsources the within hub. the Note hub. also Note in Figurealso in 11Figure that the11 that frequency the frequency stabilizes stabilizes at a relatively at a relatively lower value lower when value there when is anthere uneven is an shareuneven of share the fast of activethe fast power active adjustment. power adjustment. As shown As in shown [12], in this [12], issue this can issue be solved can be bysolved performing by performing an optimal an optimal and coordinated and coordinated tuning tuning of the of FAPR the FAPR strategies strategies acting acting on the on Electronics 2021, 10, x FOR PEER REVIEW 9 of 11 controllablethe controllable devices. devices. This This aspect aspect is alsois also relevant relevan fort for the the design design of of an an optimal optimal transitiontransition between primary andand secondarysecondary frequencyfrequency control.control.

FigureFigure 11.11. Frequency comparisoncomparison plotsplots inin renewablerenewable energyenergy systemsystem conclusions.conclusions.

6. Conclusions In this paper, the value of FAPR strategies for fast, active power–frequency support by renewable energy hubs was examined. Droop-based FAPR, droop derivative-based FAPR, and VSP-based FAPR were implemented and tested by performing a real-time EMT simulation on a hub comprising a solar PV farm and a PEM electrolyzer. The simulations conducted on a futuristic situation (year 2030) performed in a test system of the transmission network covering the north of the Netherlands showed that VSP-based FAPR is the most attractive option to quickly adjust the active power output of the solar PV farm and the PEM electrolyzer, resulting in the significant improvement of the RoCoF and the maximum frequency deviation. The simultaneous deployment of VSP-FAPR in each controllable active power source within the hub should be optimally tuned and coordinated to achieve a desired steady state of the frequency response after the occurrence of the maximum frequency deviation. Future studies shall investigate this aspect, as well as the design of an optimal transition between primary and secondary frequency control. The influence of emerging methods for data exchange and dynamic state estimation, e.g., [17,18], on the optimal deployment of FAPR shall also be investigated.

Author Contributions: Conceptualization, J.R.T., E.R., Z.A., N.V.K.; formal analysis, J.R.T., N.V.K., E.R., Z.A.; investigation, N.V.K., E.R., Z.A., J.R.T.; methodology, N.V.K., E.R., Z.A., J.R.T.; resources, N.V.K., J.R.T., E.A.; software, N.V.K., E.R., Z.A., J.R.T.; supervision, J.R.T., E.R., P.P. and M.v.d.M.; validation, N.V.K., E.R., J.R.T., Z.A.; visualization, J.R.T., E.R. Z.A. and N.V.K.; writing—original draft, N.V.K., J.R.T., E.R.; writing—review and editing, J.R.T., N.V.K., Z.A., E.R., P.P. and M.v.d.M. All authors have read and agreed to the published version of the manuscript. Funding: This research work has received funding from the European Union’s Connecting Europe Facility (CEF) program under the grant agreement No INEA/CEF/SYN/A2016/1336043–TSO2020 Project (Electric “Transmission and Storage Options” along TEN-E and TEN-T corridors for 2020). This paper reflects only the authors’ views, and the European Commission is not responsible for any use that may be made of the information it contains. The APC was funded by Delft University of Technology. Acknowledgments: This work has received funding from the European Union’s Connecting Europe Facility (CEF) program under the grant agreement No INEA/CEF/SYN/A2016/1336043–TSO2020 Project (Electric “Transmission and Storage Options” along TEN-E and TEN-T corridors for 2020). This paper reflects only the authors’ views, and the European Commission is not responsible for any use that may be made of the information it contains.

Electronics 2021, 10, 1651 9 of 10

6. Conclusions In this paper, the value of FAPR strategies for fast, active power–frequency support by renewable energy hubs was examined. Droop-based FAPR, droop derivative-based FAPR, and VSP-based FAPR were implemented and tested by performing a real-time EMT simulation on a hub comprising a solar PV farm and a PEM electrolyzer. The simulations conducted on a futuristic situation (year 2030) performed in a test system of the transmission network covering the north of the Netherlands showed that VSP- based FAPR is the most attractive option to quickly adjust the active power output of the solar PV farm and the PEM electrolyzer, resulting in the signiﬁcant improvement of the RoCoF and the maximum frequency deviation. The simultaneous deployment of VSP-FAPR in each controllable active power source within the hub should be optimally tuned and coordinated to achieve a desired steady state of the frequency response after the occurrence of the maximum frequency deviation. Future studies shall investigate this aspect, as well as the design of an optimal transition between primary and secondary frequency control. The inﬂuence of emerging methods for data exchange and dynamic state estimation, e.g., [17,18], on the optimal deployment of FAPR shall also be investigated.

Author Contributions: Conceptualization, J.R.T., E.R., Z.A., N.V.K.; formal analysis, J.R.T., N.V.K., E.R., Z.A.; investigation, N.V.K., E.R., Z.A., J.R.T.; methodology, N.V.K., E.R., Z.A., J.R.T.; resources, N.V.K., J.R.T., E.A.; software, N.V.K., E.R., Z.A., J.R.T.; supervision, J.R.T., E.R., P.P. and M.v.d.M.; validation, N.V.K., E.R., J.R.T., Z.A.; visualization, J.R.T., E.R., Z.A. and N.V.K.; writing—original draft, N.V.K., J.R.T., E.R.; writing—review and editing, J.R.T., N.V.K., Z.A., E.R., P.P. and M.v.d.M. All authors have read and agreed to the published version of the manuscript. Funding: This research work has received funding from the European Union’s Connecting Europe Facility (CEF) program under the grant agreement No INEA/CEF/SYN/A2016/1336043–TSO2020 Project (Electric “Transmission and Storage Options” along TEN-E and TEN-T corridors for 2020). This paper reflects only the authors’ views, and the European Commission is not responsible for any use that may be made of the information it contains. The APC was funded by Delft University of Technology. Acknowledgments: This work has received funding from the European Union’s Connecting Europe Facility (CEF) program under the grant agreement No INEA/CEF/SYN/A2016/1336043–TSO2020 Project (Electric “Transmission and Storage Options” along TEN-E and TEN-T corridors for 2020). This paper reflects only the authors’ views, and the European Commission is not responsible for any use that may be made of the information it contains. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Nomenclature

RoCoF Rate of Change of Frequency BPMS Battery Power Management System BESS Battery Energy Storage System AGC Automatic Generation Control

References 1. Rakhshani, E.; Gusain, D.; Sewdien, V.; Torres, J.L.R.; van der Meijden, M.A.M.M. A Key Performance Indicator to Assess the Frequency Stability of Wind Generation Dominated Power System. IEEE Access 2019, 7, 130957–130969. [CrossRef] 2. Kundur, P. Power System Stability and Control; McGraw-Hill: New York NY, USA, 1994. 3. Hatziargyriou, N.; Milanovic, J.; Rahmann, C.; Ajjarapu, V.; Canizares, C.; Erlich, I.; Hill, D.; Hiskens, I.; Kamwa, I.; Pal, B.; et al. Deﬁnition and Classiﬁcation of Power System Stability—Revisited & Extended. IEEE Trans. Power Syst. 2021, 36, 3271–3281. [CrossRef] 4. Hong, Q.; Khan, M.A.U.; Henderson, C.; Egea-Àlvarez, A.; Tzelepis, D.; Booth, C. Addressing Frequency Control Challenges in Future Low-Inertia Power Systems: A Great Britain Perspective. Engineering 2021, in press. [CrossRef] Electronics 2021, 10, 1651 10 of 10

5. Rakhshani, E.; Perilla, A.; Veerakumar, N.; Ahmad, Z.; Torres, J.L.R.; van der Meijden, M.A.M.M. Analysis and tuning methodology of fapi controllers for maximising the share of grid-connected wind generations. IET Renew. Power Gener. 2020, 14, 3816–3823. [CrossRef] 6. Sanchez, F.; Gonzalez-Longatt, F.; Torres, J.L.R. Multi-objective optimal provision of fast frequency response from EV clusters. IET Gener. Transm. Distrib. 2020, 14, 5580–5587. [CrossRef] 7. Rakhshani, E.; Rouzbehi, K.; Sánchez, A.J.; Tobar, A.C.; Pouresmaeil, E. Integration of Large Scale PV-Based Generation into Power Systems: A Survey. Energy 2019, 12, 1425. [CrossRef] 8. Low, F.; Power, I.; Using, S.; Technique, C. Primary Frequency Response Enhancement for Future Low Inertia Power Systems Using Hybrid Control Technique. Energies 2018, 11, 699. [CrossRef] 9. Rakhshani, E.; Remon, D.; Rodriguez, P. Effects of PLL and Frequency Measurements on LFC Problem in Multi-Area HVDC Interconnected Systems. Int. J. Electr. Power Energy. Syst. 2016, 81, 140–152. [CrossRef] 10. Tuinema, B.W.; Adabi, E.; Ayivor, P.K.S.; Suárez, V.G.; Liu, L.; Perilla, A.; Ahmad, Z.; Torres, J.L.R.; van der Meijden, M.A.M.M.; Palensky, P. Modelling of large-sized electrolysers for real-time simulation and study of the possibility of frequency support by electrolysers. IET Gener. Transm. Distrib. 2020, 14, 1985–1992. [CrossRef] 11. Dreidy, M.; Mokhlis, H.; Mekhilef, S. Inertia response and frequency control techniques for renewable energy sources: A review. Renew. Sustain. Energy Rev. 2017, 69, 144. [CrossRef] 12. Zhang, C.; Rakhshani, E.; Veerakumar, N.; Torres, J.L.R.; Palensky, P. Modeling and Optimal Tuning of Hybrid ESS Supporting Fast Active Power Regulation of Fully Decoupled Wind Power Generators. IEEE Access 2021, 9, 46409–46421. [CrossRef] 13. Torres, J.L.R.; Tuinema, B.W.; Adabi, M.E.; Ahmad, Z.; Suárez, V.G.; Ayivor, P.K.S.; Kumar, N.V.; Liu, L.; Perilla, A.; Alshehri, F.A.; et al. TSO2020 Activity 2—Final Report: Stability Analysis of an International Electricity System connected to Regional and Local Sustainable Gas Systems; TSO2020 Project (Electric “Transmission and Storage Options” along TEN-E and TEN-T Corridors for 2020); December 2019. Available online: http://tso2020.eu/wp-content/uploads/2020/01/TSO2020_Final_Report_TUD.pdf (accessed on 11 July 2021). 14. Veerakumar, N.; Ahmad, Z.; Adabi, M.E.; Torres, J.R.; Palensky, P.; van der Meijden, M.; Gonzalez-Longatt, F. Fast Active Power-Frequency Support Methods by Large Scale Electrolyzers for Multi-Energy Systems. In Proceedings of the 2020 IEEE PES Innovative Smart Grid Technologies Europe (ISGT-Europe), The Hague, The Netherlands, 26–28 October 2020; pp. 151–155. [CrossRef] 15. Torres, J.L.R.; Ahmad, Z.; Kumar, N.V.; Rakhshani, E.; Adabi, E.; Palensky, P.; van der Meijden, M.A.M.M. Power Hardware-in- the-Loop-Based Performance Analysis of Different Converter Controllers for Fast Active Power Regulation in Low-Inertia Power Systems. Energies 2021, 14, 3274. [CrossRef] 16. Robert, C. Review of International Grid Codes; Lawrence Berkeley National Laboratory: Berkeley, CA, USA, 2018. 17. Noor-A-Rahim, M.; Khyam, M.O.; Li, X.; Pesch, D. Sensor Fusion and State Estimation of IoT Enabled Wind Energy Conversion System. Sensors 2019, 19, 1566. [CrossRef][PubMed] 18. Enescu, F.M.; Ionescu, V.M.; Marinescu, C.N.; ¸Stirbu,C. System for monitoring and controlling renewable energy sources. In Proceedings of the 2017 9th International Conference on Electronics, Computers and Artiﬁcial Intelligence (ECAI), Targoviste, Romania, 29 June–1 July 2017; pp. 1–6. [CrossRef]