energies Article Article Large Scale Integration: LargeIssues Scale Renewable and Solutions Energy Integration: Issues and Solutions G V Brahmendra Kumar 1, Ratnam Kamala Sarojini 1, K Palanisamy 1,*, Padmanaban G. V. BrahmendraSanjeevikumar Kumar 12 and, Ratnam Jens Bo Kamala Holm-Nielsen Sarojini 21 , K. Palanisamy 1,*, 2, 2 Padmanaban1 Sanjeevikumar School of Electrical *Engineering,and Jens Vellore Bo Holm-Nielsen Institute of Technology, Vellore 632014, India; 1 School of [email protected] Engineering, Vellore Institute (G.V.B.K.); of Technology, [email protected] Vellore 632014, India; (R.K.S.); [email protected] (K.P.) [email protected] Center for Bioenergy and (G.V.B.K.); Green Engineering, [email protected] Department of (R.K.S.) Energy Technology, Aalborg University, 2 Center for6700 Bioenergy Esbjerg, and Denmark; Green Engineering, [email protected] Department (P.S.); [email protected] of Energy Technology, (J.B.H.-N.) Aalborg University, 6700 Esbjerg,* Denmark; Correspondence: [email protected] [email protected]; [email protected]; Tel. No.: +45-7168-2084 * Correspondence: [email protected] (K.P.); [email protected] (P.S.) Received: 16 April 2019; Accepted: 21 May 2019; Published: date  Received: 16 April 2019; Accepted: 21 May 2019; Published: 24 May 2019  Abstract: In recent years, many applications have been developed for the integration of renewable Abstract: Inenergy recent sources years, many (RES) applications into the grid have in beenorder developed to satisfy forthe the demand integration requirement of renewable of a clean and energy sourcesreliable (RES) electricity into the generation. grid in order Increasing to satisfy the demandnumber of requirement RES creates of uncertainty a clean and in reliable load and power electricity generation.supply generation, Increasing which the also number presents of RES an createsadditional uncertainty strain on in the load system. and power These supplyuncertainties will generation,affect which the also voltage presents and an frequency additional variation, strain on stability, the system. protection, These uncertainties and safety issues will a ffatect fault the levels. RES voltage andpresent frequency non-linear variation, characteristics, stability, protection, which requires and safety effective issues atcoordination fault levels. REScontrol present methods. This non-linear characteristics,paper presents whichthe stability requires issues effective and coordinationsolutions associated control methods. with the Thisintegration paper presents of RES within the the stabilitygrid. issues and solutions associated with the integration of RES within the grid.

Keywords:Keywords:renewable renewable energy sources; energy energy sources; storage energy system; storage voltage; system; frequency; voltage; frequency; grid integration grid integration

1. Introduction1. Introduction The majorityThe of majority countries of acrosscountries the across world the are world concentrating are concentrating on RES, due on toRES, an due increase to an of increase of environmentalenvironmental concerns and concerns depletion and ofdepletion fossil fuels of fossil [1]. Forfuel example,s [1]. For Chinaexample, is aiming China foris aiming RES to for RES to represent somewhererepresent somewhere around 35 percent around of 35 consumption percent of byconsum 2030.ption India hasby 2030. set an India ambitious has set RES an target ambitious RES of 175 GWtarget [2]. The of European175 GW [2]. Union The European and the United Union States and the also Un haveited targets States also regarding have targets RES [3 ].regarding Some RES [3]. countries successfullySome countries integrated successfully large sharesintegrated of RES large in theshares power of gridRES inin 2017,the power and most grid countries in 2017, and most have set targetscountries to receive have set their targets power to generation receive their from power RES generation by 80% in 2050,from asRES is shownby 80% in in Figure 2050, 1as. is shown in Figure 1.

Figure 1. Renewable energy share and targets by countries in 2017 and 2050 [4]. Energies 2019, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/energies

Energies 2019, 12, 1996; doi:10.3390/en12101996 www.mdpi.com/journal/energies Energies 2019, 12, x FOR PEER REVIEW 2 of 17 Energies 2019, 12, 1996 2 of 17 Figure 1. Renewable energy share and targets by countries in 2017 and 2050 [4].

Due Table to 1. the The irregularities comparison of and renewable fluctuating energy features sources (R ofES) RES, with new synchronous challenges generator to the (SG) grid [5]. over a unified anticipated generation have been created. The RES outputs are influenced by meteorological Characteristics Solar Wind SG conditions. These characteristics affect the power system efficacy, power quality, system reliability, load Fluctuations More Less No management, security, and safety in various ways and are also very significant factors to be viewed in Cost for large scale More Medium Low to medium the integration of RES within the grid [6]. This influence is commonly observed with solar and wind Maintenance cost Minimum More Moderate energy, but geothermal, hydropower and energy resources are more anticipated and have Inertia No inertia Low inertia High irrelevant difficulties in their association with the grid [7]. The comparison of RES with a synchronous Very Low Low to medium More generator (SG) is shown in Table1. Annual growth in power industry Very High More More

Table 1. The comparison of renewable energy sources (RES) with synchronous generator (SG) [5]. Due to the irregularities and fluctuating features of RES, new challenges to the grid over a unified anticipatedCharacteristics generation have been created. Solar The RES outputs Wind are influenced SG by meteorological conditions. These characteristicsFluctuations affect the power More system efficacy, Less power quality, No system reliability, ,Cost security, for large scaleand safety in various More ways and Medium are also very Lowsignificant to medium factors to be viewed in the integrationMaintenance of RES cost within the grid Minimum [6]. This influence More is commonly Moderate observed with solar Inertia No inertia Low inertia High and wind energy, but geothermal, hydropower and biomass energy resources are more anticipated Capacity factor Very Low Low to medium More and have Annualirrelevant growth difficulties in power in industry their associationVery High with the grid More [7]. The comparison More of RES with a synchronous generator (SG) is shown in Table 1.

The RESRES integratedintegrated with with the the grid grid is is shown shown in in Figure Figu2re. Figure2. Figure2 is incorporated2 is incorporated with with RES, RES, grid andgrid electronicand electronic components. components. The power The power generation generation from RES from to RES the gridto the is unidirectional.grid is unidirectional. The RES The requires RES therequires converters the converters to be connected to be connected to the grid. to Thesethe grid converters. These converters will achieve will effi achievecient operation efficient and operation obtain theand energy obtain quality the energy requirements quality relatedrequirements to the harmonicsrelated to level.the harmonics Also, this allowslevel. theAlso, integration this allows of RESthe inintegration a power inverterof RES in with a power high energyinverter features with high and energy safety. features and safety.

Figure 2. RES integration with grid.

The remainderremainder of of this this paper paper is organizedis organized as follows:as follows: Section Section2 considers 2 considers the grid the integration grid integration issues withissues a with high sharea high of share RES, of and RES, solutions and solutions for variable for variable RES is discussedRES is discussed in Section in3 Section. The Energy 3. The Storage Energy SystemStorage (ESS)System support (ESS) issupport explained is inexplained Section4 ,in and Section followed 4, and by smartfollowed grid featuresby smart in grid voltage features control in withvoltage RES control in Section with5 .RES Section in Section6 is the 5. conclusion. Section 6 is the conclusion. 2. Grid Integration Issues with a High Share of RES 2. Grid Integration Issues with a High Share of RES As the power generation from RES increases, the installed capacity of power converters increases. As the power generation from RES increases, the installed capacity of power converters By utilizing the large scale of solar energy, the RES system is able to supply power to the grid. Thus, increases. By utilizing the large scale of solar energy, the RES system is able to supply power to the by increasing the capacity of the RES, the grid-connected RES will create a negative impact on the grid. Thus, by increasing the capacity of the RES, the grid-connected RES will create a negative grid when fault or disturbance occurs. Hence, the Point of Common Coupling (PCC) voltage drop is impact on the grid when fault or disturbance occurs. Hence, the Point of Common Coupling (PCC) triggered by a fault in grid power and the RES will become off-grid across a wide area range. Moreover, voltage drop is triggered by a fault in grid power and the RES will become off-grid across a wide these fault impacts cause the grid voltage and frequency collapse, also affecting the safe, stable and area range. Moreover, these fault impacts cause the grid voltage and frequency collapse, also reliable operation of the grid and even triggering large economic losses [8]. affecting the safe, stable and reliable operation of the grid and even triggering large economic losses [8].

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2.1. Impact of Large-Scale Integration of RES on Frequency The frequency of a power system must be preservedpreserved near to its nominal value (either 50 Hz or 60 Hz based on the grid). The The frequency frequency deviations will only arise when there is a mismatch between generation and load. A A stiff stiff power system preser preservesves the frequency subsequent to a contingency event [[9].9]. The frequency of the power system is maintained at the nominal value only when the active power of generation and demand is balanced as indicated in Figure3 3.. IfIf thethe demanddemand isis moremore thanthan thethe generation, thenthen thethe frequencyfrequency decreases decreases from from the the nominal nominal value. value. In In the the case case of surplusof surplus generation, generation, the systemthe system frequency frequency increases. increases. The kineticThe kinetic energy energy (KE) stored(KE) stored in the rotorin the of rotor the rotatingof the rotating machines machines present inpresent the power in the system power contribute system contribute to inertia. to The inerti inertiaa. The in the inertia power in systemthe power regulates system the regulates frequency the in demandfrequency generation in demand imbalances. generation Ifimbalances. the inertia If is the more iner intia the is powermore in system, the power then system, it is less then sensitive it is less to smallsensitive power to small imbalances. power imbalances. The inertia in The the inertia power in system the power provides system energy provides for some energy time for to some reduce time the frequencyto reduce the deviations. frequency deviations.

Figure 3. Power balance in a power system network.

In order to balance the power generation and de demand,mand, several control techniques are employed in a power system in multiple time frames as shownshown in Figure4 4.. TheThe frequencyfrequency responseresponse ofof aa powerpower system is comprised of the following steps [[10]:10]:

1. InertialInertial response response (a (a few few seconds). seconds). 2. PrimaryPrimary frequency frequency response—G response—Governorovernor Response (1–10 s).

3. SecondarySecondary frequency frequency response—Area response—Area Governor Control response (seconds to minutes). When the power imbalance occurs, the frequency starts to fall. The Rate of Change of Frequency (ROCOF)When is the more power at the imbalance initial stage. occurs, ROCOF the frequency is associated starts with to fall. how The much Rate ofinertia Change is present of Frequency in the (ROCOF)system; the is moreinertia at theslows initial down stage. the ROCOFinitial frequenc is associatedy deviation, with how and much this inertia is called is present the inertial in the system;frequency the response. inertia slows The downgovernor the initialresponse frequency is the deviation,primary frequency and this istechnique called the which inertial acts frequency within response.the first few The seconds governor (typically response 10–30 is the s) primary after a fr frequencyequency event technique and whichseeks to acts reduce within the the frequency first few secondsdeviation. (typically As the 10–30frequency s) after reaches a frequency a minimum event and value seeks (the to reducefrequency the frequencynadir); then deviation. the governor As the frequencycontrol is activated. reaches a Hence, minimum the valueactive (the power frequency output nadir);increases then and the the governor frequency control settles is at activated. a point Hence,slightly the below active the power nominal output value increases [11]. and the frequency settles at a point slightly below the nominal valueAs [11 the]. penetration level of RES increases, the frequency deviations are more frequent. As these RES areAs theconnected penetration to the level grid of through RES increases, a power the electr frequencyonic inverter, deviations substituting are more the frequent. conventional As these SG RESwith arepower connected electronic to the inverters grid through will adecrease power electronic the inertia inverter, of the substituting power system. the conventional To handle the SG frequency stability issues raised from the low inertia and reserve power, new frequency control techniques need to be employed for RES to participate in a frequency regulation process.

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with power electronic inverters will decrease the inertia of the power system. To handle the frequency stability issues raised from the low inertia and reserve power, new frequency control techniques need Energiesto be employed 2019, 12, x FOR for PEER RES toREVIEW participate in a frequency regulation process. 4 of 17

FigureFigure 4. 4. FrequencyFrequency response response in in a a power power system system network. network. 2.2. Voltage Rise and Fluctuation 2.2. Voltage Rise and Fluctuation It is considered that electricity is distributed at the consumer’s terminal within the tolerable limit. It is considered that electricity is distributed at the consumer’s terminal within the tolerable The normal allowable voltage range is 6% of the nominal value [12]. When large RES is integrated limit. The normal allowable voltage range± is ±6% of the nominal value [12]. When large RES is with lightly loaded feeders, the impact is unbearable. Whenever there is any change in load, the voltage integrated with lightly loaded feeders, the impact is unbearable. Whenever there is any change in fluctuation occurs at PCC. The voltage fluctuations and dips occur due to earth leakage faults and earth load, the voltage fluctuation occurs at PCC. The voltage fluctuations and dips occur due to earth short-circuits located in the electrical power system (EPS). These faults weaken the voltage quality at leakage faults and earth short-circuits located in the electrical power system (EPS). These faults PCC, based on fault situations. This is a significant element, particularly for solar and wind energy weaken the voltage quality at PCC, based on fault situations. This is a significant element, sources which possess irregular characteristics due to the wind speed disparity and solar irradiance particularly for solar and wind energy sources which possess irregular characteristics due to the that changes with time. The sensitivity of both electrical and electronic appliances that contributes to wind speed disparity and solar irradiance that changes with time. The sensitivity of both electrical the life span deficiency of maximum devices is due to the voltage deviation [13]. and electronic appliances that contributes to the life span deficiency of maximum devices is due to theImpact voltage of RES deviation on Voltage [13]. Drops in the Grid

ImpactVoltage of RES is on a significantVoltage Drops factor in inthe EPS. Grid Hence, voltage control requirements are essential in EPS for both transmission and distribution levels. The source voltage and voltage drop at the feeder is Voltage is a significant factor in EPS. Hence, voltage control requirements are essential in EPS determined at the end of the feeder [14]. The voltage drop in the feeder is due to the conductor for both transmission and distribution levels. The source voltage and voltage drop at the feeder is impedance, current flow, and load. Also, the drop should not be lower when in a peak load situation, determined at the end of the feeder [14]. The voltage drop in the feeder is due to the conductor and it should not be more than the maximum voltage in a light load condition. impedance, current flow, and load. Also, the drop should not be lower when in a peak load situation, and it should not be more than theR maximumLN(PL PG voltage) + XLN (inQ aL lightQG )load condition. ∆V = V1 V2 = − − (1) − R ()(PP−+V XQQ2 − ) Δ= − =LN L G LN L G VVV12 (1) Equation (1) shows the injection of reactive powerV2 from RES within the grid; the RES will continually diminish the drop at the feeder. Where ∆V is the voltage between bus 1 & 2, P and Q are Equation (1) shows the injection of reactive power from RES within the grid; the RES will active and reactive power, and RLN & XLN are the line resistance and reactance, respectively. The grid continuallyis incorporated diminish with the RES drop and at the the load feeder. is shown Where in Δ FigureV is the5. voltage If the supply between power bus 1 is & lower 2, P and than Q theare activedemand, and the reactive RES will power, inject and power RLN & into XLN the are grid. the line Hence, resistance this process and reactance, is subject respectively. to RES active The/reactive grid ispower incorporated that is relevant with RES to the and load theactive load /isreactive shown power in Figure and 5. line If the X/R supply ratio [15 power]. is lower than the demand, the RES will inject power into the grid. Hence, this process is subject to RES active/reactive power that is relevant to the load active/reactive power and line X/R ratio [15].

Energies 2019, 12, x FOR PEER REVIEW 5 of 17 Energies 2019, 12, 1996 5 of 17 Energies 2019, 12, x FOR PEER REVIEW 5 of 17

Figure 5. Grid connected system with RES and load.

Figure 5. Grid connected system with RES and load. 3. Solutions for Variable RESFigure 5. Grid connected system with RES and load. 3. Solutions for Variable RES 3. SolutionsVarious forcontrol Variable techniques RES need to be employed to increase the power generation from RES and toVarious decrease control the negative techniques impact need of to the be RES employed on the topower increase grid. the This power section generation gives a brief from overview RES and toof decreasetheVarious frequency the control negative and voltagetechniques impact control of need the techniques RESto be on employed the for power RES. to grid. increase This sectionthe power gives generation a brief overview from RES of theand frequency to decrease and the voltage negative control impact techniques of the RES for on RES. the power grid. This section gives a brief overview 3.1.of the Frequency frequency Control and voltageTechniques control for RES techniques for RES. 3.1. Frequency Control Techniques for RES 3.1. FrequencyThe frequency Control control Techniques techniques for RES employed for RES are shown in Figure 6. The frequency controlThe techniques frequency used control for techniques both wind employed and solar for are RES discussed are shown in this in Figure section.6. TheFor frequencyRES-based control power techniquesplants,The the frequency frequency used for bothcontrol can windbe controlledtechniques and solar by employed are reservin discussedg fo ther inRESactive this are section.power shown by For inusing RES-basedFigure the de-load6. The power frequencyoperation plants, theorcontrol by frequency using techniques the can energy be used controlled storage for both system by wind reserving and(ESS). solar theGe nerally, activeare discussed power Inertia byin and this using frequency section. the de-load For control RES-based operation methods power or for by usingRESplants, are the the categorized energy frequency storage in can two systembe ways:controlled (ESS). control Generally,by reservintechniques Inertiag the to active andde-load frequency power the byRES, control using and the methodscontrol de-load practices for operation RES arefor categorizedRESor by with using ESS. inthe two energy ways: storage control system techniques (ESS). to de-loadGenerally, the RES,Inertia and and control frequency practices control for RES methods with ESS. for RES are categorized in two ways: control techniques to de-load the RES, and control practices for RES with ESS.

Figure 6. Frequency control techniques for RES. Figure 6. Frequency control techniques for RES. 3.1.1. Control techniques UsedFigure for Wind 6. Frequency TurbinesTurbines control techniques for RES. The frequency control techniques for the can be classified in three ways, i.e., 1. 3.1.1.The Control frequency techniques control Used techniques for Wind for Turbines the wind turbine can be classified in three ways, i.e., 1. Inertia control,control, 2.2. Droop control, 3.3. De-loading control. The frequency control techniques for the wind turbine can be classified in three ways, i.e., 1. 3.1.2. Inertia Control 3.1.2.Inertia Inertia control, Control 2. Droop control, 3. De-loading control. The inertia of the wind turbines needs to be emulatedemulated to maintain the frequency stability under the3.1.2. highhigh Inertia penetrationpenetration Control ofof windwind powerpower generators.generators. The inertia of the wind turbines can be emulated by eitherThe “hidden” inertia inertiaof the wind emulationemulation turbines oror byby needs thethe fastfastto be powerpower emulated reservereserve to maintain emulation.emulation. the frequency stability under the high penetration of generators. The inertia of the wind turbines can be emulated by (a) Hidden inertia emulation (a)either Hidden “hidden” inertia inertia emulation emulation or by the fast power reserve emulation. The emulation of hidden inertia present in the wind turbines is used to reduce the frequency The emulation of hidden inertia present in the wind turbines is used to reduce the frequency deviation(a) Hidden and inertia to maintain emulation the frequency stability. The suitable control algorithm for the power deviation and to maintain the frequency stability. The suitable control algorithm for the power electronic converter for the wind turbines allows the wind turbine to release the KE stored in the electronicThe emulation converter offor hidden the wind inertia turbines present allows in the the wind wind turbines turbine isto used release to reducethe KE thestored frequency in the rotating blades. The KE stored in the blades of the wind turbine helps to regulate the frequency under rotatingdeviation blades. and to The maintain KE stored the infrequency the blades stability. of the Thewind suitable turbine control helps toalgorithm regulate forthe thefrequency power unbalance condition through its inertial response [16]. There are two ways to emulate the inertial underelectronic unbalance converter condition for the throughwind turbines its inertial allows re sponsethe wind [16]. turbine There to are release two waysthe KE to storedemulate in thethe rotating blades. The KE stored in the blades of the wind turbine helps to regulate the frequency under unbalance condition through its inertial response [16]. There are two ways to emulate the

Energies 2019, 12, x FOR PEER REVIEW 6 of 17

Energies 2019, 12, 1996 6 of 17 inertialEnergies 2019 response,, 12, x FOR either PEER REVIEWby considering the ROCOF alone or by incorporating both ROCOF6 ofand 17 frequency deviation. Figure 7 shows the control diagram for the inertia emulation, considering only ROCOFresponse,inertial response,to eitherrelease by theeither considering KE by stored considering thein the ROCOF wind the aloneturbine.ROCOF or The byalone incorporating inertia or by constant incorporating both (H) ROCOF is used both andto ROCOF express frequency andthe inertialdeviation.frequency characteristics. deviation. Figure7 shows Figure The the“hidden” 7 shows control inertiathe diagram control of wind for diag the turbinesram inertia for thecan emulation, inertiabe known emulation, considering as, considering only ROCOF only ROCOF to release the KE stored in the wind turbine. The inertia constant (H) is used to express the to release the KE stored in the wind turbine. TheJ inertiaω 2 constant (H) is used to express the inertial characteristics.inertial characteristics. The “hidden” The “hidden” inertia ofinertia wind Hof turbines wind= nomturbines can be known can be as,known as, (2) 2S 2 ω2 Jω nom where J the inertia of the wind turbine is, SH is== the nomVA rating of the machine, ω is the nominal(2)(2) 2SS nom angular frequency. J S ω where Jthethe inertia inertia of of the the wind wind turbine turbine is, is,is S the is VA the rating VA rating of the of machine, the machine,ωnom is thenom nominal is the nominal angular frequency.angular frequency.

Figure 7. Hidden inertia emulation control [17].

The inertial active powerFigure control 7. Hidden signal inertia Pinertia emulation of the control hidden [17]. [17 ].inertial emulation control is known as, The inertial active power control signal PinertiaPof the hidden inertial emulation control is known as, The inertial active power control signal inertia of theω hidden inertial emulation control is d sys known as, PH= 2*ω * (3) inertia sys dωdtsys Pinertia = 2H ωsys ω (3) ∗ ∗ ddt sys ω PH= 2*ω * (3) where sys the angular frequency of theinertia system is, r sys is the reference angular frequency. where ωsys the angular frequency of the system is, ωr is the referencedt angular frequency. TheThe inertia inertia control control algorithm algorithm with with both both ROCOF ROCOF and and frequency frequency deviation deviation is is shown shown in in Figure Figure 8.8. where ω the angular frequency of the system is, ω is the reference angular frequency. InIn the the power powersys balanced balanced condition, condition, the the active active set set powerr is is monitored by the MPPT controller. Under powerpowerThe imbalanceimbalance/frequency inertia control/frequency algorithm disturbancedisturbance with both conditions conditions ROCOF the theand control controlfrequency algorithm algorithm deviation acts acts and is shownand produces produces in Figure the extra the 8. extrainertialIn the inertial power power powerbalanced signal. signal. In condition, this In controller, this thecontroller, active the inertial set the power inertial power is ispowermonitored calculated is calculated by from the both MPPT from the controller.both ROCOF the loopROCOF Under and loopfrequencypower and imbalance/frequency frequency deviation deviation loop. The disturbanceloop. inertial The power inertial conditions calculated power the calculated from control the Figurefromalgorithm the8 is Figure known acts 8and as,is known produces as, the extra inertial power signal. In this controller, thedf inertial power is calculated from both the ROCOF loop and frequency deviation loop. The inertial=+Δ podwer f calculated from the Figure 8 is known as, PinertiaP =KKfK I+ K droop∆ f (4)(4) inertia I dt droop =+Δdf where KKand K K are the inertia andPinertia droopingKKf I gains respectively. droop (4) where I I anddroop droop are the inertia and droopingdt gains respectively.

where K I and Kdroop are the inertia and drooping gains respectively.

FigureFigure 8. 8. InertiaInertia emulation emulation control control with with droop droop control control [18]. [18].

Figure 8. Inertia emulation control with droop control [18].

Energies 2019, 12, x FOR PEER REVIEW 7 of 17

Energies 2019, 12, 1996 7 of 17 EnergiesThe 2019 wind, 12, x FORgenerators PEER REVIEW can store and release KE instantly compared to the SG, due to the 7power of 17 electronic converter controller. The variable speed wind turbines can participate more in the frequencyTheThe wind wind regulation generators generators by releasing can can store store more an andd KErelease release than KE KEfixed instantly instantly speed comparedwind compared turbines to to thethe and SG, SG, SG due due[19]. to to the the power power electronicelectronic converterconverter controller.controller. The The variable variable speed speed wind wind turbines turbines can participate can participate more in themore frequency in the frequencyregulation(b) Fast Power regulation by releasing Reserve bymore releasing KE than more fixed KE than speed fixed wind speed turbines wind and turbines SG [19 and]. SG [19].

(b)(b) Fast FastUsually, Power Power the Reserve Reserve emulated inertia for the wind turbines can be determined based on the frequency deviation or ROCOF, as illustrated in the above section. Whereas the fast power reserve is defined as Usually, the emulated inertia for the wind turbines can be determined based on the frequency the constantUsually, activethe emulated power supportinertia fo regardlessr the wind of turbines the wind can speed be determined [16]. The fast based power on thecharacteristics frequency deviation or ROCOF, as illustrated in the above section. Whereas the fast power reserve is defined as deviationare shown or in ROCOF, Figure 9.as Theillustrated fast power in the reserve above issect theion. temporary Whereas power,the fast releasedpower reserve from the is defined KE stored as the constant active power support regardless of the wind speed [16]. The fast power characteristics are thein theconstant rotating active blades power of the support wind turbine.regardless of the wind speed [16]. The fast power characteristics areshown shown in Figure in Figure9. The 9. The fast powerfast power reserve reserve is the is temporary the temporary power, power, released released from thefrom KE the stored KE stored in the inrotating the rotating blades blades of the of wind the turbine.wind turbine.

Figure 9. Fast power reserve characteristics.

The control diagram of fastFigureFigure power 9. 9. FastFast reverse power power is reserve reserveshown characteristics. characteristics.in Figure 10. This fast power reserve can be realized by regulating the set value of the rotor speed. This is given by, TheThe control control diagram diagram of of fast fast power power reverse reverse is is show shownn in in Figure Figure 10.10. This This fast fast power power reserve reserve can can be be realizedrealized by by regulating regulating the the set set value value of of the the rotor rotor=−11 speed. ωω22 This This is is given given by, by, PJJConst,,0, t r r t (5) 221 1 =11222 2 PJJPConst,=−t ωωJωr,0 Jωr,t (5)(5) where t (t < tmax) is the lasting time ofConst the,,0, t fast22 power2 r −reserve2 r tsince the beginning of the frequency event, ω is the rotor speed at the start and ω is the rotor speed at t, P is the constant where t (tr,0< tmax) is the lasting time of the fast powerrt, reserve since the beginningConst,t of the frequency where t (t < tmax) is the lasting time of the fast power reserve since the beginning of the frequency event,active ωpowerr,0 is theavailable rotor speed at t, hence at the startthe refence and ωr ,tangularis the rotor speed speed in th atist ,methodPConst,t iscan the be constant calculated active as event, ω is the rotor speed at the start and ω is the rotor speed at t, P is the constant powerω availabler,0 at t, hence the refence angular speedrt, in this method can be calculatedConst, t as ω . ref . re f active power available at t, hence the refence angular speed in this method can be calculated as ω s ref . 2 PPConstConst,t ωωωre f ==−ω 2 (6)(6) ref rr,0,0 − J 2 PConst, t ωω=−2 (6) The fastfast power power reserve reserve control control operates operatesref when rwhen,0 the frequency theJ frequency deviation deviation is more thanis more the thresholdthan the value.threshold This value. control This delivers control the delivers extra power the extra in the power frequency in the event frequency through event the KE through stored in the the KE rotor stored and canin the beThe rotor known fast and aspower “over-production”.can bereserve known control as “over-production”. To operates recover the when KE after Tothe recover thefrequency event the is KE knowndeviation after asthe “under-production”. isevent more is knownthan the as thresholdThe“under-production”. shifting value. of an This over-production controlThe shifting delivers of state an the over-production toextra an power under-production in thestate frequency to an state under-production event must bethrough in a sloped the state KE manner,must stored be intoin the avoida sloped rotor a sudden andmanner, can dip betoin avoidknown the activea assudden “over-production”. power dip [ 20in ].the active To power recover [20]. the KE after the event is known as “under-production”. The shifting of an over-production state to an under-production state must be in a sloped manner, to avoid a sudden dip in the active power [20].

Figure 10. Fast power reserve control.

3.1.3. Droop Control for FrequencyFigure Regulation 10. Fast power reserve control.

3.1.3. Droop Control for Frequency Regulation

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Energies3.1.3. 2019The Droop, 12droop, x FOR Control control PEER forREVIEW of Frequency a wind turbine Regulation adjusts the active power response based on the frequency8 of 17 deviation. The droop controller shown in Figure 11 decreases the frequency nadir. Regulated active powerTheThe droopand droop frequency control control of is a oflinearly wind a wind turbine related turbine adjusts and adjusts it is the shown theactive active in power Figure power response 12. response based based on onthe thefrequency frequency deviation.deviation. The The droop droop controller controller shown shown in Figure in Figure 11 decreases11 decreases the thefrequency frequency nadir. nadir. Regulated Regulated active active powerpower and and frequency frequency is linearly is linearly related related and and it is it shown is shown in Figure in Figure 12. 12 .

Figure 11. Droop control for wind turbine.

When the frequency dropsFigure Figurefrom 11. Droop f 11.ref toDroop control fcal, the control for wind wind for generator turbine. wind turbine. increases the output of power from P0 to P1 to regulate the frequency deviation. Hence, the active power regulated by the droop control When the frequency drops from fref to fcal, the wind generator increases the output of power from canWhen be given the frequency as, drops from fref to fcal, the wind generator increases the output of power from P0 to P1 to regulate the frequency deviation. Hence, the active power regulated by the droop control P0 to P1 to regulate the frequency deviation. Hence, the activef − powerf regulated by the droop control can be given as, Δ= − =−meas nom can be given as, PPP10 (7) fmeas fnom ∆P = P P = R− (7) 1 − 0 f− − fR Δ=PPP − =−meas nom (7) where R is the droop coefficient, fmeas10 and P1 are the measured frequency and wind turbine where R is the droop coefficient, fmeas and P1 are the measuredR frequency and wind turbine output outputpower, power, respectively, respectively, while f nomwhileand fPnom0 are and the initialP0 are operating the initial points. operating points. where R is the droop coefficient, f and P are the measured frequency and wind turbine TheThe analytical analytical method method for for estimating estimatingmeas the the1 influence influence of of inertia inertia and and droop droop responses responses from from wind wind outputforfor frequency frequencypower, respectively, control control was was while presented presented fnom [21]. [and21]. The TheP0 droop aredroop the control controlinitial foroperating for wind wind turbine turbinepoints. is is shown shown in in Figure Figure 12. 12 . InIn Theorder analytical toto useuse the themethod wind wind turbinefor turbine estimating in in frequency frequency the influence regulation regulation of underinertia under higher and higher droop penetration, penetration, responses the adaptive fromthe adaptive wind gains forgains forfrequency both for theboth control inertia, the inertia, was and presented droopand droop controller [21]. controller The was droop proposedwas control proposed infor [ 22wind in] by[22] turbine Yuan-Kang by Yuan-Kang is shown Wu. Wu.in The Figure The inertia inertia 12. and In andorderdroop droop to gains use gains arethe alteredare wind altered fromturbine from time in time tofrequency time to ti dependingme redependinggulation on the underon frequencythe higherfrequency imbalance.penetration, imbalance. the adaptive gains for both the inertia, and droop controller was proposed in [22] by Yuan-Kang Wu. The inertia and droop gains are altered from time to time depending on the frequency imbalance.

FigureFigure 12. 12. DroopDroop characteristics. characteristics.

3.1.4. De-Loaded Operation in Wind Turbines 3.1.4. De-Loaded Operation in WindFigure Turbines 12. Droop characteristics. The control techniques presented in this section would help to eliminate the adverse impact of The control techniques presented in this section would help to eliminate the adverse impact of 3.1.4.the De-Loaded higher RES Operation penetration in Wind level Turbines on the frequency stability. This section addresses the inertia and the higher RES penetration level on the frequency stability. This section addresses the inertia and frequency control techniques with the de-loaded operation of RES. The reserve power in RES-based frequencyThe control control techniques techniques presented with the in de-loadedthis section operation would help of RES. to eliminate The reserve the adversepower in impact RES-based of generators from de-loaded operation can be utilized for the inertial response, and primary frequency thegenerators higher RES from penetration de-loaded level operation on the can frequency be utilized stability. for the Thisinertial section response, addresses and primary the inertia frequency and support. Generally, the wind turbines are operated with a maximum power point tracking technique frequencysupport. control Generally, techniques the wind with turbines the de-loaded are operated operation with a of maximum RES. The powerreserve po powerint tracking in RES-based technique and this does not contribute to frequency regulation. To maintain the stability of the power system, the generatorsand this fromdoes de-loadednot contribute operation to frequency can be utilized regulati foron. the To inertialmaintain response, the stability and primaryof the power frequency system, wind turbines need to participate in the frequency regulation with the increasing penetration level of support.the wind Generally, turbines the need wind to turbines participate are operatedin the freq withuency a maximum regulation power with potheint increasing tracking technique penetration wind in the power system. The de-loaded control techniques enable the wind turbines in frequency andlevel this ofdoes wind not in contribute the power to system.frequency The regulati de-loadedon. To control maintain techniques the stability enable of the powerwind turbines system, in regulation. The de-loaded operation of wind generators for the fast frequency reserve was initially thefrequency wind turbines regulation. need Theto participate de-loaded inoperation the freq ofuency wind regulation generators with for thethe fast increasing frequency penetration reserve was levelinitially of wind proposed in the inpower [23]. system.Some of Thethe authorsde-loaded [9] proposedcontrol techniques maintaining enable the activethe wind power turbines reserve in for frequency regulation. The de-loaded operation of wind generators for the fast frequency reserve was initially proposed in [23]. Some of the authors [9] proposed maintaining the active power reserve for

Energies 2019, 12, x FOR PEER REVIEW 9 of 17 Energies 2019, 12, 1996 9 of 17 highEnergies wind 2019, 12speeds,, x FOR PEER and REVIEW the reserve being unavailable in lower wind speeds. Another way9 of to17 maintain the reserve power for the wind generators is by de-loading them in low wind speeds. proposed in [23]. Some of the authors [9] proposed maintaining the active power reserve for high wind Although,high wind thespeeds, reserve and is thenot reserveavailable being above unavai the wind-ratedlable in lower speeds wind [24]. speeds. The authorAnother of way[25–28] to speeds, and the reserve being unavailable in lower wind speeds. Another way to maintain the reserve proposedmaintain thethe reservede-loaded power operation for the of winda wind generators generator is over by de-loadingthe entire speed them range,in low either wind in speeds. lower power for the wind generators is by de-loading them in low wind speeds. Although, the reserve is not windAlthough, speeds the or reserve in higher is not wind available speeds. above The thepower wind-rated and rotor speeds characteristics [24]. The forauthor the ofde-loaded [25–28] available above the wind-rated speeds [24]. The author of [25–28] proposed the de-loaded operation of operationproposed ofthe wind de-loaded turbine operation are shown of ina windFigure generator 13. over the entire speed range, either in lower winda wind speeds generator or in over higher the entire wind speed speeds. range, The either power in lowerand rotor wind characteristics speeds or in higher for the wind de-loaded speeds. Theoperation power of and wind rotor turbine characteristics are shown for in the Figure de-loaded 13. operation of wind turbine are shown in Figure 13.

Figure 13. De-loaded operation for wind turbine.

(a) Speed Control Figure 13. De-loaded operation for wind turbine.

(a) SpeedThe Speed de-loading Control Control operation of the wind turbine can be realized by changing the operating point from the maximum power point (Pmpp) to a sub-optimal power point (PSuboptimal). The power can be The de-loading operation of the wind turbine ca cann be realized by changing the operating point varied from Psub to Pmpp by regulating the rotor speed. The speed control method controls the tip from the maximum power point (Pmpp) to a sub-optimal power point (PSuboptimal). The power can be speedfrom the ratio maximum (λ) and it poweris known point as, (Pmpp) to a sub-optimal power point (PSuboptimal). The power can be varied from Psub to Pmpp by regulating the rotor speed. The speed control method controls the tip speed varied from Psub to Pmpp by regulating the rotor speed. The speed control method controls the tip ratio (λ) and it is known as, ω speed ratio (λ) and it is known as, r R λ = ω R (8) λ = vr (8) ωvR λ = r (8) where RRthethe rotor rotor radius radius and andν νis is is is the the velocity velocity of of wind. wind.v By controlling the the speed speed ratio ratio ( (λ)) thethe operatingoperating pointpoint ofof thethe windwind turbineturbine can be altered and is shownwhere in R Figure the rotor 1414. .radius and ν is is the velocity of wind. By controlling the speed ratio ( λ ) the operating point of the wind turbine can be altered and is shown in Figure 14.

Figure 14. SpeedSpeed control for wind turbine.

Therefore, the reference powerFigure given 14. Speed forfor thethe control windwind for turbineturbine wind turbine. cancan bebe knownknown asas [[29],29], " # Therefore, the reference power given for the wind turbineωωωr,del − canωr, measbe known as [29], Pre f =+= Pdel + (Pmax −Pdel) rdel,,− rmeas (9) PPref del()* PPmax − del ∗ω ωr,max (9) ωωr,del − ωωrdel,,max− r PP=+()* PP − rdel,, rmeas where Pmax is maximum power,refPdel is del de-loadedmax power, del ωωωr,max is− rotor speed at Pmax, ωr,del is de-loaded(9) rdel,,maxω r max ω whererotor speed Pmax at isP maximumdel. power, Pdel is de-loaded power, r,max is rotor speed at P , rdel, is de-loadedIn the rotor rotor speed speed at control P . of wind turbines, there are two different possibilities to regulate the where P is maximum power,del P is de-loaded power, ω is rotor speed at Pmax, ω is active powermax output: over-speedingdel and under-speeding of ther turbine.,max In case of over-speedrdel mode,, In the rotor speed control of wind turbines, there are two different possibilities to regulate the de-loadedthe wind turbine rotor speed delivers at P the. KE until the operating point has reached the maximum power point active power output: over-speedingdel and under-speeding of the turbine. In case of over-speed mode, (P ). Whereas, in under-speed mode the wind turbine absorbs the KE until the operating point has theMPP windIn the turbine rotor speed delivers control the KE of winduntil theturbines, operating there point are two has differentreached thepossibilities maximum to powerregulate point the activereached power the maximum output: over-speeding power point [and30]. under-speedin The under-speedingg of the of turbine. the rotor In controlcase of over-speed results in stability mode, ( P ). Whereas, in under-speed mode the wind turbine absorbs the KE until the operating point problems.theM PPwind turbine Hence, delivers over-speeding the KE of until the rotorthe operating control is point used. has reached the maximum power point

( PMPP ). Whereas, in under-speed mode the wind turbine absorbs the KE until the operating point

Energies 2019, 12, x FOR PEER REVIEW 10 of 17 Energies 2019, 12, x FOR PEER REVIEW 10 of 17 has reached the maximum power point [30]. The under-speeding of the rotor control results in hasEnergies reached2019, 12 ,the 1996 maximum power point [30]. The under-speeding of the rotor control results10 of in 17 stability problems. Hence, over-speeding of the rotor control is used. stability problems. Hence, over-speeding of the rotor control is used. The active power reference from Equation (9) uses a de-loaded power extraction curve shown The active power reference from Equation (9) uses a de-loaded power extraction curve shown in FigureThe active 14. This power technique reference regulates from Equation output of (9) the uses active a de-loaded power from power the extraction wind turbine curve under shown a in Figure 14. This technique regulates output of the active power from the wind turbine under a infrequency Figure 14 event.. This Generally, technique the regulates speed control output technique of the active is appropriat power frome for the low wind wind turbine speeds. under a frequency event. Generally, the speed control technique is appropriate for low wind speeds. frequencyThe rotor event. speed Generally, control the of speedthe wind control turbine technique is presented is appropriate in [23] and for regulates low wind the speeds. active power The rotor speed control of the wind turbine is presented in [23] and regulates the active power outputThe under rotor speeda frequency control event. of the In wind this turbinepaper, the is presented author utilized in [23] andthe optimum regulates power the active extraction power output under a frequency event. In this paper, the author utilized the optimum power extraction outputcurve to under regulate a frequency the active event. power In thisof a paper,wind turbin the authore. The utilized power thetransferred optimum to power the grid extraction is dependent curve curve to regulate the active power of a wind turbine. The power transferred to the grid is dependent toon regulate the slip thefor activea doubly-fed power ofinduction a wind turbine. generator. The If power the slip transferred is increased, to the then grid the is power dependent transferred on the on the slip for a doubly-fed . If the slip is increased, then the power transferred slipfrom for the a doubly-fedwind generator induction to the generator. grid is increased. If the slip is increased, then the power transferred from the from the wind generator to the grid is increased. wind generator to the grid is increased. (b) Pitch Angle control (b) Pitch Pitch Angle Angle control control Another type of de-loading control for the wind turbines is created by changing the blade angle, Another type of de-loading control for the wind tu turbinesrbines is is created created by by changing changing the the blade blade angle, angle, known as “pitch angle control”. The pitch angle control can be applied to both variable speed and known as “pitch angle control”. The The pitch angle control can be applied to both variable speed and fixed speed wind turbines. Figure 15 illustrates the power-speed characteristics of a wind turbine for fixedfixed speed wind turbines. Figure 1515 illustratesillustrates th thee power-speed characteristics of a wind turbine for different pitch angles. differentdifferent pitch angles.

Figure 15. Pitch angle controller. Figure 15. PitchPitch angle angle controller. controller. To de-load the wind turbine in pitch angle control, the pitch angle (beta) needs be increased to To de-load the wind turbine in pitch angle control,control, the pitch angle (beta) needs be increased to receive the active power reserve which is shown in Figure 16. Whenever the frequency event occurs, receive the active power reserve which is shown in Figure 16.16. Whenever Whenever the the frequency frequency event event occurs, occurs, the pitch angle controller increases the value of beta and the operating point without changing the the pitch angle controller increases the value of betabeta and the operating point without changing the rotor speed. In pitch angle control, the pitch angle of the wind turbine is set at a sub-optimal point rotor speed.speed. InIn pitchpitch angleangle control, control, the the pitch pitch angle angle of of the the wind wind turbine turbine is setis set at a at sub-optimal a sub-optimal point point and and reaches optimum value under frequency events to deliver/absorb active power. Pitch angle andreaches reaches optimum optimum value value under under frequency frequency events events to deliver to /absorbdeliver/absorb active power. active Pitch power. angle Pitch control angle is control is suited to high wind speed conditions [31]. controlsuited to is highsuited wind to high speed wind conditions speed conditions [31]. [31]. Generally, the de-loading control can be selected depending upon the wind speeds. In low Generally, thethe de-loadingde-loading control control can can be be selected selected depending depending upon upon the windthe wind speeds. speeds. In low In wind low wind speeds, the de-loading is realized by the rotor speed control. The coordinated control of both windspeeds, speeds, the de-loading the de-loading is realized is realized by the by rotor the speedrotor speed control. control. The coordinated The coordinated control control of both of pitchboth pitch angle and rotor speed control is needed in medium wind speed conditions. The pitch angle pitchangle angle and rotor and speedrotor speed control control is needed is needed in medium in medium wind speed wind conditions.speed conditions. The pitch The angle pitch control angle control alone is used in high wind speed conditions. controlalone is alone used inis used high windin high speed wind conditions. speed conditions.

FigureFigure 16.16. De-loaded curvescurves withwith pitchpitch angleangle controlcontrol forfor windwind turbine.turbine. Figure 16. De-loaded curves with pitch angle control for wind turbine. 3.1.5. Solar Photovoltaic Array in Frequency Regulation 3.1.5. Solar Photovoltaic Array in Frequency Regulation 3.1.5.The Solar installed Photovoltaic photovoltaic Array in (PV) Frequency generators Regulation do not contribute to the active reserve power for frequency regulation. All the PV systems are operated at the maximum power point using MPPT

Energies 2019, 12, x FOR PEER REVIEW 11 of 17

EnergiesThe2019 installed, 12, 1996 photovoltaic (PV) generators do not contribute to the active reserve power11 offor 17 frequency regulation. All the PV systems are operated at the maximum power point using MPPT algorithms and do not alter their active power output when load variation occurs. The PV systems algorithms and do not alter their active power output when load variation occurs. The PV systems also need to participate in the frequency regulation to maintain the power system stability, with an also need to participate in the frequency regulation to maintain the power system stability, with an increasing penetration level of PV system in the power system. The frequency regulation is increasing penetration level of PV system in the power system. The frequency regulation is employed employed in two ways for the PV generators: in two ways for the PV generators: 1. De-loaded operation of PV. 2.1. UseDe-loaded of ESS. operation of PV. 2. Use of ESS. In solar photovoltaic arrays, both the inertial and primary frequency response can be implementedIn solar photovoltaic by using de-loaded arrays, both operation the inertial of PV and and primary ESS with frequency proper control response algorithms. can be implemented by using de-loaded operation of PV and ESS with proper control algorithms. 3.1.6. De-Loaded Operation of a PV Generator [32] 3.1.6. De-Loaded Operation of a PV Generator [32] In the de-loaded operation of PV, the power electronic converters need to inject the required amountIn theof power de-loaded under operation imbalance of PV,conditions the power depend electronicing on convertersthe control needsignal to generated inject the in required Figure 17.amount of power under imbalance conditions depending on the control signal generated in Figure 17.

Figure 17. De-loadedDe-loaded operation operation control control for solar photovoltaic (PV).

The solarsolar powerpower plants plants need need to beto operated be operated under under a sub-optimal a sub-optimal powerpoint power below point the below maximum the maximumpower point power to maintain point to the maintain active power the active reserve power for the reserve PV plants, for the as PV isshown plants, in as Figure is shown 18. Whenin Figure the frequency event occurs, the operating voltage of the PV is decreased from the V to V , to increase 18. When the frequency event occurs, the operating voltage of the PV is decreaseddel MPP from the Vdel the active power output. This reserve power will not be released until the frequency is deviated. When toV , to increase the active power output. This reserve power will not be released until the theM frequencyPP deviation occurs, the DC-link voltage is changed based on the frequency. The active frequencypower reserve is deviated. available When under the de-loaded frequency operation deviation is knownoccurs, as,the DC-link voltage is changed based on the frequency. The active power reserve available under de-loaded operation is known as, Preserve ==−PMPP Pdel (10) PPPreserve MPP− del (10) The voltage reference calculated in the de-loaded operation is, The voltage reference calculated in the de-loaded operation is,

Vdc,re=+−Δ f = VMPP + Vdel Vdc∆ f (11) VVVVfdc, ref MPP del− dc (11) where,Energies 2019Vdc ,is 12 the, x FOR DC-link PEER REVIEW voltage. 12 of 17 where, Vdc is the DC-link voltage.

3.2. Voltage Control Techniques The control methods for voltage are discussed in two ways: (1) Low-voltage ride through (LVRT) and (2) High-voltage ride through (HVRT). The power disturbance in the grid is higher because of the output power from RES is irregular. Hence, LVRT should be attributed to the large capacity of grid-connected RES. The main purpose of LVRT is restricting the PCC current of the RES inverter, and power elements of RES inverter should not be damaged and tripped. The RES inverter is operated at 1.1 times the rated current in a short time, and it can supply a reactive current to support the grid voltage recovery. Thus, the control strategy of LVRT for RES is particularly dependent upon controlling the current output of the inverter during a grid fault [33]. Hence, to

Figure 18. Frequency response in power system network. Figure 18. Frequency response in power system network. improve the power quality and stable operation of the grid, the RES is associated with the power system to serve the grid recovery from grid fault and to sustain the regulation of grid voltage and frequency. Developing the penetration of (DG) systems into the grid, one of the major problems is sudden tripping of DG from the grid during faults occurring in the system such as power outages and voltage flickers. Hence, DG units need to support the grid during fault conditions. The reactive power support diagram is shown in Figure 19. According to the theory of instantaneous reactive power, the active and reactive currents are controlled by varying the amplitude and phase of the output voltage of the inverter. In daytime situations, the active power output and the reactive power compensation (RPC) of the system are obtained concurrently [34,35]. When the PV power is not available, the RPC characteristic of the RES can be employed to enhance the utilization factor of the system. The main factors for RPC in a system are: (1) Regulating the voltage, (2) Improving the stability of the system, (3) Minimizing power losses, (4) Effective utilization of machines associated with the system. During the LVRT, the RES has the ability to be associated with the grid when the voltage at PCC drops to a prescribed cut-out point. When the drop is below the prescribed point, the RES can switch from the grid. Hence, a certain amount of reactive power is needed to provide effective voltage support to overcome this fault situation [36].

Figure 19. Reactive power support diagram.

There are various causes for the PCC voltage to be above/below rated values, such as the load has been suddenly disconnected, earth faults, and the irrational control strategies. During HVRT conditions, The RES should consume a required reactive power in order to make the PCC voltage become lower. The amount of absorbing power is determined by the capacity of the inverter and

Energies 2019, 12, x FOR PEER REVIEW 12 of 17

Energies 2019, 12, 1996 12 of 17

3.2. Voltage Control Techniques The control methods for voltage are discussed in two ways: (1) Low-voltage ride through (LVRT) and (2) High-voltage ride through (HVRT). The power disturbance in the grid is higher because of the output power from RES is irregular. Hence, LVRT should be attributed to the large capacity of grid-connected RES. The main purpose of LVRT is restricting the PCC current of the RES inverter, and power elements of RES inverter should not be damaged and tripped. The RES inverter is operated at 1.1 times the rated current in a short time, and it can supply a reactive current to support the grid voltage recovery. Thus, theFigure control 18. Frequency strategy of response LVRT for in RESpower is particularlysystem network. dependent upon controlling the current output of the inverter during a grid fault [33]. Hence, to improve the power quality and improvestable operation the power of thequality grid, and the RESstable is operation associated of with the thegrid, power the RES system is associated to serve the with grid the recovery power systemfrom grid to faultserve and the togrid sustain recovery the regulation from grid offault grid and voltage to sustain and frequency. the regulation of grid voltage and frequency.Developing the penetration of distributed generation (DG) systems into the grid, one of the majorDeveloping problems isthe sudden penetration tripping of of di DGstributed from the generation grid during (DG) faults systems occurring into inthe the grid, system one suchof the as majorpower problems outages and is sudden voltage tripping flickers. of Hence, DG from DG the units grid need during to support faults theoccurring grid during in the faultsystem conditions. such as powerThe reactive outages power and support voltage diagram flickers. is shownHence, inDG Figure units 19 .need According to support to the theorythe grid of instantaneousduring fault conditions.reactive power, The thereactive active power and reactive support currents diagram are is shown controlled in Figure by varying 19. According the amplitude to the and theory phase of instantaneousof the outputvoltage reactive of thepower, inverter. the active In daytime and situations,reactive currents the active are power controlled output by and varying the reactive the amplitudepower compensation and phase (RPC)of the ofoutput the system voltage are of obtainedthe inverter. concurrently In daytime [34 situations,,35]. When the the active PV power power is outputnot available, and the the reactive RPC characteristic power compensation of the RES (RPC) can beof employedthe system toare enhance obtained the concurrently utilization factor [34,35]. of Whenthe system. the PV power is not available, the RPC characteristic of the RES can be employed to enhance the utilizationThe main factor factors of for the RPC system. in a system are: The main factors for RPC in a system are: (1) Regulating the voltage, (1) Regulating the voltage, (2) Improving the stability of the system, (2) Improving the stability of the system, (3) Minimizing power losses, (3) Minimizing power losses, (4) Effective utilization of machines associated with the system. (4) Effective utilization of machines associated with the system. During the LVRT, LVRT, the RES has the ability to be a associatedssociated with the grid when the voltage at PCC drops to a prescribed cut-out point. When When the the drop drop is is below below the the prescribed prescribed point, the RES can switch fromfrom thethe grid.grid. Hence, Hence, a certaina certain amount amount of reactiveof reacti powerve power is needed is needed to provide to provide effective effective voltage supportvoltage supportto overcome to overcome this fault this situation fault situation [36]. [36].

Figure 19. ReactiveReactive power power support support diagram.

There are various causes for the PCC voltage to be above/below above/below rated values, such as the load has been suddenly disconnected, earth faults, and the irrational control strategies.strategies. During During HVRT conditions, The The RES RES should should consume consume a a required required reactive reactive power power in in order to make the PCC voltage become lower. The The amount amount of of absorbing absorbing power power is is determined determined by by the the capacity of of the inverter and voltage at the above level of the prescribed cut-off point. Hence, the active power supplied from RES should not be changed, so the RES inverter can utilize its capacity to consume reactive power [37].

Energies 2019, 12, x FOR PEER REVIEW 13 of 17 Energies 2019, 12, 1996 13 of 17 voltage at the above level of the prescribed cut-off point. Hence, the active power supplied from RES should not be changed, so the RES inverter can utilize its capacity to consume reactive power [37]. TheThe voltage voltage requirements requirements during during the the VRT VRT at at P PCCCC are shown in Figure 20.20. The The HVRT HVRT handles thethe grid grid faults, faults, which which depends depends on on voltage voltage rise. rise. The The HVRT HVRT functionality functionality is is used used for for each each of of the the three three voltagevoltage phases. phases. Hence, Hence, the the functionality functionality is is actuated actuated when when the the voltage voltage rises rises higher higher than than the the voltage levellevel determined determined by by the the utility utility grid. grid. The The control control requirements requirements for for LVRT LVRT and and HVRT HVRT equations equations are are presentedpresented in in Equations (12)–(14).

FigureFigure 20. VoltageVoltage requisites requisites during during the the low-voltage low-voltage ride ride through through (LVRT) (LVRT) and high-voltage and high-voltage ride through ride throughHVRT at HVRT PCC [ 38at ].PCC [38].

TheThe equation equation is is to to satisfy satisfy the the demand demand requirement requirement as follows,

222+≤2 2 '2 IIdq+ IIII0 max (12)(12) d q ≤ max where Imax is maximum reference current from RES system, and Id and Iq are the active and reactive where Imax is maximum reference current from RES system, and Id and Iq are the active and reactive currents.currents. During a faultfault situation,situation, the the reactive reactive support support should should meet meet the the demand demand of Equationof Equation (12), (12), and andthe activethe active and and reactive reactive currents currents are setare as set follows, as follows,  I  Iddset,set, =   q  IId =  2'22 2 (13)(13) ororminmin(I Id0, I Imax0 −I Iq )  dq0, max−

q  2 '22 2 IKIImax0 − ()KId,re f H I = max− drefH , (14)(14) Iq,re f H = K qrefH, K where Id0 is the pre-fault current and IdrefH is the reactive reference current. The active power control whereactivities Id0 is are the presented pre-fault in current Table2 .and IdrefH is the reactive reference current. The active power control activities are presented in Table 2. Table 2. Active power control activities [38]. Table 2. Active power control activities [38]. Time Working State Principle Time (0, t)Working steady State state Basic control principlePrinciple (0, t) (t, t1) steady steady state state to fault state Id is Basic from control (12) principle (t, t1) (t1, t2) steady state recovery to fault state state recovery rate set betweenId is fromt1 to(12)t2 (t1, t2) (t2, ~) recovery steady state state Basic recovery control rate principle set between t1 to t2 (t2, ~) steady state Basic control principle The control strategy of LVRT and HVRT is shown in Figure 21. In this [38], the control strategy is decided by the voltage at PCC. The Idref and Iqref are generated various working positions, and in that way to understand the LVRT and HVRT.

Energies 2019, 12, x FOR PEER REVIEW 14 of 17

The control strategy of LVRT and HVRT is shown in Figure 21. In this [38], the control Energies 2019, 12, 1996 14 of 17 strategy

FigureFigure 21. 21. ControlControl diagram diagram of of the the LVRT LVRT and HVRT [38]. [38].

4. ESS Support 4. ESS Support The above section considers the the various controll controllersers for for both solar and wind and discusses the maintenance of of the power system stability stability without without ESS. ESS. RES RES is is intermittent in in nature and may lead toto stability stability issues issues if if we we completely completely rely rely on on these these sources sources for for frequency frequency stability. Hence, Hence, ESS ESS needs needs to to be used along with with the the RES RES to to maintain maintain the the stability stability of of power power system system under under high high penetration penetration of of RES. RES. The frequency of the power system can be balanced by using an energyenergy storagestorage device.device. Whenever therethere is an imbalance between the the demand and and gene generation,ration, the the control control algorithm algorithm acts acts at at the the ESS ESS to to deliver the required amountamount ofof thethe activeactive power. power. The The ESS ESS support support for for RES RES issues issues diagram diagram is is shown shown in inFigure Figure 22 .22. The performanceperformance of of ESS ESS in thein smartthe is angrid eff ortis towardsan effort balancing towards generation, balancingconsumption, generation, consumption,reduction of variations reduction inof RES,variations and alsoin RES, delivers and al highso delivers quality high supply quality and supply reliability and of reliability the supply. of theThere supply. has been There an has evolution been an of evolution the enhanced of the intelligent enhanced electricity intelligent network electricity called network “smart called grid”, “smart and it grid”,has a prominentand it has contesta prominent of supporting contest allof thesupporting sources connectedall the sources with econnectedffective load with control, effective powered load control,from large powered penetration from of large non-dispatchable penetration of RES. non- Thesedispatchable irregular RES. energy These sources irregular are given energy higher sources levels areof grid given perception. higher levels Hence, of grid ESS isperception. the most significant Hence, ESS smart is the grid most advanced significant element smart to grid produce advanced stable elementpower at to the produce grid level. stable ESS power produces at the grid vital level. benefactions ESS produces in defeating vital benefactions the complexity in defeating of irregular the complexityvariation created of irregular by RES. variation It compensates created the by variation RES. It compensates and lessens the the variance variation among and supplylessens andthe variancedemand [among39]. ESS supply consists and of pumpeddemand hydro-storage,[39]. ESS consists compressed of pumped air energyhydro-storage, storage (CAES), compressed flywheel, air energysuperconducting storage (CAES), magnetic flywheel, energy superconductin storage (SMES),g battery,magnetic and energy capacitors storage that (SMES), are supposed battery, toand be capacitorswidespread that in RESare supposed integrated to to be the widespread grid. ESS employsin RES integrated a power transformation to the grid. ESS system employs to integratea power transformationinto the system system network; to integrate they can into add the or system receive network; both active they andcan add reactive or receive power both to balance active and for reactivevoltage changespower to in balance a short for or mediumvoltage changes duration. in Thea short ESS or integration medium withduration. the grid The willESS increaseintegration the withpower the quality, grid will reliability, increase voltage the power support, quality, backup reliability, power and voltage decrease support, losses. backup When thepower irregular and decreaseenergy source losses. reaches When higherthe irregular levels energy of grid source penetration, reaches ESS higher is the levels solution of grid for providingpenetration, a reliable ESS is energy supply [40,41]. Hence, proper coordination is needed for micro-grid (MG) and ESS operations. The main control methods among MG and ESS are the voltage, frequency, and active and reactive Energies 2019, 12, x FOR PEER REVIEW 15 of 17 Energies 2019, 12, 1996 15 of 17 the solution for providing a reliable energy supply [40,41]. Hence, proper coordination is needed for micro-grid (MG) and ESS operations. The main control methods among MG and ESS are the voltage, powerfrequency, controllers. and active Thus, and ESS reactive can sustain power a contro balancellers. within Thus, the ESS generation can sustain and a load balance at the within instant the of generationoperation, andand canload also at the produce instant a of firming operation, potential and ofcan RES also [42 produce]. a firming potential of RES [42].

Figure 22. System (ESS) support for RES issues. 5. Smart Grid Features in Voltage Control with RES 5. Smart Grid Features in Voltage Control with RES Smart grid is being increasingly used in practice by electric utilities and involves the use Smart grid is being increasingly used in practice by electric utilities and involves the use of of information, communication and control abilities to enhance the performance of the grid. information, communication and control abilities to enhance the performance of the grid. The The fundamental concept of a smart grid involves developing analysis, control, monitoring abilities fundamental concept of a smart grid involves developing analysis, control, monitoring abilities of of the traditional grid system, and reducing energy consumption. These objects are feasible over a the traditional grid system, and reducing energy consumption. These objects are feasible over a system that can yield accurate and precise monitoring of the grid. A promising selection to minimize system that can yield accurate and precise monitoring of the grid. A promising selection to minimize the challenges created by variations in RES is to add ESS into the grid. Another approach is to achieve the challenges created by variations in RES is to add ESS into the grid. Another approach is to flexible operation in energy consumption by employing demand side integration (DSI) or the usage achieve flexible operation in energy consumption by employing demand side integration (DSI) or of the micro-grid system. Moreover, the RES, ESS, and DSI are listed as one of the DERs. Hence, the usage of the micro-grid system. Moreover, the RES, ESS, and DSI are listed as one of the DERs. combining various aspects of certain sources is collectively essential in enhancing the utility of RES in Hence, combining various aspects of certain sources is collectively essential in enhancing the utility the energy market [43]. of RES in the energy market [43]. 6. Conclusions 6. Conclusions The large-scale integration of RES into the power grid will have an impact on the stability of The large-scale integration of RES into the power grid will have an impact on the stability of the the power system. Both the frequency and voltage stability are highly affected by integrating large power system. Both the frequency and voltage stability are highly affected by integrating large scale scale RES into the grid. This paper presented the issues related to high integration of RES in detail RES into the grid. This paper presented the issues related to high integration of RES in detail and and reviewed their possible solutions available in the literature. To minimize frequency and voltage reviewed their possible solutions available in the literature. To minimize frequency and voltage related issues, the several control techniques that are applied to wind and solar-based generators related issues, the several control techniques that are applied to wind and solar-based generators are are summarized. Furthermore, ESS support could be used to maintain the stability and reduce the summarized. Furthermore, ESS support could be used to maintain the stability and reduce the negative impacts related to grid integration as discussed. negative impacts related to grid integration as discussed. Author Contributions: Initial idea purpose, data collection, formal analysis, original draft writing and editing: AuthorsG.V.B.K. andContributions: R.K.S.; resources, Initial supervision,idea purpose, review data collection, and editing: formal K.P., analysis, P.S., and original J.B.H.-N. draft writing and editing: G.V.B.K.Funding: andNo R.K.S.; source resources, of funding supervision, was attained review for this and research editing: activity. K.P., P.S., and J.B.H.-N. Acknowledgments:Funding: No source Theof funding authors was would attained like tofor acknowledge this research theactivity. support and technical expertise received from the center for Bioenergy and Green Engineering, Department of Energy Technology, Aalborg University, Esbjerg, Acknowledgments:Denmark which made The this authors publication would possible. like to acknowledge the support and technical expertise received from theConflicts center of for Interest: BioenergyThe authorsand Green declare Engineering, no conflict De of interest.partment of Energy Technology, Aalborg University, Esbjerg, Denmark which made this publication possible.

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