Electrical Power and Energy Systems 44 (2013) 115–122

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Electrical Power and Energy Systems

journal homepage: www.elsevier .com/locate / i jepes

Energy storage system-based power control for grid-connected wind power farm

Baoming Ge a,c,⇑, Wenliang Wang a, Daqiang Bi b, Craig B. Rogers c, Fang Zheng Peng c, Aníbal T. de Almeida d,Haitham Abu-Rub e

a School of Electrical Engineering, Beijing Jiaotong University, Haidian District, Beijing 100044, Chinab Department of Electrical Engineering, Tsinghua University, Beijing 100086, Chinac Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824, USAd Department of Electrical and Computer Engineering, University of Coimbra, 3030-290 Coimbra, Portugale Department of Electrical and Computer Engineering, Texas A & M University at Qatar, Doha 23874, Qatar

a r t i c l e i n f o

Article history:Received 29 December 2010Received in revised form 23 June 2012Accepted 5 July 2012Available online 9 August 2012

Keywords:Energy storagePower converterPower controlVRBWind power generation

0142-0615/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.ijepes.2012.07.021

⇑ Corresponding author at: School of Electrical EUniversity, Haidian District, Beijing 100044, China.

E-mail address: gebaoming@tsinghua.org.cn (B. Ge

a b s t r a c t

Wind power is characterized as intermittent with stochastic fluctuations, which can result in deviation ofgrid frequency and voltage when the wind power ratio is high enough. These effects have a definiteimpact on stability and power quality of grid operation. This paper proposes an energy storage system(ESS) based power control for a grid-connected wind power system to improve power quality and stabil-ity of the power system. Vanadium redox flow battery (VRB), as an environmentally-friendly battery pro-vided with many advantages, is employed in the ESS. A dynamic mathematical model of VRB is built byusing an equivalent circuit, and its charging and discharging characteristics are analyzed. The VRB’s stablevoltage is available in a wide range (around 20–80% state of charge), which is suitable for utilization of asingle-stage AC/DC converter in the VRB-based ESS. With a proposed energy storage control method,VRB-based ESS is added at the exit of the grid-connected wind farm to filter fluctuations of wind power,which ensures that smooth power will be injected into the grid and which improves power quality of thepower system. Simulations and experiments are carried out to verify the proposed power control methodfor these grid-connected wind power systems. The grid-connected wind farm with VRB-based ESS, windspeed (characterized as gust and stochastic wind), and wind turbines are modeled in simulations. Simu-lation results show that the grid-injected active power from the wind farm is effectively smoothed, andreactive power support can be provided for the grid by the designed VRB-based ESS. Experimental veri-fication is achieved with a low power bench, where a RT-LAB real-time simulation platform and a directtorque controlled induction motor simulate a real wind turbine and a wind speed model is built in the RT-LAB real-time simulation platform. The experimental results verify the proposed scheme through dem-onstrating a stable and smooth power flow injected into the grid though the wind power fluctuated.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, wind power generation technology is develop-ing rapidly and is becoming more mature [1–3]. Wind velocity pre-sents intermittent and stochastic characteristics, which leads torelatively large fluctuations of wind power. Power fluctuationscan result in deviation of grid frequency and voltage [4], and affectstability and power quality of grid operation [5]. If wind powerdoes achieve 20% or more of total system power, peak capacityand safe operation of the grid will face enormous challenges. Now-adays, with a growing number of large-scale grid-connected windfarms and the continuous extension of installed capacity, the windpower ratio is becoming higher. Therefore, the fluctuations of wind

ll rights reserved.

ngineering, Beijing Jiaotong

).

power should be overcome urgently to avoid a degradation of thegrid’s performance.

All over the world, researchers have proposed several solutionsto smooth wind power fluctuations. For example, for a given hourof the day, wind-deficient farms could be compensated by wind-benefiting farms [6], so a general planning method was proposedin [6] to minimize the variance of aggregated wind farm poweroutput by optimally distributing a predetermined number of windturbines over a preselected number of potential wind farming sites,in which the objective is to facilitate high wind power penetrationthrough the search for steadier overall power output. Kassem et al.proposed a hybrid power system by combining the continuouslyavailable diesel power and locally available, pollution-free windenergy [7–9], of which the main goal is to reduce fuel consumptionand in this way to reduce system operating costs and environmen-tal impact. A wind turbine generator was controlled with a voltagesource converter to smooth power fluctuations in [10], and pitch

Fig. 1. VRB operating principle.

116 B. Ge et al. / Electrical Power and Energy Systems 44 (2013) 115–122

control of the turbine blades was employed for the same purposein [11], but their abilities and control ranges are limited due toreducing wind power acquisition. FACTS were also used to main-tain grid voltage stability at the wind power access point by adjust-ing reactive power [12–14], but it cannot smooth active powerfluctuations [15].

Large-scale energy storage technology provides an effective ap-proach for large-scale grid-connected wind farms to improve thewind power quality [16–26], which not only can smooth activepower but also can regulate reactive power. As a result, large-scalewind farms with energy storage systems (ESSs) can be easily andreliably connected to the conventional grid. There are many energystorage schemes for this purpose, for example, the compressed airenergy storage (CAES) system is a mature and reliable bulk energystorage technique with promising potential to accommodate highwind power penetration in power systems [16,18]. It operates withfast adaptability and low cost, but requires a suitable sealed under-ground geological cavern for air storage, as a result of that it is lo-cated far from user/load. Flywheel based ESS is used in [22] toimprove power quality and stability of the wind farms. Supercon-ducting magnetic-based ESS is achieved in [23,24] to smooth thefluctuations of wind power. Super-capacitors are employed in[25,26] to adjust wind farm output power. Practical applicationsof flywheel, super-conduction, and super-capacitors in the ESSfor wind power are restrained due to their high cost or low capac-ity. At present, lead-acid batteries are widely used because of theirmature technology and low price, but cycle life is very limited. So-dium sulfur batteries with their high energy density, high effi-ciency of charge/discharge and long cycle life, are primarilysuitable for large-scale non-mobile applications such as grid en-ergy storage [27–29]. However, it requires a high operation tem-perature of 280–360 �C and has the highly corrosive nature ofsodium polysulfide. Also, hybrid power generation systems useESS to improve power quality of the grid, for example, diesel gen-erators and renewable energy sources (such as wind power andphotovoltaic power units) are deployed at different locations ofthe system, and electric double layer capacitors as energy storageare utilized to play the main role to control the system’s powerquality and system frequency [17]. Operation schedule of thermalgenerators, wind generators, photovoltaic power generation sys-tems, and batteries are optimized through considering the trans-mission constraint to reduce operational cost of thermal units [19].

The vanadium redox flow battery (VRB) is well suited for appli-cations of large-scale power energy storage when compared toother energy storage batteries, because of its large capacity, longlife, low materials cost, low maintenance requirements, and fastresponse, etc. [30,31]. VRB-based ESS has been applied to windpower projects in Hokkaido of Japan, in Australia, etc. Currently,VRB has already started to achieve its commercial operation andis expected to play an important role in wind power system andother renewable energy source areas.

In this paper, VRB-based ESS is dirctly added at the exit of awind farm to regulate grid-injected power, which will absorb windpower fluctuations, provide an amount of reactive power support,and effectively improve power quality and stability of the powersystem. The experimental results and simulation results verifythe proposed scheme.

Fig. 2. Equivalent circuit model of VRB.

2. VRB model

2.1. Operating principle of VRB

As shown in Fig. 1 [31,32], VRB is an electrochemical cell di-vided into two compartments by an ionic membrane where batteryreaction takes place. Positive and negative vanadium electrolytes

are stored in two tanks. A pump per compartment will achievethe electrolyte circulation between the tank and the cell, whichwill improve battery performance and efficiency.

Total power available is related to electrode area of the cell andtotal energy stored in VRB, which depends on both state of charge(SOC) and amount of active chemical substances. Simplified elec-trode reaction processes are as follows:

(1) For positive electrode, it is

V4þ � e� ¢Charge

DischargeV5þ

(1) For negative electrode, it is

V3þ þ e� ¢Charge

DischargeV2þ

2.2. VRB modeling

VRB model based on an equivalent circuit [31,32] takes into ac-count physical and mathematical characteristics, as shown inFig. 2. The proposed model has the following characteristics: r

the SOC is modeled as a dynamically updated variable; s the stackvoltage is modeled as a controlled voltage source, and the powerflowing through this source will impact the SOC; t the variablepump loss model, as a controlled current source, is controlled bythe pump loss current Ipump that is related to the SOC and the cur-rent Istack flowing through the battery stack. VRB power loss

Fig. 3. VRB SOC during a charge-discharge cycle.

Fig. 4. VRB voltage during a charge-discharge cycle.

B. Ge et al. / Electrical Power and Energy Systems 44 (2013) 115–122 117

includes two parts: (1) one is from the equivalent internal resis-tances Rreactor and Rresistive and (2) one is parasitic loss due to theparasitic resistance Rfixed, and the pump loss.

VRB equivalent circuit parameters are calculated on the basis ofthe estimated losses for the worst case. When the SOC of 20% is as-sumed as the worst case, we estimate 15% internal loss and 6% par-asitic loss, and resultant total 21% loss. Therefore, the VRB willprovide a rated power PN with 21% loss [32]. If the cell stack’s out-put power is

Pstack ¼PN

1� 21%ð1Þ

A single cell stack voltage Vcell is directly related to the SOC, asfollows:

V cell ¼ Vequilibrium þ 2k � lg SOC1� SOC

� �ð2Þ

where k is a constant related to temperature impact on the batteryoperation, k = 0.059; Vequilibrium is a standard electromotive forcedifference of each cell, Vequilibrium = 1.25 V.

The parasitic loss, which is related to the pump operation, isseparated into fixed and variable losses as follows

Pparasitic ¼ Pfixed þ Ppump ¼ Pfixed þ k0Istack

SOC

� �ð3Þ

The parasitic resistance Rfixed and the pump loss current are calcu-lated as

Rfixed ¼V2

b

Pfixedð4Þ

Ipump ¼ k0Istack

SOC

� �=Vb ð5Þ

where Vb is the output terminal voltage of VRB; k0 is a constant re-lated to pump loss.

The internal resistance loss of 15% can be approximately di-vided into two parts, i.e., the loss of 9% from Rreactor and the lossof 6% from Rresistive. Each cell has 6 F capacitance, i.e., Celectrodes = 6F. The single cell voltage is very low, so a high voltage VRB needs anumber of cells connected in series.

The SOC is defined as

SOC ¼ Energy in VRBTotal Energy Capacity

ð6Þ

One way to track the SOC is by

SOCt ¼ SOCt�1 þ DSOC ð7Þ

DSOC ¼ DEEN¼ Pstack � Dt

EN¼ Istack � Vb � Dt

PN � TNð8Þ

where SOCt and SOCt�1 are the SOC at the instants of t and t � 1,respectively; DSOC is the SOC variation in a time step Dt; TN isthe time when total energy EN is charged into the battery at a powerPN.

2.3. Charge–discharge characteristics of VRB

The equivalent circuit model based simulations are employed tostudy the charge–discharge characteristics of VRB. The parametersare listed as: rated power PN = 270 kW, rated capacityEN = 405 kW h, initial voltage value VN = 810 V, the cell numbern = 648, Rreactor = 0.174 X, Rresistive = 0.116 X, Rfixed = 60.5 X.

Fig. 3 shows the SOC variation in a charge–discharge cycle,where the VRB is charged at a constant current of 320 A for the first1.5 h, and discharged at the same current for another 1.5 h later.

Fig. 4 shows the open-circuit voltage Vs and the operation terminalvoltage Vb during a charge–discharge cycle.

It can be seen from Fig. 4 that, in the charging and dischargingprocess of VRB, Vs is continuously variable with the SOC; Vb

changes with Vs and the voltage drop on the VRB internalresistance.

There is a voltage difference between Vb and Vs, during a contin-uous charging or discharging process. At the switching instantfrom charging to discharging, Vb is discontinuous due to the differ-ent current direction and voltage drop on the internal resistance.Moreover, during 0–20% SOC and 20–100% SOC, Vb and Vs dramat-ically vary whether charging or discharging. However, VRB voltageis stable during 20–80% SOC, which provides a possibility for VRBto work in the range of 20–80% SOC in order to avoid over-chargeor over-discharge in practical applications.

VRB-based ESS could have two kinds of power converter config-urations, i.e., (1) two-stage power converter through using AC/DCand DC/DC converters to achieve charging and discharging man-agement between AC grid and VRB [31] and (2) single-stage powerconverter through using AC/DC converter only. The former willkeep a constant dc-link voltage for AC/DC conversion due to theutilization of DC/DC converter, but this extra DC/DC converter de-creases overall efficiency and reliability, and also causes a highercost. These disadvantages prevent the two-stage power converterin applications of high power systems, although it is favored bysome low power systems. The single-stage power converter pre-sents high efficiency, simple structure, and low cost, which is suit-able for high-power energy storage systems. In addition, theaforementioned VRB’s wide stable voltage range encourages thesingle-stage AC/DC converter’s application to VRB-based ESS, be-cause an intrinsic wide stable dc voltage will make an extra DC/DC converter unnecessary. Therefore, this paper only uses an AC/DC converter to control VRB charging and discharging.

3. Wind farm with VRB-based ESS

3.1. System structure

As shown in Fig. 5, VRB-based ESS is directly added at the exit ofgrid-connected wind farm to regulate the grid-injected power,

Fig. 5. Configuration of wind farm with VRB-based ESS.

118 B. Ge et al. / Electrical Power and Energy Systems 44 (2013) 115–122

which does not need to change the existing status of the grid-con-nected wind farm. Since a wind farm consists of many generationunits, each generation unit may contribute random complemen-tary power to the total power, which mitigates the fluctuation oftotal grid-injected power. As a result, the proposed scheme re-quires a smaller total VRB capacity when compared to the distrib-uted installation of VRB-based ESS at the exit or dc-link bus ofevery wind generation unit. The resultant benefits include lowmaintenance, improved system reliability, and low cost, etc.

The configuration of Fig. 5 includes 10 generator units, with atotal installed capacity of 25 MW. Each unit is a direct-drive per-manent magnet synchronous wind turbine, with the rated capacityof 2.5 MW. The VRB-based ESS consists of 30 VRB energy storageunits, with a total rated power of 8.1 MW, and the rated powerof 270 kW per unit. To simplify the illustration of the proposed sys-tem, the paper supposes that every generation unit is the same inthe wind farm, and every VRB-based ESS is also the same.

Every direct-drive wind turbine mainly includes the wind tur-bine, permanent magnet synchronous generator (PMSG), dual-PWM converters, and inductors, through a 690 V/10 kV step-uptransformer connected to the point of common coupling (PCC).As the aforementioned reason, this paper uses a single-stage AC/DC converter as power converter to control VRB charging and dis-charging, and the AC/DC converter’s AC side is connected to theexit of the wind farm through a 380 V/10 kV step-up transformer.There are local loads at the grid side of PCC.

3.2. Direct-drive wind turbine control system

Fig. 6 shows the control strategy of a dual-PWM converter foreach direct-drive wind turbine [33]. The decoupling control of tor-que and reactive power can be achieved by controlling the d-axisand q-axis current components of the generator-side converter,respectively. The active power P and reactive power Q flowing intothe grid can be controlled by the d-axis and q-axis current compo-nents of the grid-side converter, respectively. It is very convenientto adjust the power factor and make the system provide reactivepower support for the grid; also the maximum power point track-ing (MPPT) with hill-climbing method will ensure the capture ofmaximum wind energy [34–36].

3.3. VRB control system

The control principle of the VRB-based ESS is shown in Fig. 7.Active power and reactive power of the VRB-based ESS are

controlled by a bi-directional AC/DC converter, through two givenreferences denoted as P�ref and Q �ref , respectively.

Every direct-drive wind turbine could operate at unity powerfactor by controlling the grid-side converter during normal opera-tion of the grid. If the grid needs reactive power support, the VRB-based ESS provides the required reactive power to the grid by con-trolling the AC/DC converter, and for this case a given reactivepower Q �ref will be set rather than a zero reactive power reference.

In output active power of wind farm, power components over0.01 Hz will badly degrade the power quality of the power system[37]. Therefore, VRB-based ESS is designed to filter this part ofpower fluctuations through a first-order Butterworth High Pass Fil-ter (HPF) in this paper. The designed HPF transfer function GW(s) isexpressed by

GWðsÞ ¼16s

1þ 16sð9Þ

As shown in Fig. 7, PW represents output active power of the windfarm, which is filtered by the HPF GW(s), as a result of the given ac-tive power reference P�ref for the VRB-based ESS. The system opera-tion should avoid the VRB have any over-charge or over-discharge,since it will degrade the VRB performance. Therefore, an energymanagement unit is necessary to ensure the VRB’s safe operation,which will control the VRB’s SOC within an allowable range. TheVRB-based ESS will stop work if the VRB terminal voltage is greaterthan the predefined upper limit or less than the predefined lowerlimit, that is, if VVRB > VVRBmax or VVRB < VVRBmin, there will beP�ref ¼ 0 and Q �ref ¼ 0. The VRB-based ESS is limited to the ratedpower if the reference power P�ref is greater than the rated powerPN. Active power and reactive power of the VRB-based ESS can becontrolled by its d-axis and q-axis current components,respectively.

4. Simulated results

The wind farm with VRB-based ESS is simulated to verify theproposed energy stored grid-connected wind power generationsystem. The parameters of every VRB are listed in Section 2.3,and the parameters of direct-drive wind turbine are as follows:air density q = 1.225 kg/m3, wind turbine radius r = 38.5 m, pitchangle b = 0�; The PMSG rated capacity PSN = 2.5 MW, pole pairsnp = 40, rated frequency fN = 15.885 Hz, stator resistanceRa = 0.001 X, d-axis and q-axis inductance Ld = Lq = 1.5 mH; The

Fig. 6. Control principle of dual-PWM converters.

Fig. 7. Control principle of VRB-based ESS.

Fig. 8. Wind speed.

Fig. 9. Rotor speed of PMSG.

Fig. 10. Active power and reactive power of a wind turbine.

Fig. 11. DC-link voltage of dual-PWM converters in wind turbine.

Fig. 12. Smoothed grid-injected active power.

Fig. 13. Grid-injected reactive power.

B. Ge et al. / Electrical Power and Energy Systems 44 (2013) 115–122 119

grid-side inductance L = 0.5 mH; dc-link capacitor C = 12 mF, andgiven dc-link voltage Udref ¼ U�dc = 1.2 kV.

A wind speed model characterized as gust and stochastic windand a wind turbine model are built to simulate wind power fluctu-ations. As shown in Fig. 8, the wind speed variation of the windfarm presents a gust during the time interval of 20–60 s, and a sto-chastic wind occurs during 60–100 s. The given reactive power ofVRB-based ESS is 0 MVar during 20–60 s, and 1 MVar during 60–

100 s. The PMSG speed xm, the output active power P and the reac-tive power Q of every wind turbine, the DC–link capacitor voltageUdc, the grid-injected active power, and the grid-injected reactivepower are shown in Figs. 9–13, respectively.

120 B. Ge et al. / Electrical Power and Energy Systems 44 (2013) 115–122

From Fig. 9, we can see that the rotor speed of each PMSG is ad-justed to capture the maximum wind energy through the MPPTcontrol when the wind speed is changing.

From Fig. 10, we can find that the active power fluctuations ofeach wind turbine are huge, with a maximum variation of0.8 MW, due to the variation of wind speed and the utilization ofmaximum wind energy. The output reactive power of each windturbine is kept on 0 Var by controlling the grid-side converter.Fig. 11 shows that the dc-link capacitor voltage can be well main-tained at 1.2 kV through the voltage closed-loop control of thegrid-side converter.

Fig. 12 shows that the instantaneous active power fluctuationsof wind farm reach 8 MW. When there is the VRB-based ESS, thepower fluctuations of wind farm can be rapidly smoothed, andthe power fluctuations are filtered by the VRB-based ESS, as a re-sult of a smooth grid-injected power with a maximum fluctuationof 2 MW. As shown in Fig. 12, when Pb < 0, the VRB-based ESS ab-sorbs the superfluous active power from the wind farm; whenPb > 0, the VRB-based ESS provides the extra active power to thegrid for the purpose of smoothing the grid-injected power.

Fig. 13 shows that the output reactive power of the VRB-basedESS is 0 Var during 20–60 s, and �1 MVar during 60–100 s. Reac-tive power of wind farm is kept at 0 Var during the whole opera-tion. Because of Qb < 0, the VRB-based ESS provides the reactivepower to the grid. Therefore, the VRB-based ESS would achievethe purpose to provide the reactive power compensation for thegrid.

5. Experimental verification

An experimental bench of small-scale power, due to limitedconditions, for the purpose of verifying the proposed scheme, con-sists of a 1.5-kW induction motor drive, a 1.5-kW PMSG with the

Fig. 14. Experimental results of the battery based ESS during charging anddischarging. (a) Change from charging to discharging. (b) Change from dischargingto charging.

converter and controller, a grid-side inverter with the controller,a 48-V battery based ESS with an AC/DC converter and its control-ler, and two step-up transformers with a voltage ratio of 15. The di-rect torque control is implemented in the induction motor drive,and the torque closed-loop control will ensure that the inductionmotor torque will simulate the torque of a wind turbine. The mod-els of wind speed (characterized as gust and stochastic wind) andthe wind turbine are built in a RT-LAB real-time simulation plat-form, which will send a torque reference to the induction motordrive that will output a simulated wind power. The grid-side inver-ter is connected to the grid through a step-up transformer and alsothere is a step-up transformer between the grid and AC/DC con-verter of the ESS. The grid of 220-V phase-neutral voltage and50 Hz frequency is employed in the experiments.

Fig. 14 shows the experimental results of the battery based ESSduring charging and discharging, where Fig. 14a is related tochange from charging to discharging, and Fig. 14b related tochange from discharging to charging. At the beginning, the ESSworks on the charging state, and stores the energy coming from

Fig. 15. Experimental results to smooth the grid-injected active power. (a) Activepower. (b) Voltage and current of the battery. (c) Battery voltage and three accurrents.

B. Ge et al. / Electrical Power and Energy Systems 44 (2013) 115–122 121

the induction motor drive; then the ESS works on the dischargingstate, and sends out the energy to the grid, as shown in Fig. 14a.The reactive power is kept on 0 Var because of iq = 0 during thewhole process; but the active power changes its direction of flowbecause the current id varies from a positive value to a negative va-lue. Accordingly, the phase current ia presents an identical phasewith the voltage ua when the ESS is charging, and 180� phase dif-ference when discharging. There is a reverse process in Fig. 14bwhen compared with Fig. 14a, where the ESS discharges at thebeginning, then switching to charging. The results show that theESS has fast dynamic response and good performances with unitypower factor to charge and discharge, although there is a smallcharging or discharging power of 30 W in the experiments.

Fig. 15 shows the smooth grid-injected active power due tousing the ESS. In the experiments, a stochastic wind is fulfilled inthe aforementioned wind speed model through the RT-LAB real-time simulation platform. The stochastic wind causes the wind tur-bine model to output a variable torque reference to the inductionmotor drive, as a result of the induction motor drive’s power fluc-tuations. With the MPPT control, the PMSG generates a fluctuatedactive power Pw fed to the PCC, as shown in Fig. 15a. The ESS con-nected to the PCC absorbs the superfluous active power from theinduction motor drive and compensates the insufficient power tothe PCC. Therefore, the grid is injected with a smooth power Pg

from the PCC. In Fig. 15a, Pb represents the active power of theESS, where the positive value denotes the VRB charging to absorbthe superfluous power and the negative value corresponds to dis-charge VRB for compensating the insufficient power to the grid.Fig. 15b shows the battery voltage and current. It can be seen that,the battery voltage is constant even though the battery currentchanges a lot, where the positive value of the battery current ac-cords to charging and the negative value to discharging. Thegrid-injected current ig_a, the output current ib_a of the ESS, andthe output current iw_a of the grid-connected inverter of the gener-ator unit are shown in a small time interval, as shown in Fig. 15c, toclarify the details of three currents flowing into the PCC. It can beseen that ig_a = ib_a + iw_a. During this interval, the produced activepower of the generator unit is divided into two parts, i.e., one flow-ing into the grid and another one flowing into the ESS. In this way,finally the grid-injected power is effectively smoothed, and thefluctuated part of the active power Pw is filtered out by the ESS.

6. Conclusion

VRB presents many advantages in applications to large-scalepower energy storage. Its terminal voltage remains stable whenthe SOC is within 20–80%, which encouraged this paper to employonly a single-stage AC/DC converter in the VRB-based ESS toachieve charging and discharging controls, as a result of the simplesystem structure with high efficiency. VRB-based ESS was added atthe exit of a grid-connected wind farm to improve power quality ofthe power system through filtering the fluctuations of wind power.The grid-connected wind farm with VRB-based ESS, wind speedcharacterized as gust and stochastic wind, and a wind turbine weremodeled in simulations. Simulation results showed that the grid-injected active power from the wind farm was effectivelysmoothed even if the wind speed varied drastically, also, reactivepower support could be provided for the grid by the designedVRB-based ESS with a rapid dynamic response. Experimental veri-fications were achieved in a low power bench, where a RT-LABreal-time simulation platform and a direct torque controlled induc-tion motor were employed to simulate a real wind turbine. Theexperimental results verified the proposed scheme through dem-onstrating a stable and smooth power flow injected into the grideven though wind power fluctuated.

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