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IEEE TRANSACTIONS ON SMART GRID, VOL. 5, NO. 4, JULY 2014 1961

Application of UPFC to Enhancing OscillatoryResponse of Series-Compensated

Wind Farm IntegrationsSajjad Golshannavaz, Farrokh Aminifar, Member, IEEE, and Daryoush Nazarpour

Abstract—Flexible ac transmission systems, as a key buildingblock of transmission-level smart girds, have shown effective func-tionalities in promoting the system operation security and servicereliability. Facing with series-compensated lines, subsynchronousresonance (SSR) may strike the power system by jeopardizing itsstability and mechanical facilities. The operation of such transmis-sion lines is broadening as a direct result of emergent desire to ex-ploit distant wind energy resources in large scales. This paper ver-ifies the capability of unified power flow controller (UPFC) in at-tenuating SSR in wind farm integrations. SSR is local in its nature;hence, local measurements are merely employed here for control-ling the series convertor while wide-area signals could be as wellutilized in parallel for other objectives such as inter-area oscillationdamping. An equivalent self-excited induction generator (SEIG)represents the wind farm and is connected to the system througha series-compensated line. The UPFC is located at the wind ter-minal of the linking line; thus, the needed reactive power of SEIGis produced by the shunt branch of UPFC. Both SSR types, namelygenerator effect and torsional interaction, are examined here. Sim-ulations are carried out on the IEEE first SSR benchmark modelintegrated with a SEIG wind turbine.

Index Terms—Self-excited induction generator (SEIG) effect,SSR compensation, torsional interaction (TI), unified power flowcontroller (UPFC).

I. INTRODUCTION

T HE smart transmission and distribution grids whichput forward the concept of utilizing real-time extensive

measurements has brought in-depth changes in power systemstudies and control. On the other hand from the energy resourcesviewpoint, fossil fuels seem to gradually become insufficient tohandle the human society demands. The challenge of a loomingenergy crisis has motivated the global emergence toward thewind energy as a free and rich power generating resource [1].It has been planned to serve more than 10% of the total energydemand across the world through the renewable wind powerby the end of 2020 [2]. Along with the swift progress of

Manuscript received November 08, 2013; accepted January 29, 2014. Dateof current version June 18, 2014. Paper no TSG-00837-2013.S. Golshannavaz and D. Nazarpour are with the Department of

Electrical Engineering, Urmia University, Urmia 021, Iran (e-mail:[email protected]; [email protected]).F. Aminifar is with the School of Electrical and Computer Engineering, Col-

lege of Engineering, University of Tehran, Tehran 15556-34414, Iran (e-mail:[email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TSG.2014.2304071

installed capacity of wind farms, large wind turbines (WTs)are commercially manufactured and integrated into the electricpower grids. The generated bulk power should be transmittedthrough the long transmission lines toward the consumptionpoints. Having deferred additional system upgrades, the seriescapacitive compensation is known as a practical solution toincrease the available transfer capacity [3]. However, the seriescompensation and system inherent features may bring about aserious detrimental occurrence referred to as subsynchronousresonance (SSR) which would be an origin of mechanical andelectrical instabilities [4], [5].SSR could comprise either or both torsional interactions

(TI) or induction generator (IG) effect [6]. For a radial-con-nected wind farm operating at the end of a series-compensatedtransmission line, SSR generated due to the IG effect is morelikely [7]. Also, [8] talks about the potential of SSR occurrencein series-compensated wind farm integrations.Based on practical and theoretical reports available in lit-

erature, flexible ac transmission systems (FACTS) are provenas an effective countermeasure for attenuating SSR [9]–[13].Quite few papers attempted the application of various FACTSdevices to attenuate SSR in series-compensated wind farmintegrations. In [11], SSR mitigation in a series-compensatedwind farm was examined utilizing static var compensator(SVC) as well as thyristor-controlled series capacitor (TCSC).Reference [14] has proposed a novel STATCOM controller formitigating SSR, damping oscillations, and enhancing the tran-sient stability margin in a wind park integrated to the system.Some other researchers have addressed SSR damping throughdoubly-fed induction generator (DFIG) [15], [16] and variablespeed wind energy conversion system [17]. Reference [18]demonstrated the use of unified power flow controller (UPFC)in order to improve the stability margin and also to damp outthe power fluctuations in a combined wind and wave energyproduction system connected to bulk power network. Suchcombined power generations prevalently in large scales areunder construction in some countries and installation of ded-icated FACTS devices in those systems could be technicallyand economically sensible.UPFC is the most versatile of FACTS controllers which

yields simultaneous control of all basic power system param-eters including voltage amplitude and angle, line impedance,and power flows [19]. Thanks to these salient capabilities,considerable researches have been devoted to the UPFC in therecent years. Reference [20] has investigated the SSR damping

1949-3053 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

1962 IEEE TRANSACTIONS ON SMART GRID, VOL. 5, NO. 4, JULY 2014

capability of UPFC; however, the series-compensated windfarm connection has not been accounted for. On the otherhand, the advent of modern phasor measurement unit (PMU)technology has made it possible to globally optimize the per-formance of FACTSs and hence has made them much smarterthan before [21], [22]. UPFC could respond to either or bothlocal phenomena, e.g., SSR damping, or system-wide ones, sayinter-area oscillations. In the former case, the local measure-ments would suffice while the latter application necessitatesavailability of wide-area information. These merits justify theutilization of UPFC as the supplementary equipment in powersystems and making the transmission systems much smarter.This paper assesses the contribution of the UPFC to the SSR

mitigation in wind farm connections by series-compensatedlines. A wind farm comprises numerous WTs; however, it is acommon practice in dynamic studies to represent their dynamicbehavior through an equal large WT in conjunction with aself-excited induction generator (SEIG). The IEEE first SSRbenchmark model along with a wind farm is considered herefor simulation purposes. The power sending end of the lineconnecting the wind farm to the remaining system is assumedto be equipped with a UPFC. It controls the bus voltage throughcontrolling the shunt converter and enhances the transmissionline stability margin by means of its series converter. Here,the performance of UPFC in transients particularly with theaim of SSR alleviation is of interest as well. In this way, threedifferent control strategies are considered for the UPFC. First,a subsynchronous damping (SSD) control loop is added to theshunt branch of UPFC and simulation results are obtained.Next, only the series branch is equipped with an SSD controlloop. The latter case manifests a better damping performancethan the former one. In the third case, both SSD control loopsare integrated to the main control system of the UPFC yieldingsimultaneous control of shunt and series branches. Accordingly,the whole capability of UPFC in controlling the power systemis expectedly achieved.The outline of the manuscript is as follows. Different

types of SSR are summarized and categorized in Section II.Power system configuration for studying SSR is illustrated inSection III. Section IV certifies the potential occurrence of SSRin a wind farm connection. Section V addresses the UPFC basicconfiguration and its controller design for the SSR mitigation.The performance of UPFC in mitigation of SSR is examined inSection VI. Eventually, Section VII concludes the paper.

II. SUBSYNCHRONOUS RESONANCE IN POWER SYSTEMS

This section briefly reviews the SSR fundamental mathe-matics, afterward, presents a classification of miscellaneousSSR sorts [4], [5]. The series-compensated line would includesubsynchronous currents with electrical frequency of

(1)

where is the series capacitor reactance, is the equalreactance of the transmission line, generator, and transformer,also stands for the nominal frequency of power system. The

Fig. 1. Equivalent circuit of a generic induction machine.

generated subsynchronous currents will result in rotor torque atthe complementary frequency of

(2)

The electrical resonance due to the series compensation cre-ates a rotating field on the generator stator corresponding to theresonant frequency. For the high compensation levels, the elec-trical resonance frequency, , would be less than the systemnominal frequency, . The resonant current drawn by the elec-trical network hence creates a revolving field at the subsyn-chronous frequency. For the case of synchronous generators, thegenerator rotor is rotating at synchronous frequency; the syn-chronous machine consequently acts as an IG according to thesubsynchronously rotating field. On the other hand, the powersystem such as wind farms, which is under investigation here,may comprise SEIGs. For both sorts, the slip of the machine thatis seen as an IG is given by

(3)

Since the electrical resonance frequency is less than thesystem nominal frequency, the slip is negative. Hence, the re-sistance of the rotor (at the subsynchronous frequency from thearmature terminals perspective) is negative likewise. Referringto the generic equivalent circuit of a SEIG depicted in Fig. 1,a self-excitation circumstance appears when the magnitude ofthis resistance exceeds the sum of the armature and networkresistance at the resonant frequency.Considering mechanical system’s dynamics, torsional inter-

actions (TIs) might happen in the overall electrical-mechanicalsystem. The generator rotor torsional oscillation with the fre-quency induces armature voltage components at the frequen-cies of

(4)

When the subsynchronous frequency componentcomes in closer to or equals with any of the electric resonancefrequency, , the torsional oscillation and electrical resonancewill be mutually excited resulting in SSR.

III. IEEE FIRST SSR BENCHMARK MODEL INTEGRATEDWITH WIND FARM AND UPFC

The well-known IEEE first SSR benchmark model [4] iswidely used for SSR studies. This system, with some modifi-cations, is employed here and shown in Fig. 2. Generally, thesending end of the compensated-transmission line may includea conventional power plant or renewable energy farms, saywind or sea-wave. Here, since our studies focus on only wind

GOLSHANNAVAZ et al.: APPLICATION OF UPFC TO ENHANCING OSCILLATORY RESPONSE 1963

Fig. 2. IEEE first SSR benchmark model incorporating various generations, UPFC connected to the compensated line, and wide-area data.

farm behavior and capabilities, the other types of generationsare overlooked. The feedback signals could either comprisewide-area information through utilization of PMUs dispersedover the network or be limited to merely local data capturedat the line’s sending end. Since this paper concentrates onthe SSR phenomenon which is inherently a local issue, onlylocal data are used for the closed-loop control objective. Theseries-compensated line is connected to an infinite bus whichrepresents the remaining parts of the power system.In Fig. 2, the wind farm is represented by a number of co-

herent IGs each of 0.746 MW rating which is connected to thegrid through a fixed series-compensated line. Note that for thesimulation purposes, the integration of wind farm is representedby an induction generator. In practical systems [1] and [23], de-pending on the installed capacity of wind generation, its poweroutput may reach 500 MW. Also, the series compensation levelis not usually raised to the values greater than 75%, but in thisresearch it is assumed that a high compensation level of 90%is practical likewise. Such assumption, although rather imprac-tical, are necessary to cover a broad spectrum of SSR behaviorsand to figure out the extreme cases. Analogously, this paper hassome simulation cases in which the wind power generation isequal to 500 MW or the series compensation level is adopted as90%. The conclusions raised here could be generalized for realcases where the SSR phenomenon might occur with reasonablevalues of key parameters discussed above.The mechanical system of the wind turbine consists of a two-

mass torsional system where one represents the wind turbinesystem and the other is for the set of IGs. The rated voltage andfrequency are 539 kV and 60 Hz, respectively. The transmissionline is modeled with an inductive reactance, a resistance, anda series fixed capacitive compensation reactance. The reactivepower support of generator is achieved through the shunt branchof UPFC. The complete electrical and mechanical data for thesystem are given in Table I.

TABLE IELECTRICAL AND MECHANICAL SYSTEMS DATA

As illustrated in Fig. 2, the UPFC is composed of a SSSC(VSI 2) and a STACOM (VSI 1) which are coupled togetherthrough a common DC link capacitor providing bidirectionalreal power exchanges between the series and shunt inverters.The series and shunt inverters are able to independently generateor absorb the reactive power as well. With the series reactivevoltage injection, the active series compensation is achieved.The injection of controlled shunt reactive current leads to thevoltage regulation at bus where shunt inverter is connected.Here, the UPFC is mainly considered as an SSR countermea-sure and its parameters are reported in Table II.

IV. POTENTIAL OCCURRENCE OF SSR IN THE

SERIES-COMPENSATED WIND FARM

It would be fruitful to present a brief study and discussionabout the circumstances augmenting the probability of SSRoccurrence in series compensated wind farm integrations. Thesimulations would let us recognize appropriate cases for further

1964 IEEE TRANSACTIONS ON SMART GRID, VOL. 5, NO. 4, JULY 2014

TABLE IIUPFC DATA

Fig. 3. Electromagnetic torque oscillation for different wind farm power outputvalues and 90% series compensation.

studies. Hence, in this section, the probable occurrence ofSSR is illustrated. Here, only the study of the induction gen-erator self-excitation effect is covered in the system shown inFig. 2. Simulations are carried out using PSCAD/EMTDC andMATLAB/Simulink. As the first step, the UPFC is eliminatedfrom the system so that the potential occurrence of the SSR canbe explored in a FACTS-unequipped wind farm integration. Thecontingency stimulating the SSR is a three-phase-to-groundfault located close to the infinite bus. The following discussionillustrates that the power output of the wind farm as well aslevel of the series compensation are two main factors whichmay initiate the self-excitation phenomenon.The wind farm output power value is adjusted in three dif-

ferent values: 100, 300, and 500 MW. All cases are accompa-nied with a fixed 90% series compensation. Such a high com-pensation level and output power are also considered in somemanuscripts such as [11] and [14]. The electromagnetic torqueis separately obtained for each case and the time-domain repre-sentation is shown in Fig. 3. It is observed from Fig. 3 that forthe power transfer of 100 MW, the self-excitation effect is notremarkable even with a high series compensation level. How-ever, with the increase of power transfer level, the magnitudeof oscillations are becoming larger and for the power transfergreater than 300 MW, say 500 MW, the SSR oscillation makesthe system unstable.Now, the wind farm output power is assumed to be 500 MW

constant and the series compensation level gets different values.Fig. 4 displays the electromagnetic torque for 50, 70, and 90%line series compensation. It is deduced that as more series com-pensations are supported, larger oscillation magnitudes appearand the system instability might be even rendered. Talking aboutthe IG effect, the series compensation levels greater than 75%

Fig. 4. Electromagnetic torque oscillation for different series compensationlevels and 500 MW power transfer.

with power transfer greater than 300 MW are susceptible tocause unstable SSR. Regarding torsional interactions (TIs), sim-ilar sets of simulations are conducted. It has been observed thatassociated with different series compensation levels as well asdifferent wind power production levels; there would be someTI fluctuations while being stable with passing the time. Hence,no unstable mode of TI oscillations is observed; however, theirdamping behaviors are to some extent poor.

V. UPFC CONTROLLERS FOR SSR MITIGATION

This section analyzes the UPFC control circuits in order tomitigate SSR. Two cases designated as the base case and withSSD controller are investigated. The control algorithm in thebase case corresponds to the voltage regulation and power flowadjustment while that of SSD has further objectives to keep thefrequency deviation limited.

A. Base Case

As pointed out earlier, the UPFC is composed of two invertersconnected back-to-back through a regulated voltage DC link.Since the UPFC is pretty flexible and is able to simultaneouslyaffect a couple of power system basic parameters, a diverse setof objectives could be desirable and the controller module isaccordingly designed. In the technical literature, various appli-cations of UPFC in conjunction with control algorithms havebeen elaborated. In most cases, the UPFC is intended to con-trol its bus voltage by generating or absorbing reactive powerthrough the shunt inverter as well as controlling the power flowon the transmission line by adjusting the magnitude and phaseshift of the series inverter injected voltage. This is the case spec-ulated here and the active power reference for the series inverteris the same as the generated power by the wind farm. Also, if thewind turbine is equipped with pitch angle control, it is techni-cally possible to set the reference of active power equal to valuesdetermined by the control center for improving the stability andperformance of the whole power system.The UPFC control system includes two major subsystems.

One represents the UPFC’s shunt inverter control system whichis shown in Fig. 5(a) while the other is for the UPFC’s seriesinverter control system depicted in Fig. 5(b). The UPFC’s shunt

GOLSHANNAVAZ et al.: APPLICATION OF UPFC TO ENHANCING OSCILLATORY RESPONSE 1965

Fig. 5. Block diagram of the UPFC control system. (a) Shunt inverter con-troller. (b) Series inverter controller.

Fig. 6. SSD controller (a) for shunt branch (b) for series branch.

inverter control system generates the control signals andwhich are used to compute the modulation index and

the phase angle for the shunt inverter. Similarly, the seriesinverter control system generates proper control signalsand to determine the modulation index and the phaseangle associated with the series inverter. For both shunt andseries inverter control systems, the limiters are adjusted in orderto keep the injected currents and voltages within the specifiedlimits ( , and ) [11]. The auxiliarydamping signals in Fig. 5 are equal to zero in this case.

B. UPFC With SSD Controllers

The UPFC control system described above, does not pro-vide, by itself, the essential damping of oscillations since itsprimary mission is to regulate the bus voltage and to control thepower flow in corresponding transmission line. However, theUPFC controllable signals including , and canbe modulated in a desired way to provide some other ancillaryduties such as SSD, power oscillation damping, etc.In order to achieve an effective damping of SSR, it is indis-

pensible to apply synchronized tuning of UPFC with auxiliarySSD controllers. Fig. 6 presents two controllers which are re-spectively granted to shunt and series branch control systems.Two controllers depicted by Fig. 6 generate auxiliary signals

for the main control circuits shown in Fig. 5. Referring to Fig. 5,

TABLE IIIDATA FOR SSD CONTROLLERS

with the aim of achieving effective damping of oscillations, theoutput of SSD controller is utilized to modulate in shuntconverter. In contrast, the output of SSD controller in conjunc-tion with the series branch is devised to regulate with theaim of providing the proper damping. As illustrated in Fig. 6,a gain block, a washout filter, and a lead-lag compensator com-prise the building blocks of a SSD controller. signifies theangular frequency difference and is speculated as the feedbackinput signal [24]. The design process for each SSD is such thatan additional electrical torque, which is in phase with the speeddeviation, would be produced to improve the damping of oscil-lations [25]. The gain settings for SSDs are determined suchthat a desired damping ratio for the subsynchronous oscilla-tions is generated. Also a washout filter is included suitably toeliminate the effect of SSDs in steady-state power conditions.Herein, a trial-and-error approach has been applied for tuningthe different parameters of SSDs through the simulation studiesto achieve the best damping performance. However, intelligentand heuristic algorithms might be employed as well. The gen-erated output signals by auxiliary SSD controllers are utilizedto adjust the reference settings of UPFC in order to realize theSSD objective.For the sake of comprehensive studies, three different con-

trol strategies are considered. In the first attempt, only the SSDcontroller is added to the shunt inverter control system. In thesecond trial, a single SSD controller is designed and added tothe series inverter control system. Finally, it is deemed that twoSSD controllers for both shunt and series inverter control sys-tems are operating simultaneously. In this case, the total capa-bility of UPFC could be realized. The detailed data for both SSDcontrollers are given in Table III.

VI. SSR MITIGATION USING UPFC IN THE

SERIES-COMPENSATED WIND FARM

In the following subsections, the performance of UPFC inalleviating the SSR is examined in different cases.

A. Suppressing IG Effect

In order to investigate the UPFC capability in the mitigationof IG effect, the power output of the wind farm is adjusted atthe 500 MW with a high series compensation level of 85%.Note that these assumptions, although might be impractical forreal world applications, are accommodated here to demonstratethe performance of the proposed methodology in worst condi-tions. Having approved the effectiveness of the control strategyin these adverse conditions, one can conclude the successful op-eration of the damping controllers in realistic situations where

1966 IEEE TRANSACTIONS ON SMART GRID, VOL. 5, NO. 4, JULY 2014

Fig. 7. IG-effect SSRmitigation using UPFC (a) generator rotor speed (b) elec-tromagnetic torque (c) generator terminal voltage.

the oscillations are likely much smaller. For the sake of simula-tion, following case studies are focused• Without UPFC: Corresponds to Section IV.• With UPFC: Corresponds to Section V.A.• With SSD controller on shunt branch: Just the controller ofFig. 6(a) is considered.

Fig. 8. (a) Auxiliary damping signal 1. (b) Auxiliary damping signal 2.

• With SSD controller on series branch: Just the controllerof Fig. 6(b) is incorporated.

• With both SSD controllers: Two controllers associated withshunt and series branches are functioning concurrently.

Fig. 7 displays oscillations in generator rotor speed, electro-magnetic torque, and generator terminal voltage. This figure re-veals the capability of UPFC with various controllers in mit-igating the IG effect. Referring to Fig. 7(a), it is observablethat, in contrast to the instability for the case without UPFC, theUPFC even with no specific controller prevents divergent SSRoscillations. The damping performance is, however, very poor.Existence of the SSD controller on the series branch providesa better damping compared to that with the SSD controller onthe shunt branch. This conclusion is due to the greater poten-tial of the series inverter in controlling the line power transferby affecting the line’s series impedance. As it can be seen inFig. 7(a), the best possible damping is achieved when both ofSSD controllers are performing together on both shunt and se-ries control systems. In this case, the whole capability of theUPFC is indeed utilized to get the substantial damping of os-cillations. This conclusion puts forward thinking the series in-verter as the stronger countermeasure for the local fast responsephenomena such as SSR damping and the shunt inverter for

GOLSHANNAVAZ et al.: APPLICATION OF UPFC TO ENHANCING OSCILLATORY RESPONSE 1967

Fig. 9. TI-effect SSR mitigation using UPFC. (a) Generator rotor speed. (b)Mechanical torque between mass 1 and mass 2.

the slower global phenomena such as low-frequency oscilla-tions. Fig. 7(b) and (c) demonstrates the stable behaviors of theelectromagnetic torque and terminal voltage quantities, respec-tively, when a UPFC equipped with both SSD controllers is inuse.As earlier depicted in Fig. 5(a), the auxiliary damping

signal 1 is used to modulate the shunt inverter voltage angle,. The limiter max and min values for the SSD controller in

shunt branch are adjusted at 20 and 20 degrees, respectively.Fig. 8(a) illustrates the auxiliary damping signal 1 generatedby the corresponding SSD controller. Also, Fig. 5(b) exhibitsthat the auxiliary damping signal 2 is utilized to appropriatelyalter the modulation index of the series inverter, namely, .The max and min values for SSD controller related to the seriesbranch are set to 0.4 and 0.4, respectively. Fig. 8(b) shows theauxiliary damping signal 2 which is yielded by the respectiveSSD controller.

B. Suppressing TI Effect

For the purpose of assessing the UPFC performance in mit-igating TI phenomenon, the wind farm power output is regu-lated at 50 MW with 85% series compensation to simulate theworst case. These situations are adopted based on the studiesconducted in Section IV where it was observed that for higher

values of power transfers, the IG is the most potent one and theTIs are not seen at all. The cases investigated here are as follows:• Without UPFC: Corresponds to Section IV.• With SSD controller on shunt branch: Just the controller ofFig. 6(a) is considered.

• With SSD controller on series branch: Just the controllerof Fig. 6(b) is incorporated.

• With both SSD controllers: Two controllers associated withshunt and series branches are functioning coincidently.

The generator rotor speed and the mechanical torque betweenmass 1 and mass 2 are utilized to illustrate the performance ofUPFC. Fig. 9 represents the simulation results acquired. The su-perior performance of SSD controller on series branch of UPFCthan its shunt branch is observable. On the other hand, the ideaof using two SSD controllers coordinated with UPFC conven-tional controllers provides the total exploitation of UPFC’s ca-pabilities. That is, the best damping of TI oscillations is ac-quired with the both SSD controllers operating together. Theseobservations are in accordance with those associated with theIG effect SSR; in the sequel, having damping controllers onboth series and shunt branches is the most effective solution fordamping either type of SSR oscillations.

VII. CONCLUSION

The credible occurrence of SSR in a series-compensated windfarm for different compensation levels and over a wide range ofwind power generations has been explored in this paper. It is ob-served that in bulk capacity wind integrations, the occurrence ofSSR phenomenon due to either IG or TI effects is likely. It wasshown that the IG effect SSR is the most potent one to makethe system unstable whereas the TIs hardly exhibit unstable be-havior while still stressing the mechanical systems. This studyserved a rather comprehensive exploration on the SSR miti-gating effect yielded by the UPFC utilization in the transmis-sion system. For the sake of achieving the best damping per-formance of oscillations, the idea of utilizing two SSD con-trollers coordinated with UPFC’s main controllers was inter-rogated through the extensive time domain simulations. Sim-ulation results demonstrated that the UPFC with a SSD con-troller on series branch offers more superior damping of IG ef-fect and TIs than that associated with the shunt branch. Thisfeature arises due to the stronger effect of series inverter onthe line power transfer by altering the line’s series impedance.Hence, it would be so fruitful to make use of UPFC’s seriesinverter as the controller of the fast response local SSR. How-ever, when two SSD controllers are considered to operate con-currently, the UPFC yields the best achievable damping of SSR.This study demonstrated that the idea of having a UPFC withtwo supplementary controllers would be more beneficial in at-tenuating SSR as well as enhancing power system stability en-hancement. Not surprisingly, from the SSR alleviation pointof view, UPFC outperforms other members of FACTS devicefamily which have only shunt or series branches. In contrastto SSR which is intrinsically a local phenomenon, the powersystems are facing other types of oscillations with system widedimensions. For such situations, the controllers should be fedby wide-area signals which are becoming available by growing

1968 IEEE TRANSACTIONS ON SMART GRID, VOL. 5, NO. 4, JULY 2014

PMU installations. In these applications, the most critical pointis the time-varying communication system latency. These issuesare open future research topics in the field of smart transmissiongrids.

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Sajjad Golshannavaz received the B.Sc. (Honsors) and M.Sc. (Honsors) de-grees in electrical engineering from Urmia University, Urmia, Iran, in 2009 and2011, respectively. He is currently pursuing the Ph.D. degree in the School ofElectrical and Computer Engineering, University of Tehran, Tehran, Iran.His research interests are in smart grid technologies, optimal operation

scheduling planning, distribution renewable energies, and FACTS applicationsin power systems.

Farrokh Aminifar (S’07–M’11) received the B.Sc. (Honsors) degree from theIran University of Science and Technology, Tehran, Iran, in 2005, and the M.Sc.(Honsors) and Ph.D. degrees from the Sharif University of Technology, Tehran,in 2007 and 2010, respectively, all in electrical engineering.He has been collaborating with the Robert W. Galvin Center for Electricity

Innovation with the Illinois Institute of Technology, Chicago, IL, USA, sinceMarch 2009. He is currently an Assistant Professor with the School of Elec-trical and Computer Engineering, the University of Tehran, Tehran. His researchinterests include wide-area measurement systems, reliability modeling and as-sessment, and smart grid technologies.Dr. Aminifar has served as the Guest Editor-In-Chief and Associate Editor of

three Special Issues of the IEEE TRANSACTIONS ON SMART GRID. He receivedthe IEEE Best Ph.D. Dissertation Award from the Iran Section for his researchon the probabilistic schemes for the placement of pharos measurement units. Heis the recipient of the 2013 IEEE/PSO Transactions Prize Paper Award.

Daryoush Nazarpour received the B.Sc. degree from the Iran University ofScience and Technology, Tehran, Iran, in 1982 and the M.Sc. and Ph.D. degreesfrom the Tabriz University, Tabriz, Iran, in 1988 and 2005, respectively, all inelectrical engineering.He is currently an Associate Professor with the School of Electrical and Com-

puter Engineering, the Urmia University, Urmia, Iran. His research interestsare primarily in advanced power electronic and FACTS applications in powersystems.