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  38 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 1, NO. 1, APRIL 2010 An Overview of SMES Applications in Power andEnergy Systems Mohd. Hasan Ali  , Senior Member, IEEE  , Bin Wu  , Fellow, IEEE  , and Roger A. Dougal  , Senior Member, IEEE   Abstract— Superconducting magnetic energy storage (SMES)is known to be a very good energy storage device. This articleprovides an overview and potential applications of the SMEStechnology in electrical power and energy systems. SMES iscategorized into three main groups depending on its power condi-tioningsystem,namely,thethyristor-basedSMES,voltage-source-converter-based SMES, and current-source-converter-basedSMES. An extensive bibliography is presented on the applicationsof these three types of SMES. Also, a comparison is made amongthese three types of SMES. This study provides a basic guide-line to investigate further technological development and newapplications of SMES, and thus benefits the readers, researchers,engineers, and academicians who deal with the research works inthe area of SMES.  Index Terms— Current source converter(CSC),electrical powerand energy systems, superconducting magnetic energy storage(SMES), thyristor, voltage source converter (VSC). I. I NTRODUCTION A VARIETY of storage technologies are in the marketbut the most viable are battery energy storage systems(BESS), pumped storage hydroelectric systems, and supercon-ducting magnetic energy storage (SMES) systems. Some of the disadvantages of BESS include limited life cycle, voltageand current limitations, and potential environmental hazards.Again, some of the disadvantages of pumped hydro electricare large unit sizes, topographic and environmental limitations.SMES isa largesuperconductingcoilcapableofstoringelectricenergy in the magnetic field generated by dc current flowingthrough it [1]. The real power as well as the reactive power canbe absorbed by or released from the SMES coil according tosystem power requirements. Although superconductivity wasdiscovered in 1911, SMES has been under study for electricutility energy storage application since the early 1970s [2].SMES systems have attracted the attention of both electricutilities and the military due to their fast response and highefficiency (a charge–discharge efficiency over 95%). Possibleapplications include load leveling, dynamic stability, transientstability, voltage stability, frequency regulation, transmissioncapabilityenhancement,powerqualityimprovement,automatic Manuscript received November 30, 2009; accepted February 11, 2010. Dateof publication March 18, 2010; date of current version April 19, 2010.M. H. Ali and R. A. Dougal are with the Electrical Engineering Department,UniversityofSouthCarolina,Columbia,SC29208USA( Wu is with the Department of Electrical and Computer Engineering, Ry-erson University, George Vari Engineering and Computing Center, Toronto, ONM5B 1Z2, Canada.Digital Object Identifier 10.1109/TSTE.2010.2044901 generation control, uninterruptible power supplies, etc. The onemajor advantage of the SMES coil is that it can discharge largeamounts of power for a small period of time. Also, unlimitednumber of charging and discharging cycles can be carried out[3]–[8].In SMES systems, it is the power conditioning system(PCS) that handles the power transfer between the super-conducting coil and the ac system. According to topologyconfiguration, there are three kinds of PCSs for SMES, namely,the thyristor-based PCS [9]–[18], voltage source converter(VSC)-based PCS [19]–[28], and current source converter(CSC)-based PCS [29]–[38]. The thyristor-based SMES cancontrol mainly the active power, and has a little ability to con-trol the reactive power, also the controls of active and reactivepowers are not independent [39]–[42]. On the other hand, boththe VSC- and CSC-based SMES can control both active andreactive powers independently and simultaneously. Therefore,the applications in which mainly the active power control isrequired, the thyristor-based SMES is used [43]–[52], while theapplications in which reactive power or both active and reactivepower controls are required, the VSC- [53]–[62] or CSC-basedSMES [63]–[70] is used.Thispaperattemptstopresentanoverviewandabibliographyon the SMES technology. A comprehensive set of referencesmainly published in archival journals and international confer-ences starting from the early 1970s to now on the above-men-tioned three types of SMES applications are presented. To thebest of our knowledge, it is the most up-to-date information onthe bibliography of the SMES applications in power and en-ergy systems. The potential applications and cost-effectivenessof SMES are discussed in this context. Moreover, a comparisonis made among these three types of SMES. It is hopedthis studywouldserveasabasicguidelinetoinvestigatefurthertechnolog-ical development and new applications of SMES, and thus ben-efit the readers, researchers, engineers, and academicians whodeal with the research works in the area of SMES.The organization of this paper is as follows. Section IIdescribes the overview of SMES technology. Section III de-scribes the applications of SMES in power and energy systems.In Section IV, the cost-effectiveness of SMES is discussed.Section V provides some conclusions regarding this work.II. O VERVIEW OF  SMES T ECHNOLOGY AND  C ONTROLS An SMES device is a dc current device that stores energy inthe magnetic field. The dc current flowing through a supercon-ducting wire in a large magnet creates the magnetic field. Sinceenergyisstoredascirculatingcurrent,energycanbedrawnfroman SMES unit with almost instantaneous response with energy 1949-3029/$26.00 © 2010 IEEE Authorized licensed use limited to: University of South Carolina. Downloaded on May 13,2010 at 15:13:48 UTC from IEEE Xplore. Restrictions apply.  ALI  et al. : OVERVIEW OF SMES APPLICATIONS IN POWER AND ENERGY SYSTEMS 39 Fig. 1. SMES unit with six-pulse bridge ac/dc thyristor controlled converter. stored or delivered over periods ranging from a fraction of asecond to several hours.An SMES unit consists of a large superconducting coil atthe cryogenic temperature. This temperature is maintained bya cryostat or dewar that contains helium or nitrogen liquid ves-sels. A bypass switch is used to reduce energy losses when thecoil is on standby. And it also serves other purposes such as by-passing dc coil current if utility tie is lost, removing converterfrom service, or protecting the coil if cooling is lost [71].Several factors are taken into account in the design of the coilto achieve the best possible performance of an SMES systemat the least cost [5]. These factors may include coil configura-tion, energy capability, structure, and operating temperature. Acompromise is made between each factor considering the pa-rameters of energy/mass ratio, Lorentz forces, stray magneticfield, and minimizing the losses for a reliable, stable, and eco-nomic SMES system. The coil can be configured as a solenoidor a toroid. The solenoid type [56] has been used widely dueto its simplicity and cost effectiveness. Coil inductance orPCSmaximumvoltage andcurrent ratingsdeter-mine the maximum energy/power that can be drawn or injectedby an SMES coil. The ratings of these parameters depend onthe application type of SMES. The operating temperature usedfor a superconducting device is a compromise between cost andthe operational requirements. Low-temperature superconductor(LTS) devices are available now, while high-temperature super-conductor devices are currently in the development stage.Different types of SMES technologies and their controlmethodologies are described below.  A. Thyristor-Based SMES  Fig. 1 shows the basic configuration of a thyristor-basedSMES unit, which consists of a Wye-Delta transformer, anac/dc thyristor controlled bridge converter, and a supercon-ducting coil or inductor.The converter impresses positive or negative voltage onthe superconducting coil. Charge and discharge are easilycontrolled by simply changing the delay angle that controlsthe sequential firing of the thyristors [72]–[81]. If is less than90 , the converter operates in the rectifier mode (charging).If is greater than 90 , the converter operates in the invertermode (discharging). As a result, power can be absorbed fromor released to the power system according to requirement. Atthe steady state, SMES should not consume any real or reactivepower [82]–[91]. Fig. 2. Basic configuration of VSC-based SMES system. The voltage of the dc side of the converter is expressedby(1)where is the ideal no-load maximum dc voltage of thebridge. The current and voltage of superconducting inductor arerelated as(2)where is the initial current of the inductor. The real powerabsorbed or delivered by the SMES can be given by(3)Since the bridge current is not reversible, the bridgeoutput power is uniquely a function of , which can bepositive or negative depending on . If is positive,power is transferred from the power system to the SMES unit.While if is negative, power is released from the SMES unit[92]–[101]. The energy stored in the superconducting inductoris(4)where is the initial energy in theinductor.  B. VSC-Based SMES  Fig.2 showsthebasic configuration ofthe VSC-basedSMESunit [102]–[111], which consists of a Wye-Delta transformer,a six-pulse pulse width modulation (PWM) rectifier/inverterusing insulated gate bipolar transistor (IGBT), a two-quadrantdc-dc chopper using IGBT, and a superconducting coil orinductor. The PWM converter and the dc-dc chopper are linkedby a dc link capacitor.The PWM VSC provides a power electronic interface be-tween the ac power system and the superconducting coil. Thecontrol system of the VSC is shown in Fig. 3. The proportional-integral (PI) controllers determine the reference d- and q-axis Authorized licensed use limited to: University of South Carolina. Downloaded on May 13,2010 at 15:13:48 UTC from IEEE Xplore. Restrictions apply.  40 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 1, NO. 1, APRIL 2010 Fig. 3. Control system of the VSC. currents by using the difference between the dc link voltageand reference value , and the difference betweenterminal voltage and reference value , respectively.ThereferencesignalforVSCisdeterminedbyconvertingd-andq-axis voltages which are determined by the difference betweenreference d-q axescurrents and their detected values. The PWMsignal is generated for IGBT switching by comparing the refer-ence signal which is converted to three-phase sinusoidal wavewith the triangular carrier signal. The dc voltage across the ca-pacitor is kept constant throughout by the six-pulse PWM con-verter [112]–[121].The superconducting coil is charged or discharged by atwo-quadrant dc-dc chopper. The dc-dc chopper is controlledto supply positive (IGBT is turned  ON ) or negative (IGBT isturned  OFF ) voltage to SMES coil and then the storedenergy can be charged or discharged. Therefore, the supercon-ducting coil is charged or discharged by adjusting the averagevoltage across the coil which is determined by the dutycycle of the two-quadrant dc-dc chopper. When the duty cycleis larger than 0.5 or less than 0.5, the stored energy of the coilis either charging or discharging. In order to generate the PWMgate signals for the IGBT of the chopper, the reference signal iscompared with the triangular signal [122]. C. CSC-Based SMES  Fig. 4 shows the basic configuration of the CSC-based SMESunit. The dc side of CSC is directly connected with the super-conducting coil, and its ac side is connected to thepower line. Abank of capacitors connected to a CSC input terminal is utilizedto buffer the energy stored in line inductances in the processof commutating direction of ac line current. Furthermore, thecapacitors can filter the high-order harmonics of the ac linecurrent. In CSC, through regulating the trigger signals of theswitching devices, the current in the superconducting coil canbe modulated to generate controllable three-phase PWM cur-rent at the ac side. As the SMES system is inherently a currentsystem, the transfer of both active and reactive powers betweenthe CSC and power network is very fast [36]. Fig. 4. SMES system with a CSC.Fig. 5. Block diagram of the dc current control algorithm. In case of 12-pulse CSC-based SMES, to improve the totalharmonics distortion (THD) of the ac source currents, an op-timal PWM switching strategy is used to minimize the 5th, 7th,11th, and 13th harmonics. It has been proved that the 5th, 7th,11th, and 13th harmonics can be minimized to zero with themodulation index ranging from 0.2 to 1 [37]. Compared to a6-pulse CSC, the 12-pulse CSC has smaller voltage ripples onthe dc side, which means a further reduction of the ac losses inthe SMES coil.Forthemagnettraining,a dc current controlalgorithm isapplied [37]. The block diagram is shown in Fig. 5, whereis the reference value of , PI is a proportional-integral regu-lator, is the inductance of the SMES coil, is the resistancein the dc circuit, and is the dc voltage. With the phase anglebeing fixed to zero, the dc voltage is proportional to the mod-ulation index , which determines the charging rate.  D. Comparison of Thyristor-Based, VSC-Based, and CSC-Based SMES  Table I shows a comparison of the thyristor-based,VSC-based, and CSC-based SMES. The comparison is donein terms of real and reactive powers control ability, controlstructure, THD, and SMES coil voltage ripple.III. A PPLICATIONS OF  SMES  IN  P OWER AND  E NERGY  S YSTEMS ItisthefastresponsethatmakesSMESabletoprovidebenefittomanypotentialutilityapplications.TheapplicationsofSMESare described in the following.1)  Energy storage —An SMES unit could provide the poten-tial for energy storage of up to 5000 MWh with a high re-turn efficiency (up to 95% for a large unit) and a rapid re-sponse time for dynamic change of energy flow (millisec-onds)[123]–[131].Thisaspectmakesitidealforlargevari-ations in energy requirements between daytime peak de-mand and off-peak back-down as well as large amounts of energy available for replacement of major unit trips. This Authorized licensed use limited to: University of South Carolina. Downloaded on May 13,2010 at 15:13:48 UTC from IEEE Xplore. Restrictions apply.  ALI  et al. : OVERVIEW OF SMES APPLICATIONS IN POWER AND ENERGY SYSTEMS 41 TABLE IC OMPARISON OF  SMES T ECHNOLOGIES may providefor thepotential reduction of spinning reserverequirements.2)  Load following —An SMES unit has the ability to followsystem load changes almost instantaneously which pro-vides for conventional generating units to operate at con-stant output [123], [126].3)  System stability —An SMES unit has the capability todampen out low frequency power oscillations and to stabi-lize system frequency as a result of system transients [42],[74], [96], [120], [123], [124].4)  Automatic Generation Control —An SMES unit can bethe controlling function in an AGC system to provide for aminimum of area control error (ACE) [123].5)  Spinning reserve —In case a major generating unit ormajor transmission line is forced out of service, a certainamount of generation must be kept unloaded as “spinningreserve.” An SMES unit can represent a tremendousamount of spinning reserve capacity when in the chargedmode. This lowers the costs for spinning reserve require-ments overcomparable values and methods of maintainingspinning reserve [123], [124], [126].6)  Reactive volt-ampere (VAR) control and power factorcorrection —An SMES unit can increase the stability andpower carrying capacity of a transmission system [123].7)  Blackstartcapability —AnSMESunitcanprovidepowerto start a generating unit without power from the grid. Thisprovides for grid restoration when area failures have oc-curred [123].8)  Bulkenergymanagement —AnSMESunithastheabilityto store large quantities of energy, and thus can act as astorage and transfer point for bulk quantities of energybased on the economics, potentially lowering the cost of electricity [123].9)  Transientvoltagedipimprovement —Atransientvoltagediplastingfor10–20cyclescanresultwhen amajordistur-bance on the power system occurs. SMES and associatedconverter equipment has been shown to be effective forproviding voltage support which can result in increasingthe power transfer limitations on the transmission system[127].10)  Dynamic voltage stability —Dynamic voltage instabilitycan occur when there is a major loss of generation orheavily loaded transmission line and there is insuffi-cient dynamic reactive power to support voltages. SMEShas been shown to be effective in mitigating dynamicvoltage instability by supplying real and reactive powersimultaneously supplanting loss of generation or a majortransmission line [126], [127].11)  Tie line control —When power is scheduled betweenutility control areas, it is important that the actual netpower matches closely with the scheduled power. Unfor-tunately, when generators are ramped up in one controlarea and down in the receiving control area to send power,the system load can change causing an error in the actualpower delivered. This ACE can result in inefficient useof generation. SMES can be designed with appropriate Authorized licensed use limited to: University of South Carolina. Downloaded on May 13,2010 at 15:13:48 UTC from IEEE Xplore. Restrictions apply.
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