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24IEEEpower & energy magazinemarch/april 2005by Brad Robertsand Jim McDowall1540-7977/05/$20.002005 IEEEmarch/april 2005IEEEpower & energy magazine25LLARGE-CAPACITY ELECTRICITY STORAGE SYSTEMS ARE IN DAILY USEaround the world, helping to stabilize the electricity delivery infrastructure and mini-mize the cost of meeting peak-load requirements. Water, as a storage medium, is in itssimplest form in a lake behind a dam. By adding sophisticated pumping systems, waterfrom a lake or river can be transferred to another lake (usually man-made) at the top ofa hill. The motor-driven turbines for pumping the water uphill can be reversed andbecome electrical generators when the water flows back down. These pumped-hydrosystems typically store the equivalent of hundreds of megawatt-hours of electricalenergy and, because of their large power and storage capacities, are relatively inexpen-sive on a cost-per-kilowatthour basis. These systems typically operate on a diurnalbasis: charging with inexpensive utility energy at night and discharging during periodsof peak load demand.Such systems, however, are geographically limited, and mostof the feasible sites have been exploited already. Finding newreservoirs is becoming increasingly unacceptable from an envi-ronmental point of view. With this in mind, a wide variety ofalternative storage media are being investigated and developed.Compressed air energy storage (CAES) is proposed on aboutthe same scale as pumped hydro but is similarly limited regard-ing suitable sites. Since pumped hydro is a practical and cost-effective formof bulk energy storage used throughout the world, it is naturalthat developers of smaller-scale devices would seek to emu-late its success by providing the same load-shifting capabili-ties with hours of stored electricity. Thus, on a much smallerscale, battery systems have also been proposed for bulk energystorage. These technologies, such as flow batteries (also calledregenerative fuel cells) and high-temperature batteries (such assodium-sulfur and sodium-metal chloride), are currently demon-strating their technical feasibility but are not in full industrial useyet. As such, they are still heavily subsidized by their developersand by governmental and quasi-governmental agencies.While such multimegawatthour systems attract considerable attention, real commer-cial successes are being achieved at the opposite end of the storage spectrum, with sys-tems that store modest amounts of electricity and deliver power for seconds or minutesrather than hours. The more immediate success of these systems lies in the fact thatthey are doing what conventional generation cannot doreacting instantly to systemdisturbances to safeguard critical loads or to stabilize the local grid. Bulk storage sys-tems, on the other hand, are competing with traditional generation sources and will notachieve real commercial viability until their costs are reduced to a competitive level. Inessence, their life-cycle costs must be less than those of power generators, such as gasturbines.Notable successes in electric power storage include medium voltage power quality-UPS (uninterruptible power supply) systems, delivering up to 16 MW, used to safe-guard entire industrial facilities, and the recently commissioned Golden Valley ElectricAssociation battery energy storage system (GVEA BESS), the most powerful batterysystem, discharging up to 46 MW and with a specified run time of 15 min. The common factor in these successes is that the storage devices work with, ratherthan compete against, conventional generation. This article describes the functions ofthese systems and examines other emerging applications in power storage, such as thestabilization of wind farm output. It also addresses other power storage technologies1999 ARTVILLE, LLC. that are achieving commercial successes, such as flywheelsand supercapacitors.Growing Concern for Utility Power QualityMajor industrial areas in Asia, Europe, and North Americaexperienced major power outages and disturbances in the lasttwo years. Also, in some areas there is growing concern aboutthe safety of the power grids as prime targets for terrorism.These concerns coupled with ongoing impacts of weather onutility grid performance all come at a time when productivitydemands and greater automation are making power qualityone of the most critical issues for any business.The widespread use of UPS systems began over 30 yearsago, primarily to protect data centers and telecommunica-tions facilities. These devices ranged in size from less than 1kVA to as large as 1,000 kVA and served this market well.Today, the fastest growing market for total power protectionis semiconductor fabrication housed in very large, ultracleanfactories stretched beyond the limit of conventional UPSsystems. Japan and Korea have continued to push the limitsof conventional UPS design by assembling larger and largersystems made up of low-voltage devices. Taiwan and Euro-pean semiconductor companies have innovated the use ofrotary UPS devices that utilize medium voltage to achievehigh power ratings. In addition, North Americas researchactivities in the 1990s, conducted by the U.S. Department ofEnergy (DOE) and the Electric Power Research Institute(EPRI), resulted in the commercialization of very large,high-power battery-based UPS systems.Medium-Voltage UPS SolutionsDepending on the value of lost production at a criticalprocess facility and the potential for a complete power out-age from the utility, more large critical-process users areimplementing complete UPS protection at the medium-volt-age level. The continuing development of higher power sili-con controlled rectifiers (SCRs) and insulated gate bipolartransistors (IGBTs) has allowed the power electronic devicesnecessary for full control of higher power levels to be placedin the field with capabilities up to 20 MVA. Prior to applica-tion of the new devices, large medium-voltage systems werebased on rotary or dynamic UPS configurations. These sys-tems are generally limited to a few seconds of “flywheel”energy and require very fast operation of a backup generatortypically coupled directly to the shaft of the rotating UPSdevice. New solid-state designs allow for longer ride-through times using battery technology and off-line powerinverters to achieve very high power levels and outage pro-tection up to one minute. This allows for either a battery-only scheme or coordination with a conventional backupgenerator system for long-term protection. These very large UPS systems are typically off-linedesigns utilizing medium-voltage power electronic switches(PES) in series with the load. The PES allows rapid discon-IEEEpower & energy magazinemarch/april 2005flywheel electricity storagecontinues to growUsing kinetic energy as an electricity-storagemedium has served the power industry fordecades. In the last ten years, newer high-speedflywheels have been designed and demonstratedin various applications, from electric vehicles tosatellite power systems in outer space. Severalcompanies, like Beacon Power, continue to showhow flywheel energy can serve a variety of dynam-ic energy applications.In terms of commercial success, lower-speed fly-wheels (running under 10,000 rpm) have been usedin uninterruptible power supply (UPS) applicationsup to several megawatts. These systems typicallyoffer 1520 s of ride-through during power outages,which statistically solves over 94% of all power dis-turbances (based on the EPRI 1996 DistributionPower Quality Assessment).Germany-based RWR Piller GMBH produces thelargest single unit that stores 1,100 kW of usableenergy for 15 s. This flywheel assembly weighsapproximately 10,000 pounds and operates at 3,600rpm. This unit is matched typically to a motor gener-ator power conditioner to produce a complete UPSsystem. Like all UPS systems, these units can beoperated in parallel with build up complete systemshaving ratings of several megawatts. Piller typicallyoperates up to three units in a single installation,and the flywheels can be incorporated with a dieselgenerator to accommodate longer power outages.The latest flywheel contender to achieve com-mercial success is Active Power. The Active Powerflywheel operates at 7,700 rpm and produces 240kW for 13 s. This unique device showed that thephysical size could be compressed. The flywheelassembly itself weighs only 600 lb, has a diameterof 31 in, and is less than 1-ft high. These units havebeen coupled to a static power converter system toproduce UPS systems rated up to 1,200 kVA (con-sisting of four flywheels operated in parallel).The popularity of flywheels in UPS applicationshas grown as some users choose battery-less alter-natives. In some cases, flywheels are being addedas “short-term” energy supply coupled with amore conventional lead-acid battery for outageprotection longer than the 1015 s available fromflywheels.26march/april 2005IEEEpower & energy magazinenect from the utility (typically, 24 ms) and permits theUPS inverters to be kept in standby service. UPS powerflows only when a utility disturbance occurs. This configu-ration allows for very high operating efficiency (98%),which is extremely important for high loads. Figure 1 showsthe one-line functional diagram for a 5-MVA medium-volt-age UPS system.The basic building block of each energy storage unit isa power module consisting of an IGBT inverter and anintegral IBGT battery charger, combined with a mainte-nance-free battery string as shown in Figure 2.Because of the off-line design of this system, the ratingsof power-electronic components and power circuits can bescaled down. Based on a duty cycle of 100 s of run time,these components have been sized at less than 30% of theircontinuous-current rating. A high power rating is thereforeattainable in a very small package. Up to eight power mod-ules are combined in parallel to produce an energy storageunit rated 2,500 kVA/2,000 kW (Figure 3) for a load at 480 Vor higher. Such energy storage units can be operated in par-allel to create a medium-voltage UPS system up to 20MVA/16 MW providing 30 s of power outage protection.These are installed in outdoor structures similar to standardutility equipment.ST Microelectronics SolutionConsider, for example, one major semiconductor wafermanufacturer, ST Microelectronics. The company hasfigure 1. A 5,000-kVA medium-voltage UPS one-line functional diagram.CPT36SOSOSO3EFCPTPESUp To EightPower ModulesSystemControlBatteryStringInverterBridgeBatteryChargeracCapacitorsEnergy Storage ContainerUp To EightPower ModulesSystemControlBatteryStringInverterBridgeBatteryChargeracCapacitorsEnergy Storage ContainerUtilitySourceCriticalLoadAdditional EnergyStorage ContainersSystem Switchgear6figure 2. A 313-VA/250-kW, 30 s UPS power module.figure 3.A 2,500-kVA medium-voltage outdoor UPS system.27fabrication installations in Europe, North America, andAsia. At each facility, every effort is made to ensure thatthe most reliable source of utility power is obtained. How-ever, given the extreme sensitivity of wafer-fabricationequipment, the addition of some form of power protectionequipment is mandated. In the past, wafer-fab plants have usually chosen con-ventional, low-voltage solid-state or rotary UPS systemsarranged in subsystems, with each subsystem protectingan individual section of thecleanroom floor. But that tech-nology has its limitations. Forexample, the protection ofprocess cooling system chillersis generally considered too cost-ly with this technology.ST Microelectronics facilityin Phoenix, Arizona, began theprocess of researching power pro-tection alternatives for the plantin early 1999. This process cul-minated with the August 2000installation of a 12,500-kVAuninterruptible power supply(UPS) system operating at 12,470V. Located in the utility substa-tion feeding the plant, this instal-lation is the worlds largestbattery-based UPS system operat-ing at medium voltage (Figure 4). The system capacity wasincreased to 15,000 kVA in 2002 to support load growthwithin the facility.In the four years since initial installation, the UPS has mit-igated more than 100 utility power disturbances, includingthe most severe events which occurred in 2004. For example,utility events (power interruptions and sags) lasting up to 20 soccurred during the storm season in Phoenix.This concept of larger-scale UPS has gained acceptance,and similar systems have been installed at large criticalprocess facilities around the world.The GVEA BESSThe Golden Valley Electric Association, an electric coopera-tive, provides power to a population of around 90,000 in theFairbanks, Alaska, area. The GVEA system is virtually anelectrical island, with a single intertie linking it to theAnchorage area, 400 km to the south. There are no links toeither Canada or the lower 48 states. With mostly oil-basedand some coal-fired generation, GVEA has historically triedto minimize costs by running with minimal spinning reserves.(Broadly speaking, spinning reserves are the differencebetween the total capacity of running generators and the sys-tem load.) GVEA also imports as much power as possiblefrom Anchorage to take advantage of lower-cost gas-fired andhydroelectric generation in that area. These purchases, how-28IEEEpower & energy magazinemarch/april 2005figure 5. A schematic layout of the GVEA BESS.Batteries#3#1#4#2Filter CircuitsConverterConverterTransformers138-kV GridDCAC+figure 4. A 12,500-kVA medium-voltage UPS systeminstalled in a utility substation.More large critical-process users are implementing complete UPS protection at the medium-voltage level. march/april 2005IEEEpower & energy magazineever, come with the requirement for GVEA to meet a certainlevel of spinning reserves. GVEA met this obligation byimplementing a SCADA-based load-shedding scheme calledSILOS (shed in lieu of spin).Although this system is quite sophisticated, it is ulti-mately all about dumping customers off the grid. This wasbecoming increasingly unacceptable, so in the early 1990s,GVEA sought ways to improve the reliability of its system.Following EPRI and DOE studies of various options, it wasdecided to install a battery energy storage system (BESS).The functions of this system were determined to be: VAR Support: In this mode, the BESS provides voltagesupport for the power system regardless of whether thebattery is charging or discharging. Spinning Reserve: The BESS responds to remotegeneration trips in the system, with this mode beinginitiated at a system frequency of 59.8 Hz and withthe BESS loading to full output at 59.4 Hz if systemfrequency continues to drop. Spinning reserve has thehighest priority of all modes and will interrupt anyother BESS operating mode. Power System Stabilizer: The BESS reacts to effective-ly damp power system oscillations. Automatic Scheduling: This mode is used to provideinstantaneous system support in the event of a breakertrip on either a transmission line or a local generator.The BESS has 15 independently triggered inputs,which are tied remotely to the trip circuits of breakers. Support for Scheduled Load Increases: This mode willbe initiated and terminated by SCADA and puts theBESS in a frequency and voltage regulation mode,allowing it to respond to the scheduled, but rapid, addi-tion of large motor loads. Automatic Generation Control: This is essentially amanual operating mode for the BESS.GVEA project manager Tim DeVries provided a descrip-tion of the project background and vendor selection processat Battcon 2002. A turnkey contract for the construction and29figure 6. GVEA Project Manager Tim DeVries (left) explains the finer points of the BESS design to his brother, electricianTerry DeVries.Productivity demands and greater automation are making power quality one of the most critical issues for any erconnection of the BESS was awarded in October 2001 toa consortium of ABB and Saft. The system was inauguratedin August 2003 and saw its first real discharges beginning inNovember of that year. DeVries described the constructionprocess and acceptance testing in a paper presented at Elec-tricity Energy and StorageApplications and Technologies(EESAT) 2003.The layout of the BESS is shown schematically in Fig-ure 5. The battery comprises four parallel strings of 3,440nickel-cadmium pocket-plate cells, type SBH920. The cellsare arranged in ten-cell forkliftable modules, installed in anindustrial pallet racking in a narrow-aisle configuration.The battery area has room to expand the battery to eightstrings total. The battery strings are connected to a dc bus,which leads into the converter room. Filters balance the dclink voltage, reduce voltage ripple, and eliminate parallelresonances to protect the batteries. The power conversionsystem is capable of full power circle operation and usesABBs integrated gate commutated thyristor (IGCT) tech-nology. Fuller descriptions of the system are provided inother articles. Figure 6 shows part of the installed system.The BESS has been a considerable asset to GVEA and itscustomers. In its first year of operation it discharged 55 times,preventing more than 289,000 customer disconnections.The Future of Power StorageThe successes of the last few years in perfecting the applica-tion of fast-acting power storage have been significant. Otherapplications are being developedfor example, supplement-ing transmission stability systems with a few seconds ofactive power from a storage device and finding more eco-nomical ways to stabilize wind power systemsgreatlyincreasing the value of this renewable energy source. Theseapplications are expected to work together to increase theefficiency and performance of the electric grid. The ever-increasing activities relative to different types of energy stor-age technologies seem to point to greater commercial usebecoming a reality in the near future.For Further ReadingB.P. Roberts, “Growing use of large-scale power quality pro-tection systems,” presented at the Conference on ElecticalSupply Industry, Shanghai, China, 2004.B.P. Roberts, “Semiconductor wafer FAB plant gets pre-mium utility power,” Energy User News, Jan. 2001.T.R. DeVries, “The GVEA bessChoosing a multi-million dollar system,” in Proc. Battcon 2002, Ft. Laud-erdale, FL.T.R. DeVries, “Worlds most powerful BESS is onlineand working in Alaska,” in Proc. EESAT 2003, San Francis-co, CA.J. Varley, “Alaska grid