ABB Review Special Report - ABB - ABB Group

will allow the client to boost output orprolong the useful life of the systembeyond its planned 20 years.System and project requirementsThe final specification required that thevendor provide a turnkey solution andguarantee for twenty years that the BESScould supply 40 MW for 15 minutes,with a 4 MW/min ramp-down after the15-minute mark. The system is requiredto be capable of operating in all fourquadrants (ie, the full power circle) andto provide continuous, infinitely adjustablecontrol of real and reactivepower over the entire operating range.The specification also required that theBESS be able to operate in an automaticmode, as GVEA does not plan to manthe facility.Rated output had to be provided for thefollowing power system characteristics:Nominal voltage of 138 kV (1.0 pu)Normal sustained voltage of 0.90 pu(min) and 1.1 pu (max)Normal frequency of 60 Hz, withnormal deviation of +/- 0.1 HzSustained frequency range of 59.0 Hz(min) and 60.5 Hz (max)Seven operating modesThe BESS is able to operate in sevendistinct modes:Var support: The BESS provides voltagesupport for the power systemunder steady-state and emergencyoperating conditions.Spinning reserve: In this mode, theBESS responds to remote generationtrips in the Railbelt system. It is initiatedat a system frequency of 59.8 Hz,with the BESS loading to full outputat 59.4 Hz if system frequency continuesto drop. Spinning reserve has thehighest priority of all the modes andwill interrupt any other mode theBESS is operating under.Power system stabilizer, included todamp power system oscillations.Automatic scheduling, used to provideinstantaneous system support inthe event of a breaker trip on either atransmission line or a local generator.The BESS has thirty independentlytriggered inputs, which will be tied remotelyto the trip circuits of breakers.Scheduled load increase: This is initiatedand terminated by SCADA andputs the BESS in a frequency andvoltage regulation mode to allow itto respond to the addition of largemotor loads.Automatic generation control: In thismode the BESS is capable of operatingby AGC, similar to that of rotatingmachinery.Charging: The SCADA dispatcher cancontrol the MW rate at which the BESSwill be charged and when charging isto start after a BESS discharge.The batteryThe Alaskan BESS battery comprises13,760 Saft SBH 920 high-performancerechargeable nickel-cadmium cells,arranged in four parallel strings to providea nominal DC link voltage of 5000 Vand a storage capacity of 3680 Ah. Thecells are built into 10-cell modules formounting in a drive-in racking system.An aisle between the racks providesinstallation and service access for aswing-arm fork truck.The complete battery weighs some1300 tons and the hall in which it islocated measures 120 meters by26 meters – about the size of a soccerfield. The initial battery configurationhas four individual strings operatingin parallel, but can be expanded toaccommodate eight strings. Each stringhas 3440 cells connected in series.The battery features a pocket plate constructionwith thin, high-performanceplates. This design allows the full 20–25year life to be attained without anyloss of the beneficial characteristics ofNi-Cd batteries. The type of cell usedcan deliver 80% of its rated capacity in20 minutes.Ni-Cd pocket plate cells can withstandrepeated deep discharges with littleeffect on battery life. The graph in 1shows the cycling characteristics of theSBH battery.The chosen design has several advantages:Compact arrangement: More rackdepth can be utilized, minimizing thespace taken up by aisles.1100001000100Easy installation: 90% of the connectionsare made in the factory; onlythe inter-module connections aremade on site.Quick change-out: If there is a problemwith an individual cell, the modulecontaining that cell can be replacedby another complete modulein less than 30 minutes 2 .Minimum power losses: 99% of theinter-cell connections are made withsolid copper bars; power lossescaused by flexible cable connectionsare therefore minimized.Spill isolation: The cells sit on a plasticgrating, allowing spills to drain intothe base of the module. The traycan hold the electrolyte content of allten cells.World recordCycling characteristics of theBESS battery.No. of charge/discharge cyclesvs depth of dischargeNumber of cycles0 10 20 30 40 50 60 70 80 90Depth of discharge (%)During commissioning tests, ABB’s powerconversion system and the Saft battery setan unofficial world record by achieving a peakdischarge of 26.7 MW with just two strings inoperation, making use of the short-time overloadcapability of the battery modules. Thismakes the Alaskan BESS more than 27 percentmore powerful than the previous recordholder – a 21-MW BESS commissioned byPREPA (Puerto Rico Electric Power Authority)at Sabana Llana, Puerto Rico in 1994.Special ReportABB Review43