ABB Review Special Report - ABB - ABB Group

2Battery module, comprising 10 cells, on the moveData collection and transfer are organizedhierarchically. The lowest-leveldevice in the hierarchy is the sentryunit. There is one for each 10-cell module,and its task is to measure the modulevoltage, cell electrolyte level andcell internal temperature. Each sentryunit reports its collected data to asergeant module. Every string has itsown sergeant module, which also measuresthe string float current. In turn,the sergeant module reports its collecteddata to the supervisory computer,which analyzes and displays the data.This computer is also responsible forforwarding summary data to the HMIand is the main terminal for BESS personnelwho need to access the monitoringsystem.Optical couplers carry the data from thesentry units to the data bus, which isinsulated to withstand a minimum of5000 V. 5560 readings are taken every30 seconds – a total of 5.8 billion readingsper year. These numbers can bedoubled if required.The electrical systemFour battery strings are currently installed3 . All preparations have beenmade for extension at a later stage, withup to eight battery strings possible. Everystring (and sub-string) can be switchedoff and completely isolated from the restof the system by DC switches. In addition,two disconnectors allow separationof the battery and the DC link of theconverter when maintenance work hasto be carried out on the batteries. Theconverter can then remain in operationand provide reactive power to the gridfor voltage control. Filter circuits in theDC link eliminate the risk of resonancesat higher frequencies should any harmonicsbe generated in the grid by nonlinearloads. The voltage source converterat the heart of the electrical systemcomprises standardized PEBBs (PowerElectronic Building Blocks). One doublestackPEBB in NPC (Neutral PointClamped) connection forms a singlephaseH-bridge 4 Four H-bridges areinstalled per phase, for a total of twelvesingle bridges. The stacks are cooled bydeionized water in a closed-loop circuit.A PE liner on the inside of the modulecase (between the battery cells andmetal tray) provides the necessary insulation.Each module is fitted with a selfcontained,single-point filling system,allowing all 10 cells to be topped up ina single operation without removing themodule from the rack.Battery monitoring systemThe battery monitoring system was suppliedby Philadelphia Scientific Inc. Itmeasures, records and reports the modulevoltage, string current, cell electrolytelevel (one cell per module) andcell internal temperature (also one cellper module).Each bridge is connected to its dedicatedtransformer winding. The voltagecontributed by the bridges is added inthe transformer by series-connecting theline-side partial windings, resulting in avoltage wave shape similar to the qualitythat could be expected from rotatingmachines. Voltage limiters prevent anyovervoltages due to sudden load rejectionsor possible disturbances in theelectric grid from damaging the DC link.The converter and transformers havebeen designed and built to handle thetotal power should the battery beextended from four to eight strings ata later stage.44Special ReportABB Review