DC SystemSizing Principles

Agenda1. Application Outline2. How to build a load profile3. Battery Sizing Example4. Sizing with Software5. Battery Charger Sizing2Saft BatterySizing

The Art and Science of Battery Sizing– Battery Sizing is a Science– Building the load profile is an Art.– Different electro-chemistries vary greatly– You have more control over your battery selection thanyou think3Saft BatterySizing


Introduction to Switchgear– What is Switchgear? The combination of electrical disconnect switches,relays, lighting, controls, fuses or circuit breakers used tocontrol, protect and isolate electrical equipment Large Panels of electrical distribution circuit breakersdistribute power to a facility or grid– Why is Switchgear used?5 To de-energize equipment to allow work to be doneand clear faults down stream Fix power lines Breakers are too big to flip by hand

Application Outline - Switchgear– Three main types of switchgear applications MV (medium voltage) Utility level protection Typically 8 hr. load profileLV (low voltage) Building level protectionParalleling Two or more gensets Typically 2-8 hr. load profiles– Switchgear protects against6 faults upstream and protects equipment downstreamSaft BatterySizing

LV/MV Switchgear7 DC bus 125Vdc (normal) 48Vdc is also popular Load profile is mixed High peak currents (transient) Continuous loads (steady state) 2-8 hr. battery backup normalCurrent (A)480V to 38kV (typical)Time (hh:mm:ss)

Paralleling Switchgear120V to 600V (typical)8 DC bus 24, 48 or 125Vdc Load profile is mixed High peak currents (transient) Continuous loads (steady state) 20 min. - 4 hr. battery backup normal

The Battery’s Purpose– Batteries provide DC power to the switchgear equipment during anoutage.– Best practice is to have individual batteries for each load/application.– Duration of backup is dependent on the battery Ah capacity– Battery loads include:Trip Current Close Current Spring Motor Rewind/Charge Current Continuous Loads: Relays, Meters, Control Circuits, PLCs, Lighting, Etc.Current (A) Time (hh:mm:ss)9Saft BatterySizing

IEEE Standards– IEEE 1115 Recommended Practice for Sizing Nickel Cadmium Batteries– IEEE 485 Recommended Practice for Sizing Large Lead Acid Batteries– IEEE 118910 Recommended Practice for Selection of Valve Regulated Lead Acid Batteries For Sizing, it refers to IEEE 485 practicesSaft BatterySizing


Building Load Profiles - Switchgear– Switchgear load profiles normally comprise of four components Trip Can be simultaneous, sequential or mixed 1s (Ni-Cd) and 1min (Pb-acid)* Close Can be sequential, simultaneous or mixed 1s (Ni-Cd) and 1min (Pb-acid)* Spring motor rewind/charge Usually sequential, but can be simultaneous 6s (Ni-Cd) and 1min (Pb-acid)* minimum Continuous loads 20mins to 24hrs (8hr most common)– *Lead-Acid has a minimum sizing duration of 1min. Why?12Saft BatterySizing

Coup De Fouet13Saft BatterySizing

Trip / Close / Spring Charge– Simultaneous loads (# breakers x current) for one device operationtime 1 second minimum duration for Ni-Cd 1 minute minimum duration for Pb-acid– Sequential loads One device current for (# breakers x time) 1s minimum duration for Ni-Cd 1minute minimum duration for Pb-acid– Mixed loads # breakers x current # breakers x time e.g.51 breakers17 x trip current (simultaneous)3 x time period (sequential)14Saft BatterySizing

Load Sequencing– Load sequencing defines the total number of operations and where theyoccur during the outage / backup period– The number of operations and where they occur during the backupperiod can have a dramatic impact on battery capacity– We will look at a load profile example and examine how sequencingimpacts battery selection15Saft BatterySizing

The Voltage Window– Batteries Operate within a designed Voltage Window The upper limit should allow for battery equalize/boost charging The lower limit should allow for maximum usage during discharge.Narrow Voltage WindowWide Voltage Window The narrower the voltage window, the larger the battery capacity has to be.16Saft BatterySizing

The Voltage Window (cont.)– Lead Acid usually operates between 1.75vpc and 2.33vpc depending onconstruction– NiCad batteries typically operate between 1.00vpc and up to 1.65vpcdepending on load voltage tolerance.– Typical voltage windows for standard nominal voltages 24Vdc: 21Vdc to 30Vdc 48Vdc: 42Vdc to 58Vdc 125Vdc: 105Vdct to 140Vdc*Should be based on equipment connected to the battery.17Saft BatterySizing

Temperature Factor– Battery capacities and discharge ratings are published based on acertain temperature, usually between 68oF & 77oF.– Battery performance decreases at lower temperatures and must beaccounted for with correction factors.– Lead Acid – Temperature correction factor applied at the end of thecalculation.– NiCad – Temperature correction factor applied at each step in thecalculation.Available Capacity120%110% Sintered/PBE NiCd100%90%Lead Acid80%70%60% Pocket Plate NiCd50%-40 -30 -20 -10 0 10 20 30 40 50 60Temperature C18Saft BatterySizing

Other Factors to Consider– Design Margin Used to allow for future load growth or unknowns in the load list. Typically 1.1 - 1.15– Aging Factor Used when the requirement is for the battery to be able to perform thesame duty cycle at the end of its life as when it is new. Typically 1.25 based on the IEEE recommendation to replace a batteryafter its capacity has fallen to 80%.% Capacity1201008060Lead40Nicad20002030405060% Life19Saft BatterySizing708090100110

SIZING EXAMPLES20Saft BatterySizing

125 Vdc MV Switchgear Example– From Customer:– 20 breakers: 21Breaker Trip/Close (T/C)Spring charge motor inrush (SI)Spring charge motor run (SR)Continuous loadsTrip SequenceClose SequenceTemperatureNormal Aging (AF)Design Margin (DM)Saft BatterySizing 7A for 1s 16A for 1s 4A for 6.0s 5A for 8h Simultaneous @ t 0 and t 8hr Simultaneous after trip Temperature Controlled (room temp. 68oF) 1.25 AF 1.10 DM

Written Load Profile– Trip (20 brkrs x 7A 140A) cont. load (5) 145 Amps for .1s– Close (20 brkrs x 7A 140A) cont. load (5) 145 Amps for .2s– Spring SI (20 brkrs x 16A 320A) cont. load 325 Amps for .25s– Spring SR (20 brkrs x 4A 80A) cont. load 85 Amps for 6s– Cont. load 5A for 8h22Saft BatterySizing

Load Profile – Graphical Form (NiCad)Spring ChargeMotor Inrush325 AmpsTrip/Close Loads145 AmpsSpringCharge Run85 AmpsCont. Load5 Amps9 sec.23Saft BatterySizing

Load Profile – Step Form (NiCad)– Step 1 145A for 1sec (trip cont.)followed by:– Step 2 145A for 1sec (close cont.)– Step 3 325A for 1sec (Spring Charge Inrush cont.)– Step 4 85A for 6sec– Step 5 5A for 7hr, 59min, 42 sec– Repeat Steps 1 – 4:– Step 6 145A for 1sec (trip cont.)– Step 7 145A for 1sec (close cont.)– Step 8 325A for 1sec (Spring Charge Inrush cont.)– Step 9 85A for 6sec24Saft BatterySizing

Load Profile – Graphical Form (Lead Acid)Spring ChargeMotor Inrush325 Amps, 1 Min.Cont. Load5 Amps25Saft BatterySizing

Load Profile – Step Form (Lead Acid)– Step 1 325A for 1min (Spring Charge Inrush cont.)– Step 2 5A for 7hr, 58min– Step 3 325A for 1min (Spring Charge Inrush cont.)26Saft BatterySizing

Sizing The Old Way (Fan Curves)27Saft BatterySizing

Sizing the “New” WayThese days we use custom software!!– Drastically speeds up the battery selection process.– Eliminates calculation errors.– Ensures standards compliance by providing results in IEEE worksheetformat.– Many offer additional features:28 Battery rack selection Gassing/ventilation calculations Product data sheetsSaft BatterySizing

IEEE 485 sizing worksheet from custom software29Saft BatterySizing

BaSiCS Sizing Software (NiCad)– Step 1: Input General Information:30 Voltage Window: 105 – 140 for 125Vdc Temperature: 20oC Aging Factor: 1.25 Select Product ranges you are interested in: UP1M, SPH, SBM (typical for Swgr)Saft BatterySizing

BaSiCS (Cont.)– Step 2: Input Load Profile: Input Current and Time, click Validate. Move to next step– Step 3: CLICK: Size it31Saft BatterySizing

32Saft BatterySizing

Lead-Acid sizing33Saft BatterySizing

34Saft BatterySizing

Switchgear Sizing Conclusion– Proper load profile generation is critical to the outcome.– Don’t forget to consider temperature, design marginand aging factors.– The NiCad size will often be smaller than the Lead-Acid Lead-Acid Size: 350AH NiCad size: 130AH !!35Saft BatterySizing

Application Outline - UPS– Uninterruptible Power Supply Constant Power Battery Selection Depends on: UPS rating Power Factor Efficiency Run Time Environmental Conditions (Temperature)– Backing Up Critical Loads: IT / Commercial Loads: 5 – 15 Minutes Industrial UPS: 30 min – Design Life: 20 Years– DC Bus Voltage depends on UPS manufacturer 480 Vdc common for Commercial 125 Vdc common for Industrial36Saft BatterySizing

UPS Sizing– From Customer: 25kVA 0.85 PF 92% Efficiency 30min backup 50F Normal Aging acceptable DC Bus 125Vdc (105-140)37Saft BatterySizing

UPS BaSiCs Sizing– Step 1: Same as Switchgear (input General Information)38Saft BatterySizing

UPS sizing with Basics– Step 2: Click UPS:P sizing button (shown below)– Step 3: Enter UPS information and backup time– Step 4: Click: Size it– Selection will be based on runtime. Less than 30min H-rate 30min to 1hr M-rate Greater than 1hr L-rate39Saft BatterySizing

40Saft BatterySizing

Sizing SoftwareSaft Basics Software for all/SaftBasics.zip41Saft BatterySizing


Battery Charger SizingImportant Things to Consider!!– Continuous Load– Battery Type– Battery AH Capacity– Altitude– Design Margin43Saft BatterySizing

Battery Charger Sizing– For Example 100 AH Pocket Plate NiCad Battery Needs to recharge in 8 hours Continuous DC Load is 12 amps Design Margin is 10% Altitude is less than 3000 ft.First we need to get all the factors 44Saft BatterySizing

Battery Charger Sizing– Recharge Factor Per the table below, the recharge factor for Pocket Plate NiCadbatteries is 1.40Battery TypeRecharge FactorPocket Plate Nicad1.40Sintered/PBE Nicad1.20Lead Acid1.15– Altitude Derating45 Installation 3000 ft. no derating Installation 3000 ft. 6.7% derating per 3000 ft. Our example is less than 3000 feet so we have no derating factor.Saft BatterySizing

Battery Charger Sizing– The Formula AH x RF C CL x DM x AD RT Where:46 C Charger Current AH Battery Amp Hours RF Recharge Efficiency Factor RT Required Recharge Time CL Continuous Load DM Design Margin AD Altitude DeratingSaft BatterySizing

Battery Charger Sizing– The Calculation𝐶1001.48121.101.0C 32.45Therefore the charger should be sized at 30 amps**a 30 amp charger will deliver 33 amps in current limit47Saft BatterySizing

Thank You!!– Questions?48Saft BatterySizing

calculation. – NiCad – Temperature correction factor applied at each step in the calculation.-40-30 -20 -10 0 102030 40 5060 Temperature C 50% 60% 70% 80% 90% 100% 110% . UPS sizing with Basics Saft Battery 39 Sizing – Step 2: Click UPS:P sizing button (shown below) – Step