23 - battery sizing discharge

ETAP 5.0ETAP 5.0

Copyright 2003 Operation Technology, Inc.

Battery Sizing &Discharge

Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge Slide 2

When does a battery kick in?• Load in DC System exceeds the battery

charger capacity

• Output of the battery charger is interrupted

• AC Power is lost [generally the worst casescenario]


IEEE 485- Definitions• Battery Duty Cycle

– The load that the battery is expected to supply for aspecified period of time (sum of all individual loads)

• Cell Size– Rated capacity of a lead acid battery or number of

positive plates in a cell

• Period– Time interval for which the load is assumed to be

constant

• Vpc– Volts per cell (individual cell voltage)


Types of Loads• Constant Power

– As battery voltage decreases, their current increases.

• Constant Resistance

– As battery voltage decreases, their current alsodecreases.

• Constant Current

– Load current is invariant to battery voltage changes.


Duty Cycle Classification• Continuous Load (Non-Random)

– Non-random loads

– Normally carried by the battery charger

– Includes lighting, converters, indicating lights,communications systems, continuously operating motors

• Non-Continuous Load (Random or Non-Random)– Emergency pumps

– MOV operations

– Critical ventilation system motors

– Fire protection systems actuators


Duty Cycle Classification• Momentary Loads (Random or Non-Random)

– Loads with duration that do not exceed more thanone minute

– MOV with stroke of less than a minute

– Field flashing of generators

– Motor starting currents

– Inrush currents (solenoids, relay coils, etc)


Individual Load Duty Cycles

• Static Loads

• Motor loads

• DC Lump Loads

• Elementary Elementary Diagram

• DC to AC Inverters


Guidelines for Battery DutyCycles• All duty cycle periods (constant load) must

be higher or equal to one minute

• First identify the non-random load batteryduty cycle

• Find worst case scenario

• Include the random load duty cycle

• Final battery duty cycle becomes aconservative combination of the two


Duty Cycle Example A


Duty Cycle Example B


Battery Modeling

• Battery voltage based on manufacturerbattery discharge characteristic curves

• Enter battery characteristic curves in batterylibrary

• Interpolation / extrapolation is performed toobtain battery size and discharge voltageprofile

• Different interpolation methods to obtain Vpc


Characteristic CurvesBattery Characteristic Curves

1.00

10.00

100.00

1000.00

10.00 100.00 1000.00Amps per Positive Plate

Tim

e (m

in.)

1.51.61.671.751.781.811.841.881.921.96


Interpolation Methods


Battery Sizing Study Case• Load Summation

– Adds all the load duty cycles

– Generally more conservative for systems with moreconstant resistance loads

• Load Flow Method– Adds all the load duty cycles, but performs a load flow

calculation at every time step to account for systemlosses (more realistic results)

– Generally more conservative for systems with moreconstant power loads

– Load model type based on load type or duty cycle type


• Load Duty Cycles– 5 different system duty cycles

• Correction factors– Battery minimum temperature– Aging compensation– Initial capacity– Design margin

• Voltage Requirements– Maximum system voltage deviation (charging voltage)– Minimum system voltage deviation (depends on final

Vpc)– Battery minimum discharge voltage (final Vpc)


Battery Discharge Page

• Vd Calculation Parameters– Time step and maximum voltage limit

• Correction Factors– Same as battery sizing (no design margin)

• Load Flow Parameters– Iterations

– Precision

– Initial conditions and motor load handling


Duty Cycle Span OptionsOptions

•Two Existing Options Moved from Ini File to Study Case•New Option Added – One-minute Span for Battery only

One-minute Span for Battery only•Calculating Battery Discharging Current – Use One-MinuteSpanned Duty Cycle �To Get Conservative Battery Voltage

•Calculating System Voltage – Use Battery Voltage Calculatedabove to Run Load Flow with User Specified Duty Cycle forIndividual Loads � To Get More Accurate System VoltageProfile


Displayed Results

• Tabulated results for battery voltage, busvoltage and power flows through the entiresystem

• One-line diagram displayed results (similarto DC load flow)

• Complete set of plots that include batteryvoltage, current, discharged AH and systemflows


Things to Consider

• Obtain as many battery characteristic curvesas possible

• Obtain specific curves for battery model andtype

• Understand limitations of interpolationmethods

• Use most conservative approach for yoursystem (load flow method or loadsummation)

23 - battery sizing discharge

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