23 - Battery Sizing Discharge

ETAP 5.0 Battery Sizing & Discharge

Copyright 2003 Operation Technology, Inc.

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 case scenario]

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

Slide 2

IEEE 485- Definitions • Battery Duty Cycle – The load that the battery is expected to supply for a specified 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) Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge

Slide 3

Types of Loads • Constant Power – As battery voltage decreases, their current increases.

• Constant Resistance – As battery voltage decreases, their current also decreases.

• Constant Current – Load current is invariant to battery voltage changes.

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

Slide 4

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

Slide 5

Duty Cycle Classification • Momentary Loads (Random or Non-Random) – Loads with duration that do not exceed more than one minute – MOV with stroke of less than a minute – Field flashing of generators – Motor starting currents – Inrush currents (solenoids, relay coils, etc)

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

Slide 6

Individual Load Duty Cycles • Static Loads • Motor loads • DC Lump Loads • Elementary Elementary Diagram • DC to AC Inverters

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

Slide 7

Guidelines for Battery Duty Cycles • All duty cycle periods (constant load) must be higher or equal to one minute • First identify the non-random load battery duty cycle • Find worst case scenario • Include the random load duty cycle • Final battery duty cycle becomes a conservative combination of the two Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge

Slide 8

Duty Cycle Example A

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

Slide 9

Duty Cycle Example B

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

Slide 10

Battery Modeling • Battery voltage based on manufacturer battery discharge characteristic curves • Enter battery characteristic curves in battery library • Interpolation / extrapolation is performed to obtain battery size and discharge voltage profile • Different interpolation methods to obtain Vpc Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge

Slide 11

Characteristic Curves Battery Characteristic Curves 1.5

1000.00

1.6

Time (min.)

1.67 1.75

100.00

1.78 1.81 10.00

1.84 1.88 1.92

1.00 10.00

100.00

1000.00

1.96

Amps per Positive Plate

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

Slide 12

Interpolation Methods

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

Slide 13

Battery Sizing Study Case • Load Summation – Adds all the load duty cycles – Generally more conservative for systems with more constant resistance loads

• Load Flow Method – Adds all the load duty cycles, but performs a load flow calculation at every time step to account for system losses (more realistic results) – Generally more conservative for systems with more constant power loads – Load model type based on load type or duty cycle type Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge

Slide 14

• 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) Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge

Slide 15

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

Slide 16

Duty Cycle Span Options Options •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-Minute Spanned Duty Cycle To Get Conservative Battery Voltage •Calculating System Voltage – Use Battery Voltage Calculated above to Run Load Flow with User Specified Duty Cycle for Individual Loads To Get More Accurate System Voltage Profile

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

Slide 17

Displayed Results • Tabulated results for battery voltage, bus voltage and power flows through the entire system • One-line diagram displayed results (similar to DC load flow) • Complete set of plots that include battery voltage, current, discharged AH and system flows

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

Slide 18

Things to Consider • Obtain as many battery characteristic curves as possible • Obtain specific curves for battery model and type • Understand limitations of interpolation methods • Use most conservative approach for your system (load flow method or load summation) Copyright 2003 Operation Technology, Inc. – Workshop Notes: Battery Sizing & Discharge

Slide 19