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# Chapter 26 battery sizing and discharge analysis norestriction

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### Chapter 26 battery sizing and discharge analysis norestriction

1. 1. In the above sample curves, the set of curves may apply to batteries of different sizes or to only one size.In ETAP PowerStation, you specify a set of characteristic curves for a givensize. If you want to use a given set of curves for batteries of different sizes, youcan indicate this in the Battery Method Case Editor. 26.4.3 Battery Sizing Sizing StudyThe battery sizing calculation includes determining the number of cellsto meet the system voltage requirement and determining the battery sizeand number of strings to meet the load duty cycle requirement.Number of CellsThe number of cells should be determined to satisfy systemminimum and maximum voltage requirements:1. When charging the battery, the voltage to be applied to the battery should notbe greater than the maximum system voltage.
2. 2. Let N be the number of cells. The voltage requirements can be given inthe following equation V sys, ≤N≤ V sys, V mincell, V cell, max disch ch Where Vsys,min is the minimum system voltage that is equal to the nominal voltage of the battery terminal bus multiplied by the minimum system voltage deviationVsys,maxis the maximum system voltage that is equal to the nominal voltage of the entered in the Battery Sizing Study Case Editor.battery terminal bus multiplied by the maximum system voltage deviationVcell,disch is the battery discharge voltage in V/Cell entered in theBattery Sizing Study Case Editor.entered in the Battery Sizing Study Case Editor.It is clear that the number of cells of the battery is dependent on the four values for voltage requiremententered in the Battery Sizing Study Case Editor. It can happen that forsome incompatible values, we cannot determine a value for N to satisfythe above equation. When this situation occurs, ETAP will display amessage indicating that it cannot determine the number of cells. In practical cases, there is often a range of values that N can take tosatisfy the above equation. In thiscase, ETAP will select the value for N that results in the battery ratedvoltage being closest to its terminal bus nominal voltage.
3. 3. Cell SizeIn determining the battery size, ETAP will find the smallest size that can provide sufficient power for thespecified duty cycle. The capacity of a battery can be increasedeither by using a larger size or by adding more strings. Since ETAPallows you to enter different characteristic curves for different sizesof batteries, in the battery sizing calculation, the program starts withone string and the smallest size available for the calculation. If it failsto meet the load requirement, the program first increases the size andperforms calculations with the characteristic curves for the new size.When no available sizes can meet the load requirement for the givennumber of strings, it then increases the string number and performsthe calculation with the smallest size again. This process continuesuntil a battery size and a string number are found to meet the loadrequirement.Load Sections in Battery Duty CycleA battery duty cycle generally can be represented as a squarewaveform. It consists of a number of time periods, with a constant current value during a period. The figure below shows a sampleduty i S i a= ∑ It P jcycle for battery.1 consists of six periods, designated as P1, P2, j=… P6. A load section Si is a combination of a number of loadperiods, defined as:
4. 4. In the sample duty cycle there are six load sections. Load Sections for A Sample Battery Duty CycleDetermination of Cell Size Based on Battery Characteristic CurvesBased on a given set of battery characteristic curves, we can determine the required battery size for a specified duty cycle. Let F represent cell size. It is F= Max Fi i=1,..Sm equal to:where Sm is the total number of load sections and Fi is the sizecalculated for the ith load section. The calculation of Fi depends on thewhere Ap is the load current value in period P. RT is the value obtained from thetype of characteristic curve, which is the number of amperes that each positivebattery battery library curves.
5. 5. Random Load and Non-Random LoadIn general, the duty cycle for a battery consists of random loads andnon-random loads. The program determines the cells for random andnon-random loads separately in the same way as described in theprevious section. The sum of the two cell size values is the uncorrectedcell size for the given duty cycle. Adjusting Factors In the Battery Sizing Study Case Editor, you can select several adjusting factors to be considered in calculating battery size. These factors include temperature factor, design margin factor, agingCalculation CycleIt is clear from the equations for determining cell size that the cell size iscalculated based on a given set of battery characteristic curves, which is for agiven cell size. If the calculated cell size is different from the onecorresponding to the characteristic curves used. We have to do the calculation
6. 6. 26.4.4 Battery Discharging Calculation MethodThe purpose of battery discharge calculation is to determine batteryperformance for a specified duty cycle. One of the key parameters for batteryperformance is the battery terminal voltage. When the battery is supplyingthe load as the sole source, it should be able to maintain voltage level for thewhole period of the specified duty cycle.Battery Characteristic Curves for Voltage InterpolationThe terminal voltage of a battery is dependent on the current drawing from thebattery and the ampere-hour capacity contained in the battery. Thisrelationship is described by the battery characteristic curve and is verynonlinear. In ETAP, the battery characteristics are described in the batterylibrary as discrete points. Because no closed form equation is available todescribe the battery characteristics, numerical interpolation methods have toBattery Combined Duty Cyclebe used to find the points missing in the curves. Apparently, the more curvesWhen the load powered by the battery includes random load, the random load are entered in the battery library, the more accurate the calculated results will should be added to the non-random load at the worst point, which is the time the battery has the lowest be.voltage value when only the non-random load is considered. To identify thistime moment, the program first performs battery discharge calculationexcluding the random load. It then determine the worst point, add the randomload to the non-random load and perform discharge calculation from the timewhen the random load takes effect all the way to the end of battery duty cycle.
7. 7. Battery Voltage CalculationAn iterative process is conducted to calculated battery discharge voltage values. A battery voltage valueis reported at each time step specified in the battery sizing study caseand at each moment when there is a change in the load duty cycle. Bychanging the step size from the battery sizing study case, the user canadjust the level of detail information on discharge calculation to bereported.If the battery duty cycle is calculated by the load current summation method, the battery current willchange only when there is a change in any load duty cycle. When theload flow method is selected in the study case, even if there is nochange in the load duty cycle, the battery current will change due todecrease in the battery voltage. In this case the battery current iscalculated by a full load flow calculation, considering different typesof loads and system losses. In this load flow calculation, the battery ismodeled as a constant voltage source with the voltage calculated in theprevious step. The calculated battery current will be used in thecurrent step for battery voltage calculation.Along with battery voltage and current, the battery discharge program also calculates battery dischargecapacity. When there is change in the load current, two values ofvoltageWhen the battery is calculated using load flow method, the batteryand current are calculated, at t – discharge calculation also provides aand t+, one for before the load change and one for after the load change.
8. 8. 26.5Required Data26.5.1 SourceIn battery sizing calculation, the only source is the battery to be sized. Batteries may only besized/discharged one at a time asspecified in the study case. A UPS may be considered as a load to the Batterysystem when its input bus is not connected to an energized bus.•ID•Bus connection data•Battery library type data. This information is used to retrieve librarydata for calculations. If only the battery discharge calculation is conducted, the following additional information is also •Battery number of plates and Capacity. required: •Number of cells •Number of Strings •SC page battery external resistance.
9. 9. 26.5.2 Load UPS When a UPS is not connected to an energized input AC bus, it is considered a load in battery sizing calculations.•ID•Bus connection data•DC rated voltage kW and kVA. Duty Cycle Page•If no duty cycle data is entered, this load will be assumed to be zero. DC Motor•ID•Bus connection data•Quantity•Rated voltage•kW or HP and Efficiency. Duty Cycle Page
10. 10. Lumped Load•ID•Bus connection data•Rated voltage kW Rating•Duty Cycle Page•If no duty cycle data is entered, this load will be assumed to be zero. Static Load •ID •Bus connection data kW Rating. •Rated voltage •Duty Cycle Page •If no duty cycle data is entered, this load will be assumed to be zero. Elementary Diagram (ED) Load •ID •Bus connection data •Rated voltage kW Rating. •Duty Cycle Page •If no duty cycle data is entered, this load will be assumed to be zero.
11. 11. Inverter •ID •Bus connection data •DC rated voltage •kVA, PF, DC kW rating •Duty Cycle Page •If no duty cycle data is entered, this load will be assumed to be zero.26.5.3 BranchDC Cable •ID •Bus connection data •Cable length •Resistance and Inductance and cable length units DC Impedance •ID •Bus connection data •Resistance and inductance impedance information.
12. 12. Tie PD (CB, Fuse, & Single-Throw & Double-ThrowSwitches) •ID •Bus connection data DC Converter •ID •Bus connection data •kW Rating and Rated kV Input and output. Library •Library type data •Battery characteristic curve data Study Case When you initiate a battery sizing calculation, PowerStation uses the study case currently selected from the Study Case Toolbar. Every field in the Study Case Editor is set to its default value. However, it is important to set the values in the study case correctly to meet your calculation requirements.
13. 13. The battery sizing calculation results are reported graphically on the one-line diagram, in plots and in the Crystal Reports format. The graphical one-line display shows the number of cells, number of strings, cell size, etc. You can use the Display Options Editor to specify the content to be displayed. 26.6 Output ReportsThe Crystal Reports format provides you with detailed information for abattery sizing study. You canutilize the Battery Sizing Report Manager to help you view the output 26.6.1 Battery Sizing Report Managerreport.To open the Battery Sizing Report Manager, simply click on the ViewOutput File button on the BatterySizing Study Toolbar.The editor includes four pages (Complete, Input, Result, andSummary)representing different sections of the output report. The ReportManager allows you to select formatsavailable for different portions of the report and view it via CrystalReports. There are several fields and buttons common to every page, asdescribed below.
14. 14. Output Report NameThis field displaysName to the output report you want to view. Project File the name This field displays the name of the project file based on which report was Help generated, along with the directory where the project file is located. Click on this button to access Help. OK / CancelClick on the OK button to dismiss the editor and bring up the CrystalReports view to show the selected portion of the output report. If noselection is made, it will simply dismiss the editor. Click on the Cancel Complete Report Pagebutton to dismiss the editor without viewing the report.In this page there is only one format available, Complete, which brings up the complete report for thebattery sizing study. The complete report includes input data, results,and summary reports.
15. 15. Input Page This page allows you to select formats to view different input data, grouped according to type. They include the following available formats: •Battery Characteristics •Branch Connection •Bus and Connected Load •Cable •Cover •DC Converter Impedance Inverter •Load Duty Cycle •UPS
16. 16. Result PageThis page allows you to select formats to view the result portion of the output report, including Calculation Results, Battery LoadProfile, and Battery Characteristics. The Calculation Results portionprints the uncorrected cell size for each load section in non-randomload and random load. The Battery Load Profile is the battery dutycycle generated based on load duty cycles. The Battery Characteristics are mostly data entered by the user