Chapter 24 - DC Short-Circuit Analysis

Chapter 24DC Short-Circuit AnalysisIn order to assure the safe operation of DC systems, whenever there is any changes in the system related to sources, loads, and power transmission components, a DC Short-Circuit Analysis must be carried out to evaluate system conditions under a fault and assess protective device ratings. A complete short-circuit calculation should provide details of fault current variations at the fault location as well as for contributing branches, from the initiation of the fault to its end. Due to the complexities involved in source behaviors and the non-linearity characteristics of the equipment, such calculations are very extensive and therefore the maximum short-circuit current is often calculated instead for examination of protective device ratings.DC Short-Circuit Analysis Introduction In compliance with IEEE Std. 946, the PowerStation DC short-circuit program calculates the total fault current, current contributions from different sources, and the rise time constant of the total fault current. It can conduct calculations on both radial and looped systems. The fault under consideration is a short-circuit between the positive and the negative terminals at the fault location. The contributing sources to the short-circuit current include charger/rectifier, UPS, battery, and DC motor. These sources can be modeled as a constant current source or a constant voltage source behind an impedance. For a charger/rectifier source, the AC system equivalent impedance on the AC side can also be considered.For each DC protective device, PowerStation calculates the bus fault current as well as the maximum current that flows through the device and flags the user in an outstanding color for underrated devices. The calculation results are reported in a Crystal Reports format as well as in a one-line diagram display. The Crystal Reports format provides detailed information about the study, including all the input data used in the calculation, fault current, contributions from different sources, and device rating validation summary, etc. The format and content of the Crystal Reports output report can be customized by the user. The one-line diagram display provides you with a direct visual representation of the system under fault conditions. It displays the short-circuit current at the faulted bus, fault current contributions on surrounding branches, as well as the system voltage profile under the fault.24.1 Study ToolbarThe DC Short-Circuit Study Toolbar will appear on the screen when you are in DC Short-Circuit Study mode.Run DC Short-Circuit AnalysisClick on this button to run a DC short-circuit calculation.Display OptionsClick on this button to customize the information and results annotations displayed on the one-line diagram in DC Short-Circuit mode.DC Short-Circuit Report ManagerClick on this button to open the DC Short-Circuit Report Manager. Here you can specify the Crystal Reports format for your output reports. A detailed explanation of the DC Short-Circuit Report Manager is in the Output Reports section.Halt Current CalculationClick on the Stop Sign button to halt the current calculation.Get On-Line DataIf the ETAP key installed on your computer has the on-line feature, you can copy the online data from the on-line presentation to the current presentation.Get Archived DataIf the ETAP key installed on your computer has the on-line feature, you can copy the archived data to the current presentation.24.2 Study Case EditorThe DC Short-Circuit Study Case Editor contains parameter settings required to perform a short-circuit calculation. The calculation results are dependent on these settings. When a new study case is created, ETAP PowerStation provides you with the default parameters. However, you want to check these parameters to make sure that they are set as required.The DC Short-Circuit Study Case Editor contains two pages: the Info page and the Source Model page. In the Info page, you can select faulted buses and specify contribution level, etc. In the Source Model page, you specify the type of model for chargers and batteries, as well as what loads need to be considered in a study.24.2.1 Info PageStudy Case IDIDEnter a unique alphanumeric ID with a maximum of 12 characters.ReportSpecify the contribution level the report should encompass.Bus SelectionHere you can select which buses to Fault, Don’t Fault, or click on the All Buses check box to fault all buses. Note that you can fault buses (or remove faults) directly from the one-line diagram by right clicking on the desired bus.Remarks 2nd LineYou can enter up to 120 alphanumeric characters in this remark box. Information entered here will be printed on the second line of every output report page header. These remarks can provide specific information regarding each study case. Note that the first line of the header information is global for all study cases and entered in the Project Information Editor.24.2.2 Source Model PageThis page allows you to specify the type of models you want the program to use in a short-circuit calculation.Charger Contributions Based onA charger can be represented as a constant current source or a constant voltage source behind impedance. As a constant current source, it injects a constant current into the system when a fault occurs.Editor SelectionClick on this option to select the model type as specified in the editor for individual chargers.Fixed SC ContributionClick on this option to use the constant current model for all the charges in the system.AC System ImpedanceClick on this option to use the constant voltage model for all the charges in the system.Battery Contributions Based onA battery can be represented as a constant current source or a constant voltage source behind impedance. As a constant current source, it injects a constant current into the system when a fault occurs. The current injected into the system is equal to a constant multiplied by its 1-minute discharge rate.Editor SelectionClick on this option to select the model type as specified in the editor for individual batteries.Constant Current (K*1-Min-Rating*String)Click on this option to use the constant current model for all the batteries in the system.Voc Behind Battery ImpedanceClick on this option to use the constant voltage model for all the batteries in the system.Motor Internal VoltageA motor, or the motor load portion of a lump load, is modeled as a constant voltage source behind an impedance. You can specify the internal voltage value by selecting one of the following two options: 100% of Motor Rated VoltageClick on this option to use the motor rated voltage as the internal voltage.Percent of Motor Rated VoltageClick on this option to specify the motor internal voltage in percent based on the motor rated voltage. Short-Circuit Contributions Based onThis section provides you with an option to skip certain load elements in a short-circuit analysis. Note that static loads are also considered in a DC short-circuit analysis and their presence reduces total fault current.Load Status OnlySelect this option to include loads in the short-circuit study based on load status. For the current system configuration, loads that have either the Continuous or Intermittent status will be considered in the study. Loads that have the Spare status will be excluded from the study. Note that when this option is selected all of the elementary diagram loads will be included in the study.Load Category OnlySelect this option to use the loading percent to determine which loads will be included in the short-circuit calculation. Once this option is selected, you can specify a loading category in the loading category selection box. All loads that have non-zero loading percent for the selected loading category will be included in the short-circuit calculation.Use Both Above OptionsSelect this option to use both load status and loading category to determine loads to be included in the short-circuit calculation. When this option is selected, all the loads that satisfy either or both of the above two criterions will be included in the short-circuit study.24.3 Display OptionsThe DC Short-Circuit Analysis Display Options consist of a Results page and three pages for AC, AC-DC, and DC info annotations. Note that the colors and displayed annotations selected for each study are specific to that study.24.3.1 Results PageColorThe drop down list allows you to select a color for displaying calculation results on the one-line diagram. Show UnitsWhen this box is checked the unit for the calculation results will be displayed on the one-line diagram along with the results.VoltageBusClick on this check box to display bus voltage on the one-line diagram.Bus Voltage Unit SelectionFrom the drop down list you can select to display bus voltage in percent or volt.Display Faulted BusFault Current Rise Time-ConstantClick on this option to display the fault current rise time-constant in ms for faulted buses.Equivalent Fault RClick on this option to display the equivalent fault resistance in ohms for faulted buses.Display ContributionConverter, Battery, & LoadClick on any or all of these check boxes to display short-circuit contribution from these components on the one-line diagram.24.3.2 AC PageThis page includes options for displaying info annotations for AC elements.ColorSelect the color for information annotations to be displayed on the one-line diagram.IDSelect the check boxes under this heading to display the ID of the selected AC elements on the one-line diagram.RatingSelect the check boxes under this heading to display the ratings of the selected AC elements on the one-line diagram.Device Type RatingGen. (Generator) kW / MWPower Grid (Utility) MVAscMotor HP / kWLoad kVA / MVAPanel Connection Type (# of Phases - # of Wires)Transformer kVA / MVABranch, Impedance Base MVABranch, Reactor Continuous AmpsCable / Line # of Cables - # of Conductor / Cable - SizeBracingBus kANode Bus Bracing (kA)CB Rated Interrupting (kA)Fuse Interrupting(ka)Relay 50/51 for Overcurrent RelaysPT & CT Transformer Rated Turn RatiokVSelect the check boxes under this heading to display the rated or nominal voltages of the selected elements on the one-line diagram.For cables/lines, the kV check box is replaced by the button. Click on this button to display the cable/line conductor type on the one-line diagram.ASelect the check boxes under this heading to display the ampere ratings (continuous or full-load ampere) of the selected elements on the one-line diagram.For cables/lines, the Amp check box is replaced by the button. Click on this button to display the cable/line length on the one-line diagram.ZSelect the check boxes under this heading to display the rated impedance of the selected AC elements on the one-line diagram.Device Type ImpedanceGenerator Subtransient reactance Xd”Power Grid (Utility) Positive Sequence Impedance in % of 100 MVA (R + j X)LRCMotor %Transformer Positive Sequence Impedance (R + j X per unit length)Branch, Impedance Impedance in ohms or %Branch, Reactor Impedance in ohmsCable / Line Positive Sequence Impedance (R + j X in ohms or per unit length)D-YSelect the check boxes under this heading to display the connection types of the selected elements on the one-line diagram.For transformers, the operating tap setting for primary, secondary, and tertiary windings are also displayed. The operating tap setting consists of the fixed taps plus the tap position of the LTC. Composite MotorClick on this check box to display the AC composite motor IDs on the one-line diagram, then select the color in which the IDs will be displayed.Use Default OptionsClick on this check box to use PowerStation’s default display options.24.3.3 AC-DC PageThis page includes options for displaying info annotations for AC-DC elements and composite networks. ColorSelect the color for information annotations to be displayed on the one-line diagram.IDSelect the check boxes under this heading to display the IDs of the selected AC-DC elements on the one-line diagram.RatingSelect the check boxes under this heading to display the ratings of the selected AC-DC elements on the one-line diagram.Device Type RatingCharger AC kVA & DC kW (or MVA / MW)Inverter DC kW & AC kVA (or MW / MVA)UPS kVAVFD HP / kWkVClick on the check boxes under this heading to display the rated or nominal voltages of the selected elements on the one-line diagram.AClick on the check boxes under this heading to display the ampere ratings of the selected elements on the one-line diagram.Device Type AmpCharger AC FLA & DC FLAInverter DC FLA & AC FLAUPS Input, output, & DC FLAComposite NetworkClick on this check box to display the composite network IDs on the one-line diagram, then select the color in which the IDs will be displayed.Use Default OptionsClick on this check box to use PowerStation’s default display options.24.3.4 DC PageThis page includes options for displaying info annotations for DC elements.ColorSelect the color for information annotations to be displayed on the one-line diagram.IDSelect the check boxes under this heading to display the IDs of the selected DC elements on the one-line diagram.DC Short-Circuit Analysis Display Options RatingSelect the check boxes under this heading to display the ratings of the selected DC elements on the one-line diagram.Device Type RatingHourBattery AmpereMotor HP / kWLoad kW / MWElementary Diagram kW / MWConverter kW / MWCable # of Cables - # of Conductor / Cable - SizekVSelect the check boxes under this heading to display the rated or nominal voltages of the selected elements on the one-line diagram.For cables, the kV check box is replaced by the button. Click on this button to display the conductor type on the one-line diagram.ASelect the check boxes under this heading to display the ampere ratings of the selected elements on the one-line diagram.For cables, the Amp check box is replaced by the button. Click on this button to display the cable length (one way) on the one-line diagram.ZSelect the check boxes under this heading to display the impedance values of the cables and impedance branches on the one-line diagram.Composite MotorClick on this check box to display the DC composite motor IDs on the one-line diagram, then select the color in which the IDs will be displayed.Use Default OptionsClick on this check box to use PowerStation’s default display options.24.4 Calculation MethodsThe PowerStation DC short-circuit program can perform fault analysis for a radial or a looped system. It calculates the maximum system fault current and contributions from individual sources. The fault under consideration is assumed to be a short-circuit between the positive and negative terminals at the fault location. Fault current contributing sources include chargers/rectifiers, UPS, batteries, and DC motors. These sources can be modeled either as constant current sources or constant voltage sources behind an impedance, as specified by the user. It is assumed that these sources will reach their maximum contribution level at the same time, which results in a conservative solution. The program also calculates the rise time of fault current based on the equivalent R and L at the fault location. When calculating short-circuit current, inductance values for all of the system components are neglected. These inductance values are used in calculating fault current rise time.24.4.1 Procedure for DC Short-Circuit CalculationIn a DC short-circuit calculation, a contributing source may be represented by different models, either as a voltage source or as a current source. Even the sources that are represented as constant voltage sources may have different per unit values. This is different from the AC short-circuit calculation by the IEEE method, where a prefault voltage is specified and a circuit network is solved to find the fault current. In the DC short-circuit calculation, a two-step procedure is adopted that applies the superposition theorem to calculate fault current. The two steps are voltage profile calculation and short-circuit current calculation. In the first step of the calculation, the short-circuit current sources such as charger, UPS, battery, and motor are modeled as specified in the study case editor and individual element editors. They may be modeled as constant current sources or as constant voltage sources behind an impedance. Based on this system, a load flow calculation is conducted to determine system voltage profile and current flows. These voltage values will be used in the second step as the prefault voltage for short current calculation.In the second step of the calculation, the program calculates fault current and contributions for each bus to be faulted with the bus voltage calculated in the first step as the prefault voltage.In addition to fault current, the program also calculates the equivalent R and L at the faulted bus, based on the separate R and L network. Using the equivalent R and L, it calculates the current rise time constant for the fault.24.4.2 Short-Circuit Current Rise Time Constant CalculationThe short-circuit current reaches its maximum value at a rate depending on the system configuration and the resistance and inductance values of all the elements in the system. For a radial system, it depends on the system R/L ratio, which is simple to calculate. However, for a looped network with multiple sources, it is rather complicated to determine the rise time constant of the short-circuit current.PowerStation calculates the rise time constant based on the equivalent R and L at the fault location.24.4.3 Device Rating EvaluationOne of the major purposes of conducting a short-circuit calculation is to evaluate device rating under fault conditions, such as bus rating and protective device ratings. For each DC protective device, PowerStation calculates the bus fault current and the maximum current that flows through the device. The program then compares the device rating against the maximum through current. If an underrated condition occurs, PowerStation will flag the underrated condition in the text report as well as in the one-line display. 24.4.4 Component ModelsChargerA charger can be represented as a constant current source or a constant voltage source behind an impedance. As a constant current source, it injects into the system a constant current equal to its rated current multiplied by the Imax specified in the Rating page of the charger editor.When modeled as a constant voltage source behind an impedance, the rated voltage is used as the internal voltage. The AC system Z specified in the SC page of the Charger Editor is converted to the DC side and used as the impedance in the model.UPSA UPS (Uninterruptible Power Supply) is represented as a constant current source. It injects into the system a constant current equal to its rated current multiplied by the Imax specified in the Rating page of the UPS Editor.BatteryA battery can be represented as a constant current source or a constant voltage source behind an impedance. As a constant current source, it injects into the system a constant current equal to its 1 minute discharging current multiplied by a K factor specified in the SC page of the Battery Editor.When modeled as a constant voltage source behind an impedance, the internal voltage depends on the option selected in the Battery Editor. These options include using the rated voltage or the value calculated based on the battery specific gravity and minimum operating temperature.DC ConverterA DC converter is used to change the voltage level in a DC system. If a fault occurs on the output side of the system, the DC converter is modeled as a constant current source injecting into the system a constant current. This current is equal to its rated current multiplied by the Imax specified in the Rating page of the DC Converter Editor.When calculating fault current contributions, the calculation does not extend into the input side of the system. In case a DC converter has the same input and output rated voltage values, and it is involved in any loop as the only DC converter, the program stops the calculation and posts a message to inform the user.DC MotorA DC motor is modeled as a constant voltage source behind an impedance. The internal voltage value can be specified in the DC Short-Circuit Study Case Editor. The impedance is specified in the SC page of the DC Motor Editor.DC Lumped LoadA DC lumped load is modeled as a constant voltage source behind an impedance. The internal voltage value can be specified in the DC Short-Circuit Study Case Editor. The impedance is specified in the SC Imp page of the DC Lumped Load Editor.Note that only the motor loads of the lumped loads contribute short-circuit currents, i.e., if the percent motor load of a lumped load is greater than zero, the motor load part will be modeled the same as a DC motor, while the static load part will be represented as a static load with no short-circuit contribution. DC Static & Elementary Diagram LoadsDC static loads are included in short-circuit calculations. The presence of static loads provide shunt paths for short-circuit current and hence reduce the total fault current. An elementary diagram (ED) load is treated the same as a static load.DC CableIn order to achieve conservative results, in a DC short-circuit analysis, the cable resistance is calculated at the minimum temperature entered in the Cable Editor.24.5 Required Data24.5.1 SourceChargerInfo Page• Charger ID• Bus connection dataRating Page• All the data in this page are required for DC load flow calculationsSC Page• Data in the SC Contribution for DC System section• AC System Z data is required if the Based on AC System Z option is selected UPSInfo Page• UPS ID• Bus connection dataRating Page• AC rating data• DC rating data• Auction diode optionSC Imp Page• SC Contribution to DC System section dataBatteryInfo Page• Battery ID• Bus connection data• Number of stringsRating Page• Number of cellsSC Page• Battery Library type data: Rp, time constant, SG, Vpc, and 1-min-rating • Short-circuit model data• External impedance data• Voc per cell data24.5.2 LoadDC MotorInfo Page• Motor ID• Bus connection data • Configuration status • QuantityRating Page• Rating data• Load category dataSC Page• SC parametersLump LoadInfo Page• Lump load ID• Bus connection data • Configuration status Rating Page• Rating section data• Motor/static load percent • Load category dataSC ImpPage• SC parametersStatic LoadInfo Page• Static load ID• Bus connection data • Configuration status • QuantityRating Page• Rating section data• Load category dataED LoadInfo Page• ED load ID• Bus connection dataRating Page• Rating section data• Load category data24.5.3 BranchDC CableInfo Page• Cable ID• Bus connection data• Cable length• Number of cables per phase Impedance Page• Cable resistance and inductance• Units section data• Base and minimum operating temperature DC ImpedanceInfo Page• DC impedance ID• Bus connection data• Impedance resistance and inductance 24.5.4 DC ConverterInfo Page• DC converter ID• Bus connection dataRating Page• Rating section data• SC contribution data24.5.5 Protective DeviceIf the data for a protective device has been entered by the user, the DC short-circuit calculation will compare the short-circuit current against device rating and flag the user if the device is underrated.DC CBInfo Page• ID• Bus connection data• Rated V• SC kADC FuseInfo Page• ID• Bus connection dataRating Page• Rated V• Interrupting kADC Single-Throw SwitchInfo Page• ID• Bus connection data• Rated V• Momentary kADC Double-Throw SwitchInfo Page• ID• Bus connection data• Rated V• Momentary kA24.5.6 Study CaseSimilar to any other study, you are always required to run a DC short-circuit calculation. When a DC short-circuit calculation is initiated by the user, PowerStation uses the study case currently showing in the study case editor in the calculation. Every field in a study case has its default value. However, it is important to set the values in the study case correctly to meet your calculation requirements.24.6 Output ReportsThe DC short-circuit calculation results are reported both on the one-line diagram and in the Crystal Reports format. The graphical one-line diagram displays the calculated fault currents, time constant for current rise, equivalent resistance at the faulted bus, as well as fault contributions from neighboring buses. You can use the Display Options Editor to specify the content to be displayed. It also flags underrated protective devices in red.The Crystal Reports format provides you with detailed information for a DC short-circuit analysis. You can utilize the DC Short-Circuit Report Manager to help you view the output report.24.6.1 DC Short-Circuit Report ManagerTo open the DC Short-Circuit Report Manager, simply click on the View Output File button on the DC Short-Circuit Toolbar. The editor includes four pages (Complete, Input, Result, and Summary) representing different sections of the output report. The Report Manager allows you to select formats available for different portions of the report and view it via Crystal Reports. There are several fields and buttons common to every page, as described below.Output Report NameThis field displays the name of the output report you want to view.Project File NameThis field displays the name of the project file based on which report was generated, along with the directory where the project file is located.HelpClick on this button to access Help.OK / CancelClick on the OK button to dismiss the editor and bring up the Crystal Reports view to show the selected portion of the output report. If no selection is made, it will simply dismiss the editor. Click on the Cancel button to dismiss the editor without viewing the report.Complete PageOn this page there is only one format available, Complete, which brings up the complete report for the DC short-circuit study. The complete report includes input data, results, and summary reports.Input Data PageThis page allows you to select different formats for viewing input data, grouped according to type. They include:BatteryBranch ConnectionBusCableConverterCoverLoadsResult PageThis page allows you to select formats to view the short-circuit result portion of the output report.Summary PageThis page allows you to select formats to view summary reports of the output report. The only summary report format available is the Interrupting Current format.24.6.2 View Output Reports From Study Case ToolbarThis is a shortcut for the Report Manger. When you click on the View Output Report button, PowerStation automatically opens the output report, which is listed in the Study Case Toolbar with the selected format. In the picture shown below, the output report name is Untitled and the selected format is Complete.。