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Tutorial - ANSI Short Circuit Analysis
In this tutorial, you will be shown how to use several of EasyPower’s short circuit analysis features.
Short Circuit Focus
To run short circuit analysis, you must be in the
Short Circuit focus.
- Open the Simple.dez file located in the EasyPower Samples folder. Click
Maximize to fill your screen with the one-line.
- Click Short Circuit to open the Short Circuit focus.
Calculating Fault Currents
Double-click on BUS-3. The bus turns light blue and displays fault currents in symmetrical kilo-amps. As shown in
Figure 1, the motor contributes 0.863 kA and the cable 4.481 kA to the fault. The total bus fault current, shown at a forty-five
degree angle, is 5.344 kA.
Figure 1: Calculating Fault Currents
There are several different methods to fault buses while in the Short Circuit focus:
- Double-click on a bus.
- Select a bus or multiple buses (so they turn green) and click
Fault Bus(es).
Note: To select a single bus, click on it. To select multiple buses, click on each bus while holding
down the
SHIFT key.
- Click Fault Bus(es) without selecting any buses. This runs a batch fault on all the buses in your
system.
Remote Voltages and Currents
- Fault BUS-3 again.
- Select BUS-1 by clicking on it. With the
SHIFT key pressed, also select BUS-2.
- Click
Remote V/I. The remote currents are 0.896 kA on the primary side of the transformer and 4.481 kA on the secondary
side (as shown in the figure below). The remote voltages, shown at a forty-five degree angle, are 0.643 pu on BUS-1 and 0.000 pu on
BUS-2.
Figure 2: Remote V/I
Changing the ANSI Standard Time Interval
ANSI Standards list three distinct time intervals for short circuit currents. These are momentary (½ cycle), interrupting
(5 cycle), and 30 cycle. By default, EasyPower displays the momentary currents, as indicated by
Momentary selected in the toolbar.
- Click
Interrupting to view the interrupting results. (They are displayed next to the ½-Cycle
currents.) Note that the overall bus current magnitude drops to 4.826 kA due to the decay of the motor contribution.
- Click
30 Cycle to view the 30 cycle currents (see figure below). Notice that the motor
current contribution decays to zero leaving only 4.481 kA from the utility.
Figure 3: ½ -Cycle, Interrupting, and 30 Cycle Currents
Unsymmetrical Faults
The currents calculated to this point have been 3-phase currents. EasyPower also calculates unsymmetrical faults.
According to convention, single line-to-ground fault calculations assume the A-phase is faulted. For double line-to-ground and
line-to-line faults, convention says that phases B and C are faulted. Refer to
Elements of Power System Analysis by William D. Stevenson, Jr. for examples of unsymmetrical fault calculations.
Single Line-to-Ground Faults
- Click
Interrupting and
30 Cycle to remove those selections, and leave
Momentary selected.
- Click Line to Ground.
- Fault BUS-3 again. The currents will show zero (see figure below). This is because the transformer (delta-delta connection
type) and motor are not grounded. (By default, EasyPower displays A-phase currents.)
Figure 4: Single Line-to-Ground
Double Line-to-Ground Faults
- Click Double Line to Ground. The green dots in the button indicate ground fault.
- In the message that is displayed, click Yes to fault the B-phase line rather than the A-phase. All faults will now be double
line-to-ground faults.
- Double-click on BUS-3 again. The B-phase double line-to-ground fault currents are displayed (see figure below).
Figure 5: Double Line-to-Ground
Line-to-Line Faults
- Click Line to Line. Faults are now line-to-line faults. The yellow dots in the button
indicate this is not ground fault.
- Fault BUS-3 again to display the B-phase line-to-line fault currents (see figure below).
Figure 6: Line-to-Line Fault
Text Reports
- Click SC Reports from the ribbon.
- In the dialog box, select the
Create HV & LV Momentary (½-Cycle) Reports check box on the upper left and then click
OK.
- Click
3-Phase to return to 3-phase fault calculations.
- Click
Fault Bus(es). A high voltage momentary text report window is created. You can view
this report either by selecting
HV Momentary Report for ‘simple.dez (Base case)’ from the Window button or by double-clicking on the
window icon created in the lower left corner. Your text report will look similar to that in the figure below.
Figure 7: Short Circuit Text Report
- You can return to the one-line by selecting Simple.dez from the
Window menu.
Other Options
- Click SC Options from the ribbon and click the One-line Output tab.
In this dialog (shown in the figure below) you can specify
asymmetrical fault currents to be displayed on the one-line. You can also show fault currents in
Per-Unit or
MVA units and remote voltages in
kV units.
Figure 8: Short Circuit One-line Output Dialog Box
- Select the
Control tab.
Here you can specify other short circuit settings for your fault calculations. These changes take effect on all subsequent
faults after closing this dialog box.
Conclusion
This has been a brief overview of EasyPower’s short circuit program. The EasyPower Help topics cover this and other features in greater
depth. To open Help, click in the upper-right corner of the EasyPower window or press F1.