The program has three prefault-voltage options:
•Assumed flat
•From a linear network solution and
•From a power flow solution.
You can select one of these options with the Fault Simulation tab of the File | Preferences command in the Main Window. The "Assumed flat" option is the default.
Assumed “flat” option
This prefault voltage profile assumes a uniform bus voltage magnitude at 0 degrees. The default voltage magnitude is 1.0 per-unit. You can change it in the Faults tab of the File | Preferences command dialog box.
A flat prefault voltage profile requires all the generators to have a uniform positive-sequence voltage behind the generator impedance. Also, there should be no current flow in the prefault state; otherwise, the voltage drop across the branches would create a non-uniform voltage profile.
The flat voltage profile is a valid Ohm’s Law solution in the good old days when short circuit programs allowed no loads, no positive-sequence shunts (such as capacitors), and no off-nominal-tap transformers.
A uniform voltage angle of zero degrees is also assumed in the prefault state because the older short circuit programs do not model the phase shifts of wye-delta transformers and phase shifters.
A prefault state with uniform voltage magnitude and angles is seldom the correct Ohm’s Law solution in today’s short circuit programs where loads, shunts and off-nominal transformers are allowed, and phase shifts of wye-delta transformers and phase shifters are modeled.
If one were to insist on a flat prefault voltage profile in a network where load, shunts, phase shifters, wye-delta transformers, and off-nominal transformers are present, one would soon discover that the post-fault currents do not sum to zero at certain buses. The excess current is equal to the current that goes to loads, shunts, off-nominal transformers, wye-delta transformers and phase shifter.
Our solution to this problem is to insert artificial current sources in the network to counter the prefault current flows. Specifically, the artificial, ideal current sources are used:
1) In parallel with any positive-sequence shunts or loads. The output of these current sources are labeled as "fictitious current sources" in the short-circuit solution report in text.
2) At both terminals of a phase shifter, wye-delta transformer, and any transformer with off-nominal taps. The outputs of these current sources are not shown in the printed output because the current injected into one terminal of a branch is canceled by an equal but opposite current source at the opposite terminal.
These ideal current sources do not affect the total fault current because they have infinite internal impedance. However, the current sources do affect the voltage solution near the fault. For this reason, we recommend the use of the flat prefault voltage option only for applications where:
1) The user is interested primarily in the total fault current, and
2) The user is not interested in the voltage magnitude of buses or in the phase shifts across network elements.
3) The user is not interested in the operations of distance relays (which are very sensitive to voltage magnitude and angles).
One such application is the evaluation of breaker ratings.
For most other applications, the user should consider using one of the other two prefault-voltage options.
Note: We chose the “Assume flat” to be the default option because in the early days of ASPEN, most utility wanted to make sure that ASPEN’s short circuit solution is identical to that from the older short circuit programs.
“From a linear network solution” option
Under this option, the prefault bus voltages are computed explicitly by OneLiner before any fault simulation.
Each generator in the network is modeled as an internal voltage source behind the generator impedance. The generator’s internal voltage magnitude is 1.0 per unit by default, but you can change it at will. The angle of the generators’ internal voltage source – called the generator reference angle – has to be consistent with the network topology and phase-shifting properties of transformers and phase shifters. You can change it as well.
The prefault bus voltages are calculated using a matrix form of Ohm’s Law (i.e., a linear network solution), taking into account all the network elements. The user can view the prefault voltages and current flows on the one-line diagram by executing the PowerScript file prefault.bas.
The prefault bus voltage in most cases should be in the neighborhood of 1.0 per-unit. When using the “From a linear network solution” option, you need to be vigilant in making sure that the prefault voltages are reasonable. A prefault voltage that is too high or too low will give rise to wrong fault currents.
You must also make sure that not too much current is flowing in the prefault network. A high current flow in the prefault state is usually indicative of a network anomaly.
We strongly recommend that you look for network anomalies periodically with the Check | Network Anomalies command. The most common mistakes that affect the prefault voltages are these:
•Transformer taps that are too high or too low.
•Transformers are phased incorrectly. For example, a wye-wye transformer is put in parallel with a wye-delta transformer.
•The generator reference angles are not consistent with the network topology and the phase-shifting properties of transformers and phase shifters. This problem can be fixed easily with the Network | Set Generator Reference Angle command. (Note: Make sure all the transformers are phase correctly before you set the generator reference angles.)
Unlike the “Assume flat” option, no artificial current sources are used in the short circuit solution.
The short circuit solution computed with the “From a linear network solution” is suitable for nearly all applications, including the coordination of overcurrent and distance relays.
“From a Power Flow Solution” option
Under this prefault option, the program solves a non-linear power flow solution prior to any fault simulation. The power-flow solution is used as the prefault voltage profile. To use this option when your Power Flow Program is a standalone executable, you must first solve the power flow within the Power Flow Program and save the case within the OLR file. The power-flow solution is visible on the one-line when you open the file with OneLiner.
If you use this option when your Power Flow Program is in the same executable file as OneLiner, the program will automatically prompt you to solve the power flow before solving a short circuit. You can view the pre-fault solution at any time with the PFlow | View Solution on 1-Line command.
Prior to the short-circuit solution, the magnitude and angle of the generator’s internal voltage source are computed by OneLiner using (1) the generator’s MW and MW output and (2) the power-flow voltage solution at the generator terminal. The values of the internal voltage source in the generator’s info dialog box are ignored. In addition, all the constant-power and constant-current loads are converted to constant impedances for the short circuit simulation.
We strongly recommend that you look for network anomalies periodically with the Check | Network Anomalies command. The most common mistakes that affect the prefault voltages are these:
•Transformer taps that are too high or too low.
•Transformers are phased incorrectly. For example, a wye-wye transformer is put in parallel with a wye-delta transformer.
The “From a Power Flow Solution” is the prefault option that gives the most accurate and realistic prefault voltages. This option is rarely used in North America. In some parts of the world, this option is used widely.