Variations of the source impedance ratio (SIR) affect the magnitude and phase angle
of the calculated voltages and currents.
The source impedance ratio may be used to adjust the current magnitude in cases
where calculated currents exceed the output range of your current source. For
example, on a short transmission line with a low source impedance, high current
magnitudes result. Raising the source impedance ratio in ONEBUS results in a
lower calculated current combined with a lower voltage.
If the source impedance ratio is unknown, enter a source impedance ratio of one to
determine whether calculated currents are within the range of the current source. It
is desirable to keep the source impedance ratio as realistic as possible.
Line 5:
The final input is the "DIST BUS TO FAULT." This is the distance from the relay
terminal to the fault location. To obtain the voltages and currents for a fault at 80%
of the line, enter 0.80 for the per unit distance from the bus to fault.
C. Determining Fault Simulation Values - Three-Phase Faults
You can calculate three-phase fault voltages and currents with the ONEBUS program. Enter
the positive-sequence line impedance values at the prompt for zero-sequence values. The
following example uses the example relay settings:
Figure 7.6: Example ONEBUS Input Data for Three-Phase Faults
The following output should result:
Figure 7.7: Example ONEBUS Result Screen for Three-Phase Faults
Date Code 920508
Maintenance & Testing
SEL-221F, -1 Instruction Manual
7-17
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Page 161: ...SETTINGS SHEET PAGE 5 OF 6 FOR SEL 221F RELAY DATE Date Code 961208 ...
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