Date Code 20011026 ONEBUS D-1
SEL-321/321-1 Instruction Manual
APPENDIX D: ONEBUS: PROGRAM TO COMPUTE TEST
SET SETTINGS FOR TESTING DISTANCE
RELAYS
The BASIC program in this note determines voltages and currents which would appear on
distance relay terminals for ground and phase faults on a radial system with source impedance at
the same angle as line impedance. It is useful in determining test voltage and current settings for
SEL distance relays and fault locating equipment.
The program was initially designed to run on a TRS-80 Model 100 briefcase computer but may
be installed on virtually any personal computer or laptop.
The program first prompts you for the positive- and zero-sequence impedances of the trans-
mission line. Enter the data in secondary ohms for the entire length of the protected line.
Next, you may enter fault resistance, which is used in the ground-fault computations.
Enter source impedance as a per-unit value with a base of the previously-entered transmission
line data. For example, if the radial system has a source impedance of about ten percent of the
entered line impedance, enter 0.1 for the per-unit distance from the source to the bus.
Specify the distance from the bus to the fault as a fraction of the total line length. To obtain the
voltages and currents for a fault one-half the way down the line from the bus, enter 0.5 for the
distance from the bus to the fault.
After you enter this data, the program begins computations. The display then shows voltages and
currents for both an AG and BC fault. These data can be entered into any active test source.
The bottom line of the display offers you a choice of entering new impedance data (I), changing
the distance from the source to the bus (B), specifying a new fault location (F), or quitting (Q).
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