Siemens siprotec 7SD5 User Manual page 349

Fault Locating The measuring principle of the fault locator is rather similar to that of the distance pro-
Using the Single- tection function. Here, too, the device calculates the impedances.
Ended Fault
The measured value pairs of fault currents and fault voltages (in intervals of 1/20
Locator
period) are stored in a cyclic buffer and frozen shortly after the trip command is issued
before any distortion of the measured values occurs due to the opening of the circuit
breaker even with very fast circuit breakers. The filtering of the measured values and
the number of impedance calculations are automatically adapted to the number of sta-
bilized measured value pairs in the determined data window. If no sufficient data
window with stabilized values could be determined for fault location, the alarm
„Flt.Loc.invalid" is issued.
The evaluation of the measured values in the short-circuit loops is carried out after the
short-circuit has been cleared. Short-circuit loops are those which caused the trip. In
the event of tripping by the earth fault protection, the three phase–to-earth loops are
also evaluated.
Double-ended The double-ended fault locator method also considers line capacitance and line resis-
fault locator tance. It adapts the fault locating for an optimum matching between the voltages cal-
culated for the fault locating and the values measured at the line ends. It is assumed
in this context that voltages on a line cannot leap. The voltage at the presumed fault
location is calculated once with the values measured on the left side and once with
those from the right side. The actual fault location is where there is no or hardly any
difference between the voltage characteristics from the left and the right side.
The double-ended fault locating method is based on the assumption that in a line
without branches, with known currents and voltages at the inputs, the voltage can be
calculated for any location x of the line. This is true for both the left and the right line
side. Since the fault location voltage calculated from both sides must be the same, the
fault is located at the intersection of the two voltage characteristics. These character-
istics are calculated with the telegraph equation from the locally measured currents
and voltages, and the reactances per line unit. Figure 2-140 shows a simplified sche-
matic in which linear voltage characteristics are assumed.
Figure 2-140
7SD5 Manual
C53000-G1176-C169-1
Curves of voltages on a faulty line (simplified)
2.19 Fault Locator
349