Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
150
Figure 2-55 Directional characteristic with quadrature or memorized voltages
Determination of direction in case of series-compensated lines
The directional characteristics and their displacement by the source impedance apply also for lines with series
capacitors. If a short-circuit occurs behind the local series capacitors, the short-circuit voltage however revers-
es its direction until the protective spark gap has picked up (see Figure 2-56).
Figure 2-56 Voltage characteristic while a fault occurs after a series capacitor.
a) without pickup of the protective spark gap
b) with pickup of the protective spark gap
The distance protection function would thus detect a wrong fault direction. The use of memorized voltages
however ensures that the direction is correctly detected (see Figure 2-57a).
Since the voltage prior to the fault is used to determine the direction, the peak displacement of the directional
characteristics in dependence of the source impedance and infeed conditions before the fault are displaced so
far that the capacitor reactance — which is always smaller than the series reactance — does not cause the
apparent direction reversal (Figure 2-57b).
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