Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
167
Determination of direction in case of series-compensated lines
If a short-circuit occurs behind the local series capacitor, the short-circuit voltage however is inverted until the
protective spark gap PSG has picked up (see the following Figure).
Figure 2-66 Voltage characteristic for a short-circuit after a series capacitor
a) without pickup of the protective spark gap
b) with pickup of the protective spark gap
As the polarization voltage of the MHO characteristic consists of the currently measured voltage and the voltage
measured before the occurrence of the fault, it is possible that the distance protection function would detect a
wrong fault direction. To prevent spurious trippings or erroneous pickups, a memory voltage proportion of up to
80 % could be necessary. This, however, would lead to a considerable increase of the MHO characteristic. This
increase is usually not acceptable.
Therefore an additional measurement with exclusively memorized voltage is performed for applications with
series compensation. This ensures a correct direction measurement at any time (see Figure 2-2-67) and the
MHO distance zones are not increased more than necessary.
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