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2 Functions
154
7SD5 Manual
C53000-G1176-C169-1
Note
When switching onto a three-pole fault with the MHO characteristic, there will be no
voltage in the memory or unfaulted loop voltage available. To ensure fault clearance
when switching onto three-pole close-up faults, please make sure that in conjunction
with the configured MHO characteristic the instantaneous tripping function is always
enabled.
Determination of
Direction in Case of
Series-compensat-
ed Lines The displacement of the characteristics by the source impedance applies also for lines
with series capacitors. If a short-circuit occurs behind the local series capacitors, the
short-circuit voltage however reverses its direction until the protective spark gap has
picked up (see Figure 2-51).
Figure 2-51 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
The distance protection function would thus detect a wrong fault direction. However,
even in this case pickup of the correct directional MHO characteristic is ensured by
mixing in stored voltages (see Figure 2-52 a).
Since the voltage prior to the fault is mixed with the current voltage, the zeniths of the
MHO characteristic dependent on the source impedance and infeed conditions before
the fault are thus far displaced that the capacitor reactance — which is always smaller
than the series reactance — does not cause the apparent direction reversal (Figure 2-
52 b).
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