Siemens siprotec 7SA6 User Manual page 212

Distance protection

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Recognition during

Single-Pole Open

for Non-Earthed

Logic of the earth fault recognition

The earth fault recognition is modified during the single-pole open condition with

single-pole automatic reclosure in an earthed system (Figure 6-18). In this case, the

magnitudes of the currents and voltages are monitored in addition to the angles

between the currents.

Earth fault recognition during single-pole open condition

In non-earthed systems (isolated system star point or resonant-earthed by means of

a Peterson coil) the measured displacement voltage is not used for fault detection.

Furthermore, in these systems a simple earth fault is assumed initially in case of a sin-

gle-phase fault and the fault detection is suppressed in order to avoid an erroneous

pickup as a result of the earth fault initiation transients. After a time delay 7,

3+$6, which can be set, the fault detection is released again; this is necessary for

the distance protection to still be able to detect a double earth fault with one base point

on a dead-end feeder.

If, however, an earth fault is already present in the system, it is detected by the

displacement voltage detection (8! &203,62/). In this case, there is no delay

time: an earth fault now occurring in a different phase can only be due to a double

earth fault. If, apart from the displacement measurement (8! &203,62/), there

is a fault detection in more than one phase, this is also rated as a double earth fault.

In this way, double earth faults can be detected even if no or only little earth current

flows via the measuring point.

Angle criteria

for load condition

C53000-G1176-C133-1

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