2.7 Earth fault overcurrent protection in earthed systems (optional)
161
7SA522 Manual
C53000-G1176-C155-3
Figure 2-70 Directional characteristic with zero sequence power, example S
r
= setting value
S FORWARD
Selecting the Earth-
F a ul t e d P h a s e
Since the earth fault protection employs the quantities of the zero sequence system
and the negative sequence system, the faulted phase cannot be determined directly.
To enable single-pole automatic reclosure in case of high-resistance earth faults, the
earth fault protection function features a phase selector. By means of the distribution
of the currents and voltages it detects whether a fault is single-phase or multiple-
phase. If the fault is single-phase, it locates the faulted phase.
Once a multi-phase fault has been detected, a three-pole trip command is generated.
Three-pole tripping is also initiated if single-pole tripping is not permitted (due to the
setting or three-pole coupling of other internal extra functions or external devices via
binary input e.g., reclosing device).
The phase selector evaluates the phase-to-earth voltages, the phase currents and the
symmetrical components of the currents. If a single-phase fault can be detected with
certainty due to a considerable voltage collapse or a high overcurrent, the trip is initi-
ated in the concerned phase. Three-pole tripping is initiated accordingly if the currents
and/or voltages indicate a multi-phase fault.
If methods described cannot detect the fault type beyond doubt, the negative se-
quence system and the zero sequence system are ultimately filtered out of the phase
currents. The phase angle between negative sequence current and zero sequence
current is used to determine the fault type i.e. whether the fault is single-phase or
multi-phase. To this end, the phase currents are also evaluated to rectify the load
current if necessary. This method relies on the fact that in the event of a single phase
fault the fault-free phases can conduct either no fault currents at all or only such fault
currents that are approximately in phase.
The phase selector has an action time of approximately 40°ms. If the phase selector
has not made a decision during this time, three-pole tripping is initiated. Three-pole
tripping is initiated anyway as soon as a multi-pole fault has been detected, as de-
scribed above.
Figure 2-71 shows the logic diagram. The phase determined by the phase selector
can be processed selectively for each phase, for example the internal information “E/F
PICKUP L1” etc. is used for phase-selective signal transmission.
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