phase-to-ground fault, it corresponds to the per-loop impedance, including the ground
return impedance.
R
X
RLdFwd
RLdRev
LdAngle
LdAngle
LdAngle
LdAngle
R
X
RLdFwd
RLdRev
LdAngle
90%
10%
10%
LdAngle
Possible
load
LdAngle
LdAngle
ANSI12000176-1-en.vsd
ANSI12000176 V1 EN
Figure 223: Load impedance limitation with load encroachment
During the initial current change for phase-to-phase and for phase-to-ground faults,
operation may be allowed also when the apparent impedance of the load encroachment
element is located in the load area. This improves the dependability for fault at the remote
end of the line during high load. Although it is not associated to any standard event, there
is one potentially hazardous situation that should be considered. Should one phase of a
parallel circuit open a single pole, even though there is no fault, and the load current of that
phase increase, there is actually no way of distinguish this from a real fault with similar
characteristics. Should this accidental event be given precaution, the phase-to-ground
reach (RFPG) of all instantaneous zones has to be set below the emergency load for the
pole-open situation. Again, this is only for the application where there is a risk that one
breaker pole would open without a preceding fault. If this never happens, for example
when there is no parallel circuit, there is no need to change any phase-to-ground reach
according to the pole-open scenario.
8.13.4.10 Parameter setting guidelines
IMinOpPGZx and IMinOpPPZx
The ability for a specific loop and zone to issue start or trip is inhibited if the magnitude
of the input current for this loop falls below the threshold value defined by these settings.
The output of a phase-to-ground loop n is blocked if In < IMinOpPGZRV(Zx). In is the
RMS value of the fundamental current in phase n.
The output of a phase-to-phase loop mn is blocked if Imn < IMinOpPPZx. Imn is the RMS
value of the vector difference between phase currents m and n.
Section 8 1MRK 506 369-UUS -
Impedance protection
432 Line distance protection REL670 2.2 ANSI
Application manual
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