with the settings, that is, with a security margin between the distance zone and the minimum load
impedance. This has the drawback that it will reduce the sensitivity of the protection, that is, the
ability to detect resistive faults.
The IED has a built-in function which shapes the characteristic according to the right part of figure
182. The load encroachment algorithm will increase the possibility to detect high fault resistances,
especially for phase-to-ground faults at remote line end.For example, for a given setting of the
load angle
LdAngle
the resistive blinder for the zone measurement can be expanded according to
the right part of the figure 182, given higher fault resistance coverage without risk for unwanted
operation due to load encroachment. This is valid in both directions.
The use of the load encroachment feature is essential for long heavily loaded lines, where there
might be a conflict between the necessary emergency load transfer and necessary sensitivity of
the distance protection. The function can also preferably be used on heavy loaded medium long
lines. For short lines, the major concern is to get sufficient fault resistance coverage. Load
encroachment is not a major problem. Nevertheless, always set
RLdFwd
,
RldRev
and
LdAngle
according to the expected maximum load since these settings are used internally in the
function as reference points to improve the performance of the phase selection.
R
X
Zm
RldRev
R
Zm
RLdFwd
ZL
ANSI05000495_2_en.vsd
Load impedance area in
forward direction
LdAngle
LdAngle
LdAngle
LdAngle
ANSI05000495 V2 EN-US
Figure 182: Load encroachment phenomena and shaped load encroachment characteristic
7.9.7.9 Simplified logic schemes
GUID-486BDAF6-6AE2-4671-91AF-4D2F8EBE71AC v3
PHSA, PHSB,...PHSCA are internal binary logical signals from the Phase-selection element. They
correspond directly to the six loops of the distance zones and determine which loops should be
released to possibly issue a pickup or a trip.
FWA, FWB,...FWCA and RVA, RVB,...RVCA are the internal binary signals from the Directional
element. An FW signal is set true if the criteria for a forward fault or load is fulfilled for its
particular loop. The equivalent applies to the reverse (RV) signals.
The internal input 'IN present' is true if the residual current (3I0) exceeds 7% of IBase. However, if
current transformer saturation is detected, this criterion is changed to residual voltage (3V0)
exceeding 5% of VBase/sqrt(3) instead.
1MRK 502 066-UUS B Section 7
Impedance protection
357
Technical manual
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