ABB REG670 - Load Encroachment

The infeed factor (I
A
+I
B
)/I
A
can be very high, 10-20 depending on the differences in
source impedances at local and remote end.
Z <
ZL
Z <
E
SA
U
A
U
B
A
B
E
SB
I
A
I
B
R
f
p*ZL
(1-p)*ZL
Z
SA
Z
SB
IEC09000247-1-en.vsd
IEC09000247 V1 EN
Figure 82: Influence of fault current infeed from remote line end
The effect of fault current infeed from the remote line end is one of the most driving
factors to justify complementary protection for distance protection.
When the line is heavily loaded, the distance protection at the exporting end will have
a tendency to overreach. To handle this phenomenon, the IED has an adaptive built-
in algorithm, which compensates the overreach tendency of zone 1 at the exporting
end. No settings are required for this feature.
8.2.2.3 Load encroachment
In some cases the measured load impedance might enter the set zone characteristic
without any fault on the protected line. This phenomenon is called load encroachment
and it might occur when an external fault is cleared and high emergency load is
transferred onto the protected line. The effect of load encroachment is illustrated on
the left in figure
83. A load impedance within the characteristic would cause an
unwanted trip. The traditional way of avoiding this situation is to set the distance zone
resistive reach with a security margin to the minimum load impedance. The drawback
with this approach is that the sensitivity of the protection to detect resistive faults is
reduced.
The IED has a built in feature which shapes the characteristic according to the
characteristic shown in figure
83. The load encroachment algorithm will increase the
possibility to detect high fault resistances, especially for phase-to-earth faults at the
remote line end. For example, for a given setting of the load angle ArgLd, the resistive
blinder for the zone measurement can be set according to figure 83 affording higher
fault resistance coverage without risk for unwanted operation due to load
encroachment. Separate resistive blinder settings are available in forward and reverse
direction.
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
Section 8 1MRK 502 071-UEN -
Impedance protection
192 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual