Load Encroachment - ABB RELION RET670 Applications Manual

Section 8
Impedance protection
8.12.2.3
372
= × × + +
V I p Z ( I
A A L A
EQUATION1273 V1 EN-US
If we divide V by I we get Z present to the IED at A side.
A A
V I
= = × +
a
Z p Z A
A L
I
A
EQUATION1274 V2 EN-US
The infeed factor (I +I
A B
source impedances at local and remote end.
V
A
Es
A
A
Z
SA
ANSI05000217 V1 EN-US
Figure 175: 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.

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 176. 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.
×
I ) R
B f
+
I
×
B R
f
I
A
)/I can be very high, 10-20 depending on the differences in
A
p*ZL
(1-p)*ZL
I
A
ZL
21
R
f
1MRK 504 163-UUS A
(Equation 264)
(Equation 265)
VA
Es
B
I
Z
B
SB
21
en05000217_ansi.vsd
GUID-6785BF05-2775-4422-8077-A663D01C6C07 v6
Transformer protection RET670 2.2 ANSI
Application manual
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