ABB REL670 2.2 IEC, Applications Manual

Page: 246 / 908

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-US
Figure 123: Influence of fault current infeed from remote end.
The effect of fault current infeed from remote end is one of the most driving factors
for justify complementary protection to distance protection.
8.4.2.4 Load encroachment
SEMOD154453-97 v4
In some cases the load impedance might enter the zone characteristic without any
fault on the protected line. The phenomenon is called load encroachment and it
might occur when an external fault is cleared and high emergency load is
transferred on the protected line. The effect of load encroachment for the mho
circle is illustrated to the left in figure
124 . The entrance of the load impedance
inside the characteristic is of course not allowed and the way to handle this with
conventional distance protection is to consider this with the settings, that is, to have
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.
jX
R
Load
Load
Load
Load
jX
R
Load
Load
Load
Load
No
operation
en06000403.vsd
IEC06000403 V1 EN-US
Figure 124: Load encroachment phenomena and shaped load encroachment
characteristic
Section 8
1MRK 506 369-UEN B
Impedance protection
240 Line distance protection REL670 2.2 IEC
Application manual