Load Encroachment - ABB REL670 2.2 IEC Applications Manual

Section 8
Impedance protection
8.4.2.4
240
The infeed factor (I +I
A B
in source impedances at local and remote end.
U
A
E
A
SA
Z
SA
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.

Load encroachment

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
Load
Load
IEC06000403 V1 EN-US
Figure 124: Load encroachment phenomena and shaped load encroachment
characteristic
)/I can be very high, 10-20 depending on the differences
A
p*ZL
(1-p)*ZL
I
A
ZL
Z <
R
Load
R
Load
1MRK 506 369-UEN B
U
B
B
I
Z
B
Z <
f
IEC09000247-1-en.vsd
jX
Load
Load
No
operation
Line distance protection REL670 2.2 IEC
Application manual
E
SB
SB
SEMOD154453-97 v4
Load
R
Load
en06000403.vsd