Load Encroachment - ABB RELION 670 SERIES Applications Manual

Section 8
Impedance protection
8.4.2.4
256
+
V I
= = × +
Z a p Z A
A L
I I
A A
EQUATION1274 V2 EN
The infeed factor (I +I
A B
source impedances at local and remote end.
V
A
E
A
SA
Z
SA
ANSI11000086 V1 EN
Figure 125: 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 126.
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.
I
×
B R
f
)/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 506 369-UUS -
(Equation 158)
V
B
I
B
Z
B
SB
21
ANSI11000086_1_en.vsd
Line distance protection REL670 2.2 ANSI
Application manual
E
SB