Load Encroachment - ABB RELION 670 SERIES Applications Manual

1MRK 506 369-UUS -
8.13.2.3
Line distance protection REL670 2.2 ANSI
Application manual
+
V
I
= = × +
Z a p Z A
A L
I I
A
EQUATION1274 V2 EN
The infeed factor (I +I
A B
source impedances at local and remote end.
V
A
Es
A
A
Z
SA
ANSI05000217 V1 EN
Figure 187: Influence of fault current infeed from remote line end
The effect of fault current infeed from remote line end is one of the most driving factors
to justify complementary protection to 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 and
reduces the underreach at the importing end. No settings are required for this function.

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 is illustrated in the left part of figure 188. 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 function which shapes the characteristic according to the right part
of figure 188. The load encroachment algorithm will increase the possibility to detect high
fault resistances, especially for phase-to-ground faults at remote line end.For example, for
a given setting of the load angle LdAngle the resistive blinder for the zone measurement
can be expanded according to the right part of the figure 188, given higher fault resistance
I
×
B R
f
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
Section 8
Impedance protection
(Equation 340)
VA
Es
B
B
I
Z
B
SB
21
en05000217_ansi.vsd
387