ABB Relion 670 series, Applications Manual

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Z <
ZL
Z <
Es
A
V
A
VA
A B
Es
B
I
A
I
B
R
f
p*ZL
(1-p)*ZL
Z
SA
Z
SB
en05000217.vsd
IEC05000217 V1 EN-US
Figure 151: Influence of fault 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.
7.5.2.4 Load encroachment
SEMOD154680-97 v3
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 to the left in figure 152 . The entrance of the load
impedance inside the characteristic is of cause 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.
The IED has a built in function which shapes the characteristic according to the right figure 4.
The load encroachment algorithm will increase the possibility to detect high fault resistances,
especially for phase-to-earth faults at remote end. For example for a given setting of the load
angle
ARGLd for the load encroachment function, the resistive blinder for the zone
measurement can be expanded according to the right in figure 152 given higher fault
resistance coverage without risk for unwanted operation due to load encroachment. This is
valid in both directions.
The use of the load encroachment feature is essential for long heavy loaded lines, where there
might be a conflict between the necessary emergency load transfer and necessary sensitivity
of the distance protection. ZMMPDIS function can also preferably be used on heavy loaded
medium long lines. For short lines the major concern is to get sufficient fault resistance
coverage and load encroachment is not a major problem. So, for short lines, the load
encroachment function could preferable be switched off.
The settings of the parameters for load encroachment are done in the Phase selection with
load enchroachment, quadrilateral characteristic (FDPSPDIS).
Section 7 1MRK 505 343-UEN B
Impedance protection
246
Application manual