ABB REG670 - Load Encroachment

UL1
R1 + j X1
IL1
RFPE
Phase-to-earth
fault in phase L1
Phase-to-phase
fault in phase
L1-L2
Three-phase
fault or Phase-to-
phase-earth fault
(Arc + tower
resistance)
0
(R0-R1)/3 +
j (X0-X1)/3 )
IN
UL1
R1 + j X1
IL1
UL2
R1 + j X1
IL2
RFPP
UL1
R1 + j X1
IL1
UL3
R1 + j X1
IL3
0.5·RFPP
0.5·RFPP
(Arc resistance)
Phase-to-earth
element
Phase-to-phase
element L1-L2
Phase-to-phase
element L1-L3
IEC11000419-2-en.vsd
IEC11000419 V2 EN
Figure 84: Fault loop model
7.3.7.8 Load encroachment
In some cases the measured load impedance might enter the set 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 85. 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 feature which shapes the characteristic according to the
characteristic shown in
Figure 85. The load encroachment algorithm will increase the
possibility to detect high fault resistances, especially for phase-to-earth faults at
remote line end. For example, for a given setting of the load angle ArgLd, the resistive
blinder for the zone measurement can be set according to Figure 85 affording higher
fault resistance coverage without risk for unwanted operation due to load
1MRK502052-UEN B Section 7
Impedance protection
219
Technical manual