high resistance faults and must, therefore, always be complemented with other protection
function(s) that can carry out the fault clearance in this case.
8.1.2.3 Fault infeed from remote end
All transmission and most all sub transmission networks are operated meshed. Typical for
this type of network is that we will have fault infeed from remote end when fault occurs
on the protected line. The fault infeed may enlarge the fault impedance seen by the
distance protection. This effect is very important to keep in mind when both planning the
protection system and making the settings.
With reference to figure
58, we can draw the equation for the bus voltage Va at left side
as:
( )A A L A B fV I p Z I I R= × × + + ×
EQUATION1273 V1 EN (Equation 28)
If we divide Va by IA we get Z present to the IED at A side
a
A B
A L f
A A
V
I I
Z p Z R
I I
+
= = × + ×
EQUATION1274 V2 EN (Equation 29)
The infeed factor (IA+IB)/IA can be very high, 10-20 depending on the differences in
source impedances at local and remote end.
21
ZL
21
EsA
VA
VA
A
B
EsB
IA
IB
Rf
p*ZL
(1-p)*ZL
ZSA
ZSB
en05000217_ansi.vsd
ANSI05000217 V1 EN
Figure 58: Influence of fault infeed from remote end
The effect of fault current infeed from remote end is one of the most driving factors to
justify complementary protection to distance protection.
Section 8 1MRK 506 369-UUS -
Impedance protection
160 Line distance protection REL670 2.2 ANSI
Application manual
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