Another definition for effectively earthed network is when the following relationships between
the symmetrical components of the network impedances are valid, as shown in equation
and equation
X
X
0
1
3
≤ ⋅
EQUATION1269 V4 EN-US
(Equation 44)
R
X
£
0
1
EQUATION1270 V4 EN-US
(Equation 45)
The magnitude of the earth fault current in effectively earthed networks is high enough for
impedance measuring element to detect earth-fault. However, in the same way as for solid
earthed networks, distance protection has limited possibilities to detect high resistance faults
and must, therefore, always be complemented with other protection function(s) that can carry
out the fault clearance in this case.
7.1.2.3
Fault infeed from remote end
SEMOD168232-84 v2
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.
, we can draw the equation for the bus voltage Va at left side as:
(
)
A
A
L
A
B
f
V
I p Z
I
I
R
= × ×
+
+
×
EQUATION1273 V1 EN-US
(Equation 46)
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-US
(Equation 47)
The infeed factor (IA+IB)/IA can be very high, 10-20 depending on the differences in source
impedances at local and remote end.
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 71: Influence of fault infeed from remote end
1MRK 505 343-UEN B
Section 7
Impedance protection
147
Application manual
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