IEC05000216 V2 EN-US
Figure 137: High impedance earthing network
The operation of high impedance earthed networks is different compared to solid earthed
networks where all major faults have to be cleared very fast. In high impedance earthed
networks, some system operators do not clear single phase-to-earth faults immediately; they
clear the line later when it is more convenient. In case of cross-country faults, many network
operators want to selectively clear one of the two earth faults. To handle this type phenomena
Phase preference logic function (PPLPHIZ) is needed, which is not common to be used in
transmission applications.
In this type of network, it is mostly not possible to use distance protection for detection and
clearance of earth faults. The low magnitude of the earth-fault current might not give start of
the zero-sequence measurement element or the sensitivity will be too low for acceptance. For
this reason a separate high sensitive earth-fault protection is necessary to carry out the fault
clearance for single phase-to-earth fault.
7.4.2.3
Fault infeed from remote end
SEMOD154453-84 v4
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 will 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
, we can draw the equation for the bus voltage V
A
at left side as:
U
I
p Z
I
I
Rf
A
A
L
A
B
=
⋅
⋅
+
+
(
)
⋅
EQUATION1273-IEC-650 V2 EN-US
(Equation 175)
If we divide V
A
by IA we get Z present to the IED at A side
I
A
+ I
B
U
A
I
A
I
A
Z
A
= = p ·Z
L
+ ·R
f
EQUATION1274-IEC-650 V1 EN-US
(Equation 176)
The infeed factor (I
A
+I
B
)/I
A
can be very high, 10-20 depending on the differences in source
impedances at local and remote end.
Section 7
1MRK 505 343-UEN B
Impedance protection
222
Application manual
Summary of Contents for Relion 670 series
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