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1MRK 506 369-UEN B
8.3.2.2
Line distance protection REL670 2.2 IEC
Application manual
IEC05000216 V2 EN-US
Figure 108:
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 of phenomenon, a separate function
called Phase preference logic (PPLPHIZ) is needed in medium and subtransmission
network.
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 elements 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.
Fault infeed from remote end
All transmission and most all sub-transmission networks are operated meshed.
Typical for this type of network is that fault infeed from remote end will happen
when fault occurs on the protected line. The fault current 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 109, the equation for the bus voltage U
(
U
=
I
⋅ ⋅
p Z
+
I
A
A
L
A
EQUATION1273-IEC-650 V2 EN-US
If we divide U
by I
we get Z present to the IED at A side.
A
A
U
A
Z
=
= p ·Z
+
A
L
I
A
EQUATION1274-IEC-650 V1 EN-US
)
+
I
⋅
Rf
B
I
+ I
A
B
·R
f
I
A
Section 8
Impedance protection
at A side is:
A
(Equation 107)
(Equation 108)
M17048-83 v5
213
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