Where:
3I
0
is the earth-fault current (A)
I
R
is the current through the neutral point resistor (A)
I
L
is the current through the neutral point reactor (A)
I
C
is the total capacitive earth-fault current (A)
The neutral point reactor is normally designed so that it can be tuned to a position where the
reactive current balances the capacitive current from the network that is:
1
3
L
C
w
w
=
× ×
EQUATION1272 V1 EN-US
(Equation 127)
IEC05000216 V2 EN-US
Figure 123: 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.
7.3.2.2
Fault infeed from remote end
M17048-83 v5
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
, the equation for the bus voltage U
A
at A side is:
U
I
p Z
I
I
Rf
A
A
L
A
B
=
⋅
⋅
+
+
(
)
⋅
EQUATION1273-IEC-650 V2 EN-US
(Equation 128)
Section 7
1MRK 505 343-UEN B
Impedance protection
200
Application manual
Содержание Relion 670 series
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