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Where
R
0
is the zero sequence resistance
X
0
is the zero sequence reactance
X
1
is the positive sequence reactance
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 should therefore always be complemented with other protection function(s) that can carry
out the fault clearance in this case.
High impedance earthed networks
SEMOD154453-58 v4
In high impedance networks the neutral of the system transformers are connected to the earth
through high impedance, mostly a reactance in parallel with a high resistor.
This type of network is many times operated in radial, but can also be found operating meshed
networks.
What is typical for this type of network is that the magnitude of the earth-fault current is very
low compared to the short-circuit current. The voltage on the healthy phases will get a
magnitude of √3 times the phase voltage during the fault. The zero-sequence voltage (3U
0
) will
have the same magnitude in different places in the network due to low voltage drop
distribution.
The magnitude of the total fault current can be calculated according to equation
(
)
2
2
R
L
C
0
3I
I
I
I
=
+
-
EQUATION1271 V3 EN-US
(Equation 173)
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
inductive current balances the capacitive current from the network that is:
1
3
L
C
w
w
=
× ×
EQUATION1272 V1 EN-US
(Equation 174)
1MRK 505 343-UEN B
Section 7
Impedance protection
221
Application manual
Содержание Relion 670 series
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