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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 152)
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 153)
Section 8
1MRK 506 369-UEN B
Impedance protection
238
Line distance protection REL670 2.2 IEC
Application manual
Содержание REL670 2.2 IEC
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