ABB RET670 - Page 269

What is typical for this type of network is that the magnitude of the ground-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 (3V0) 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
181.
( )
2
2
R
L C
0
3I I I I= + -
EQUATION1271 V3 EN (Equation 181)
where
3I
0
is the ground-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 ground-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 (Equation 182)
IcIcIc
ILIR
en05000216.vsd
IEC05000216 V1 EN
Figure 113: High impedance grounding network
The operation of high impedance grounded networks is different compared to solid
grounded networks where all major faults have to be cleared very fast. In high
impedance grounded networks, some system operators do not clear single phase-to-
ground faults immediately; they clear the line later when it is more convenient. In case
1MRK504116-UUS C Section 3
IED application
263
Application manual