ABB RET670 - Page 325

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 grounded networks
In high impedance networks, the neutral of the system transformers are connected to
the ground 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 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 (3V
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
55.
( )
2
2
R
L C
0
3I I I I= + -
EQUATION1271 V3 EN (Equation 246)
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 reactive current balances the capacitive current from the network that is:
1
3
L
C
w
w
=
× ×
EQUATION1272 V1 EN (Equation 247)
1MRK504116-UUS C Section 3
IED application
319
Application manual