ABB REL650 Applications Manual page 122

Section 6
Impedance protection
116
The magnitude of the ground-fault current in effectively grounded networks is high
enough for impedance measuring elements to detect ground faults. However, in the same
way as for solidlygrounded 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 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 ) will have the same magnitude in different places in the network due to low voltage
0
drop distribution.
The magnitude of the total fault current can be calculated according to equation 39.
( )
2
= + -
3I I I I
2
R L C
0
EQUATION1271 V3 EN
Where:
3I is the ground-fault current (A)
0
IR is the current through the neutral point resistor (A)
IL is the current through the neutral point reactor (A)
IC 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
w
=
L
× × w
3 C
EQUATION1272 V1 EN
1MRK 506 334-UUS A
(Equation 39)
(Equation 40)
Application manual