ABB RELION 670 SERIES Applications Manual page 318

Section 8
Impedance protection
312
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 108.
( )
= + -
3I I I I
2
R
L C
0
EQUATION1271 V3 EN
Where:
3I is the ground-fault current (A)
0
I is the current through the neutral point resistor (A)
R
I is the current through the neutral point reactor (A)
L
I is the total capacitive ground-fault current (A)
C
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
ANSI05000216 V2 EN
Figure 149: High impedance grounded 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
2
1MRK 506 369-UUS -
(Equation 235)
(Equation 236)
Line distance protection REL670 2.2 ANSI
Application manual