ABB RELION RET670 Applications Manual page 376

Section 8
Impedance protection
370
£
R
R
0 1
EQUATION2123 V1 EN-US
Where
R is the resistive zero sequence of the source
0
X is the reactive zero sequence of the source
0
R is the resistive positive sequence of the source
1
X is the reactive positive sequence of the source
1
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 solidly grounded 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 often operated radially, but can also be found operating as a
meshed network.
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 (3 V ) will have the same magnitude in different places in the
0
network due to low voltage drop distribution.
The magnitude of the total fault current can be calculated according to equation262.
( )
2
= + -
3I I I I
2
R
L C
0
EQUATION1271 V3 EN-US
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)
1MRK 504 163-UUS A
(Equation 261)
GUID-02F306F5-1038-42AC-AFAE-3F8423C4C066 v5
(Equation 262)
Transformer protection RET670 2.2 ANSI
Application manual