ABB REG670 - Page 196

same way as for solidly earthed 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 earthed networks
In high impedance networks, the neutral of the system transformers are connected to
the earth 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 earth -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 (3U
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 equation37.
( )
2
2
R
L C
0
3I I I I= + -
EQUATION1271 V3 EN (Equation 37)
Where:
3I
0
is the earth-fault current (A)
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 earth-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:
1
3
L
C
w
w
=
× ×
EQUATION1272 V1 EN
(Equation 38)
Section 8 1MRK 502 071-UEN -
Impedance protection
190 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual