ABB REL670 2.2 IEC, Руководство по применению

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f
U
U
e
pn
=
max
EQUATION1268 V4 EN-US
(Equation 185)
Where:
U
max
is the highest fundamental frequency voltage on one of the healthy phases at single
phase-to-earth fault.
U
pn
is the phase-to-earth fundamental frequency voltage before fault.
Another definition for effectively earthed network is when the following
relationships between the symmetrical components of the network impedances are
valid, see equation
186 and equation 187 .
X X
0 1
3 ≤ ⋅
EQUATION1269 V4 EN-US
(Equation 186)
R X
£
0 1
EQUATION1270 V4 EN-US
(Equation 187)
The magnitude of the earth fault current in effectively earthed networks is high
enough for impedance measuring element to detect fault. However, in the same
way as for solid 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
SEMOD154680-58 v3
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 many times operated in radial, but can also be found
operating meshed.
Typically, 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 (3U0) 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 the formula
below:
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 263
Application manual