ABB Relion 670 series, Руководство по применению

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R X
£
0 1
EQUATION1270 V4 EN-US
(Equation 210)
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:
( )
2
2
R
L C
0
3I I I I
= + -
EQUATION1271 V3 EN-US
(Equation 211)
Where:
3I0 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 that is:
1
3
L
C
w
w
=
× ×
EQUATION1272 V1 EN-US
(Equation 212)
Section 7 1MRK 505 343-UEN B
Impedance protection
244
Application manual