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
and
.
0
1
X
3 X
< ×
EQUATION2122 V1 EN-US
(Equation 124)
0
1
R
R
£
EQUATION2123 V1 EN-US
(Equation 125)
Where
R
0
is the zero sequence source resistance
X
0
is the zero sequence source reactance
R
1
is the positive sequence source resistance
X
1
is the positive sequence source reactance
The magnitude of the earth-fault current in effectively earthed networks is high enough for
impedance measuring elements to detect earth faults. However, in the same way as for
solidlyearthed 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
M17048-57 v6
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
networks.
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 equation
(
)
2
2
R
L
C
0
3I
I
I
I
=
+
-
EQUATION1271 V3 EN-US
(Equation 126)
1MRK 505 343-UEN B
Section 7
Impedance protection
199
Application manual
Содержание Relion 670 series
Страница 1: ...RELION 670 SERIES Line differential protection RED670 Version 2 1 IEC Application manual...
Страница 2: ......
Страница 40: ...34...
Страница 64: ...58...
Страница 150: ...144...
Страница 406: ...400...
Страница 472: ...466...
Страница 494: ...488...
Страница 512: ...506...
Страница 524: ...518...
Страница 604: ...598...
Страница 686: ...680...
Страница 718: ...712...
Страница 722: ...716...
Страница 758: ...752...
Страница 759: ...753...