![background image](http://html1.mh-extra.com/html/abb/relion-670-series/relion-670-series_applications-manual_3052129274.webp)
Effectively earthed networks
GUID-5F4CCC18-2BAC-4140-B56C-B9002CD36318 v1
A network is defined as effectively earthed if the earth-fault factor f
e
is less than 1.4. The earth-
fault factor is defined according to equation
f
U
U
e
pn
=
max
EQUATION1268 V4 EN-US
(Equation 252)
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 source impedances are valid, see equation
and equation
0
1
X
3 X
< ×
EQUATION2122 V1 EN-US
(Equation 253)
0
1
R
R
£
EQUATION2123 V1 EN-US
(Equation 254)
Where
R
0
is the resistive zero sequence source impedance
X
0
is the reactive zero sequence source impedance
R
1
is the resistive positive sequence source impedance
X
1
is the reactive positive sequence source impedance
The magnitude of the earth-fault current in effectively earthed networks is high enough for
impedance measuring element to detect earth-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
GUID-099F53F9-7CD8-4D96-811D-7B55EF249FFF v1
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
Section 7
1MRK 505 343-UEN B
Impedance protection
268
Application manual
Summary of Contents for Relion 670 series
Page 1: ...RELION 670 SERIES Line differential protection RED670 Version 2 1 IEC Application manual...
Page 2: ......
Page 40: ...34...
Page 64: ...58...
Page 150: ...144...
Page 406: ...400...
Page 472: ...466...
Page 494: ...488...
Page 512: ...506...
Page 524: ...518...
Page 604: ...598...
Page 686: ...680...
Page 718: ...712...
Page 722: ...716...
Page 758: ...752...
Page 759: ...753...