ABB Relion 670 series, Applications Manual

Page: 440 / 760

8.6.2 Application
SEMOD171959-4 v11
In networks with high impedance earthing, the phase-to-earth fault current is significantly
smaller than the short circuit currents. Another difficulty for earth fault protection is that the
magnitude of the phase-to-earth fault current is almost independent of the fault location in
the network.
Directional residual current can be used to detect and give selective trip of phase-to-earth
faults in high impedance earthed networks. The protection uses the residual current
component 3I
0
· cos φ, where φ is the angle between the residual current and the residual
voltage (-3U
0
), compensated with a characteristic angle. Alternatively, the function can be set
to strict 3I
0
level with a check of angle φ.
Directional residual power can also be used to detect and give selective trip of phase-to-earth
faults in high impedance earthed networks. The protection uses the residual power
component 3I
0
· 3U
0
· cos φ, where φ is the angle between the residual current and the
reference residual voltage, compensated with a characteristic angle.
A normal non-directional residual current function can also be used with definite or inverse
time delay.
A backup neutral point voltage function is also available for non-directional residual
overvoltage protection.
In an isolated network, that is, the network is only coupled to earth via the capacitances
between the phase conductors and earth, the residual current always has -90º phase shift
compared to the residual voltage (3U
0
). The characteristic angle is chosen to -90º in such a
network.
In resistance earthed networks or in Petersen coil earthed, with a parallel resistor, the active
residual current component (in phase with the residual voltage) should be used for the earth
fault detection. In such networks, the characteristic angle is chosen to 0º.
As the amplitude of the residual current is independent of the fault location, the selectivity of
the earth fault protection is achieved by time selectivity.
When should the sensitive directional residual overcurrent protection be used and when
should the sensitive directional residual power protection be used? Consider the following:
• Sensitive directional residual overcurrent protection gives possibility for better sensitivity.
The setting possibilities of this function are down to 0.25 % of IBase, 1 A or 5 A. This
sensitivity is in most cases sufficient in high impedance network applications, if the
measuring CT ratio is not too high.
• Sensitive directional residual power protection gives possibility to use inverse time
characteristics. This is applicable in large high impedance earthed networks, with large
capacitive earth fault currents. In such networks, the active fault current would be small
and by using sensitive directional residual power protection, the operating quantity is
elevated. Therefore, better possibility to detect earth faults. In addition, in low impedance
earthed networks, the inverse time characteristic gives better time-selectivity in case of
high zero-resistive fault currents.
Section 8 1MRK 505 343-UEN B
Current protection
434
Application manual