ABB Relion REG670, Техническое руководство

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8.6.2 Functionality
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
Section 8
1MRK502052-UEN B
Current protection
388
Technical manual