Hitachi Relion 670 Series Applications Manual page 164

Section 8
Current protection
8.5.1 Identification
Function description
Sensitive directional residual over
current and power protection
8.5.2 Application
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 2I
where φ is the angle between the residual current and the residual voltage (-2U
a characteristic angle. Alternatively, the function can be set to strict 2I
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 2I
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 (2U
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.
158
IEC 61850
identification
SDEPSDE
). The characteristic angle is chosen to -90º in such a network.
0
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IEC 60617 ANSI/IEEE C37.2
identification device number
67N
IN>
IEC15000260 V1 EN-US
level with a check of angle φ.
0
Railway application RER670
1MRK506375-UEN Rev. N
SEMOD172025-2 v4
SEMOD171959-4 v12
· cos φ,
0
), compensated with
0
· 2U ·
0 0
Application manual