magnitude of the phase-to-ground 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-ground faults
in high impedance grounded 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 (-3V
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-ground
faults in high impedance grounded networks. The protection uses the residual power component
3I
0
· 3V
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 ground via the capacitances
between the phase conductors and ground, the residual current always has -90º phase shift
compared to the residual voltage (3V
0
). The characteristic angle is chosen to -90º in such a
network.
In resistance grounded networks or in Petersen coil grounded, with a parallel resistor, the active
residual current component (in phase with the residual voltage) should be used for the ground
fault detection. In such networks, the characteristic angle is chosen to 0º.
As the magnitude of the residual current is independent of the fault location, the selectivity of the
ground 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 grounded networks, with large
capacitive ground 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 ground faults. In addition, in low impedance grounded
networks, the inverse time characteristic gives better time-selectivity in case of high zero-
resistive fault currents.
1MRK 502 066-UUS B Section 8
Current protection
505
Technical manual
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