ABB Relion 670 Series Applications Manual page 168

Section 8
Current protection
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.
Phase
currents
Phase-
ground
voltages
IEC13000013 V1 EN-US
Figure 81: Connection of SDEPSDE to analog preprocessing function block
Overcurrent functionality uses true 3I0, i.e. sum of GRPxL1, GRPxL2 and GRPxL3. For 3I0 to be
calculated, connection is needed to all three phase inputs.
Directional and power functionality uses IN and UN. If a connection is made to GRPxN this
signal is used, else if connection is made to all inputs GRPxL1, GRPxL2 and GRPxL3 the
internally calculated sum of these inputs (3I0 and 3U0) will be used.
8.6.3 Setting guidelines
The sensitive earth fault protection is intended to be used in high impedance earthed systems,
or in systems with resistive earthing where the neutral point resistor gives an earth fault
current larger than what normal high impedance gives but smaller than the phase-to-phase
short circuit current.
In a high impedance system the fault current is assumed to be limited by the system zero
sequence shunt impedance to earth and the fault resistance only. All the series impedances in
the system are assumed to be zero.
In the setting of earth fault protection, in a high impedance earthed system, the neutral point
voltage (zero sequence voltage) and the earth fault current will be calculated at the desired
sensitivity (fault resistance). The complex neutral point voltage (zero sequence) can be
calculated as:
162
IN
UN
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IEC13000013-1-en.vsd
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