Setting Guidelines - Hitachi Relion 670 Series Applications Manual

Section 8
Current protection
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.
Phase
currents
Phase
ground
voltages
ANSI13000013 V1 EN-US
Figure 64:
Overcurrent functionality uses true 3I0, i.e. sum of GRPxA, GRPxB and GRPxC. For 3I0 to be calculated,
connection is needed to all three phase inputs.
Directional and power functionality uses IN and VN. If a connection is made to GRPxN this signal is
used, else if connection is made to all inputs GRPxA, GRPxB and GRPxC the internally calculated sum
of these inputs (3I0 and 3V0) will be used.
8.4.3

Setting guidelines

The sensitive ground-fault protection is intended to be used in high impedance grounded systems, or in
systems with resistive grounding where the neutral point resistor gives an ground-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 ground and the fault resistance only. All the series impedances in the system are
assumed to be zero.
In the setting of ground-fault protection, in a high impedance grounded system, the neutral point voltage
(zero sequence voltage) and the ground-fault current will be calculated at the desired sensitivity (fault
resistance). The complex neutral point voltage (zero sequence) can be calculated as:
132
IN
VN
Connection of SDEPSDE to analog preprocessing function block
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ANSI13000013-1-en.vsd
Phasor measurement unit RES670
Application manual
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