ABB Relion 670 Series Technical Reference Manual page 443

1MRK505222-UUS C
Technical reference manual
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 facts:
• 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 current
• In some power systems a medium size neutral point resistor is used, for example,
in low impedance grounded system. Such a resistor will give a resistive ground-
fault current component of about 200 - 400 A at a zero resistive phase-to-ground
fault. In such a system the directional residual power protection gives better
possibilities for selectivity enabled by inverse time power characteristics.
Phase
currents
IN
Phase-
ground
voltages
UN
IEC13000013 V1 EN
Figure 217: Connection of SDEPSDE to analog preprocessing function block
Over current 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 sum of these inputs (3I0 and 3U0) will be used.
Section 7
Current protection
IEC13000013-1-en.vsd
437