ABB RELION RET670, Applications Manual

Page: 563 / 1016

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
IN
VN
ANSI13000013-1-en.vsd
ANSI13000013 V1 EN-US
Figure 277: Connection of SDEPSDE to analog preprocessing function block
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.
1MRK 504 163-UUS A
Section 9
Current protection
Transformer protection RET670 2.2 ANSI 557
Application manual