ABB RELION 650 Series Applications Manual page 118

Section 7
Current protection
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 earth via the capacitances
between the phase conductors and earth, the residual current always has -90º phase shift
compared to the residual voltage (3U
network.
In resistance earthed networks or in Petersen coil earthed, with a parallel resistor, the active
residual current component (in phase with the residual voltage) should be used for the earth
fault detection. In such networks, the characteristic angle is chosen to 0º.
As the amplitude of the residual current is independent of the fault location, the selectivity of
the earth 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 earthed networks, with large
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 54: 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.
112
IN
UN
). The characteristic angle is chosen to -90º in such a
0
1MRK 504 158-UEN A
IEC13000013-1-en.vsd
Application manual