ABB Relion REQ650, Applications Manual

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6.7 Sensitive directional residual overcurrent and power
protection SDEPSDE
6.7.1 Identification
Function description IEC 61850
identification
IEC 60617
identification
ANSI/IEEE C37.2
device number
Sensitive directional residual over
current and power protection
SDEPSDE - 67N
6.7.2 Application
In networks with high impedance earthing, the phase-to-earth fault current is
significantly smaller than the short circuit currents. Another difficulty for earth-fault
protection is that the magnitude of the phase-to-earth fault current is almost
independent of the fault location in the network.
Directional residual current can be used to detect and give selective trip of phase-to-
earth faults in high impedance earthed networks. The protection uses the residual
current component 3I
0
· cos φ, where φ is the angle between the residual current and
the residual voltage (-3U
0
), compensated with a characteristic angle. Alternatively,
the function can be set to strict 3I
0
level with a check of angle 3I
0
and cos φ.
Directional residual power can also be used to detect and give selective trip of phase-
to-earth faults in high impedance earthed networks. The protection uses the residual
power component 3I
0
· 3U
0
· cos φ, where φ is the angle between the residual current
and the reference residual voltage, compensated with a characteristic angle.
A normal non-directional residual current function can also be used with definite or
inverse time delay.
A back-up neutral point voltage function is also available for non-directional sensitive
back-up 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 reference residual voltage. The characteristic angle is chosen to
-90º in such a 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.
Section 6
1MRK 505 291-UEN A
Current protection
122
Application manual