ABB RET670 Applications Manual page 458

Section 3
IED application
452
the residual voltage (-3V
function can be set to strict 3I
Directional residual power can also be used to detect and give selective trip of phase-to-
ground faults in high impedance grounded networks. The protection uses the residual
power component 3I · 3V
0
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 ground via the
capacitances between the phase conductors and ground, 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 grounded networks or in Petersen coil grounded, with a parallel resistor,
the active residual current component (in phase with the residual voltage) should be
used for the ground-fault detection. In such networks the characteristic angle is chosen
to 0º.
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.
), compensated with a characteristic angle. Alternatively, the
0
level with a check of angle 3I
0
· cos φ, where φ is the angle between the residual current
0
1MRK504116-UUS C
and cos φ.
0
Application manual