ABB RED615 Technical Manual page 302

Section 4
Protection functions
296
the protection settings to stay the same when the resonance coil is disconnected
from between the neutral point and earth.
System neutral earthing is meant to protect personnel and equipment and to reduce
interference for example in telecommunication systems. The neutral earthing sets
challenges for protection systems, especially for earth-fault protection.
In isolated networks, there is no intentional connection between the system neutral
point and earth. The only connection is through the line-to-earth capacitances (C
of phases and leakage resistances (R
mainly capacitive and has –90 degrees phase shift compared to the residual voltage
(-Uo). The characteristic angle is -90 degrees.
In resonance-earthed networks, the capacitive fault current and the inductive
resonance coil current compensate each other. The protection cannot be based on
the reactive current measurement, since the current of the compensation coil would
disturb the operation of the relays. In this case, the selectivity is based on the
measurement of the active current component. This means that the residual current
is mainly resistive and has zero phase shift compared to the residual voltage (-Uo)
and the characteristic angle is 0 degrees. Often the magnitude of this component is
small, and must be increased by means of a parallel resistor in the compensation
equipment.
In networks where the neutral point is earthed through low resistance, the
characteristic angle is also 0 degrees (for phase angle). Alternatively, Iocos(φ)
operation can be used.
In solidly earthed networks, the Characteristic angle is typically set to +60 degrees
for the phase angle. Alternatively, Iosin(φ) operation can be used with a reversal
polarizing quantity. The polarizing quantity can be rotated 180 degrees by setting
the Pol reversal parameter to "True" or by switching the polarity of the residual
voltage measurement wires. Although the Iosin(φ) operation can be used in solidly
earthed networks, the phase angle is recommended.
Connection of measuring transformers in directional earth fault
applications
The residual current Io can be measured with a core balance current transformer or
the residual connection of the phase current signals. If the neutral of the network is
either isolated or earthed with high impedance, a core balance current transformer
is recommended to be used in earth-fault protection. To ensure sufficient accuracy
of residual current measurements and consequently the selectivity of the scheme,
the core balance current transformers should have a transformation ratio of at least
70:1. Lower transformation ratios such as 50:1 or 50:5 are not recommended.
Attention should be paid to make sure the measuring transformers are connected
correctly so that DEFxPDEF is able to detect the fault current direction without
failure. As directional earth fault uses residual current and residual voltage (-Uo),
the poles of the measuring transformers must match each other and also the fault
current direction. Also the earthing of the cable sheath must be taken into notice
). This means that the residual current is
0
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0
615 series
Technical Manual