ABB RELION REX640 Technical Manual page 443

1MRS759142 C
REX640
Technical Manual
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
phases and leakage resistances (R
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 when using
Protection functions
). This means that the residual current is mainly
0
Section 4
) of
0
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