ABB RELION REX640 Technical Manual page 474

Protection functions
the
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 core balance current transformers. The following figure describes how
measuring transformers can be connected to the protection relay.
Figure 271: Connection of measuring transformers
4.2.2.11 Signals
474
Pol reversal parameter to "True" or by switching the polarity of the residual
1MRS759142 F
REX640
Technical Manual