ABB RELION REX640 Technical Manual page 430

Section 4
Protection functions
424
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Figure 226: Earth-fault situation in an isolated network
Directional earth-fault protection in a compensated network
In compensated 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 protection relays. In this case, the selectivity is based on the measurement of the
active current component. The magnitude of this component is often small and must
be increased by means of a parallel resistor in the compensation equipment. When
measuring the resistive part of the residual current, the Operation mode should be set
to "Phase angle" and the relay characteristic angle (RCA) should be set to 0 degrees.
Alternatively, the Operation mode can be set to "IoCos".
simplified equivalent circuit for a compensated network with an earth fault in phase C.
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Figure 227: Earth-fault situation in a compensated network
The Petersen coil or the earthing resistor may be temporarily out of operation. To keep
the protection scheme selective, it is necessary to update the characteristic angle
setting accordingly. This can be done with an auxiliary input in the relay which
receives a signal from an auxiliary switch of the disconnector of the Petersen coil in
1MRS759142 C
Figure 227 illustrates a
REX640
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