ABB RELION 620 Series Technical Manual page 388

Section 4
Protection functions
382
A070441 V1 EN
Figure 183: 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 relay characteristic angle
(RCA) should be set to 0 degrees and the operation criteria to "IoCos" or "Phase
angle". Figure 184 illustrates a simplified equivalent circuit for a compensated
network with an earth fault in phase C.
A070444 V2 EN
Figure 184: 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 protection relay
which receives a signal from an auxiliary switch of the disconnector of the Petersen
1MRS757644 F
620 series
Technical Manual