ABB REF615 Applications Manual page 156

Section 5
Protection functions
150
A070441 V1 EN
Figure 72: 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 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 I
The figure below describes how earth fault current is defined in compensated neutral
networks.
A070444 V2 EN
Figure 73: 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 is done with an auxiliary input in the relay which receives
a signal from an auxiliary switch of the disconnector of the Petersen coil in
1MRS756378 D
cos(φ) or phase angle.
0
REF615
Application Manual