Figure 144: Definition of the relay characteristic angle, RCA=–90 degrees in an
isolated network
Directional earth-fault protection in an isolated neutral network
In isolated networks, there is no intentional connection between the system neutral
point and earth. The only connection is through the phase-to-earth capacitances (C
0
) of phases and leakage resistances (R
0
). This means that the residual current is
mainly capacitive and has a phase shift of -90 degrees compared to the polarizing
voltage. Consequently, the relay characteristic angle (RCA) should be set to -90
degrees and the operation criteria to "IoSin" or "Phase angle". The width of the
operating sector in the phase angle criteria can be selected with the settings
Min
forward angle
,
Max forward angle
,
Min reverse angle
or
Max reverse angle
.
Figure
145
illustrates a simplified equivalent circuit for an unearthed network with an earth
fault in phase C.
For definitions of different directional earth-fault characteristics, see
Chapter 4.2.2.8 Directional earth-fault characteristics
.
Protection functions 1MRS758755 C
302 REC615 & RER615
Technical Manual
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