GUID-A052B527-CE4E-4F2F-B0E6-6A4B0C445C60 V1 EN
Figure 71: 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 line-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 residual voltage (-
U
0
). Consequently, the relay characteristic angle (RCA) should be set to -90 degrees
and the operation criteria to I
0
sin(φ) 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. The figure below describes
how earth fault current is defined in isolated neutral networks.
For definitions of different directional earth-fault characteristics,
refer to the Technical manual.
1MRS756378 D Section 5
Protection functions
REF615 149
Application Manual
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