Figure 192: 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.
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
1MRS757644 H
396
620 series
Technical Manual
