ABB RELION REX640 Technical Manual page 460

Protection functions
Figure 258: 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
) of phases and leakage resistances (R
0
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
forward angle , Max forward angle , Min reverse angle or Max reverse angle .
259
fault in phase C.
460
illustrates a simplified equivalent circuit for an unearthed network with an earth
For definitions of different directional earth-fault characteristics, see
Chapter 4.2.2.9 Directional earth-fault characteristics
). This means that the residual current is
0
1MRS759142 F
Min
Figure
.
REX640
Technical Manual