maximum torque line
Min forward angle
Min reverse angle
Min operate
current
Max forward angle
Max reverse angle
Backward
tripping zone
zero torque line
Non-tripping zone
Forward
tripping zone
Characteristic angle
= -90 deg
IG
(operating quantity)
-VG
(polarizing
quantity)
GUID-04370359-62D0-4D09-B296-D72C961293C4 V1 EN
Figure 154: Definition of the relay characteristic angle, RCA=–90 degrees in an
isolated network
Directional ground-fault protection in an isolated neutral network
In isolated networks, there is no intentional connection between the system neutral point
and ground. The only connection is through the phase-to-ground 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 155 illustrates a simplified
equivalent circuit for an ungrounded network with a ground fault in phase C.
For definitions of different directional ground-fault characteristics, see
Directional ground-fault principles.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 335
Technical Manual
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