ABB RER620, Technical Manual

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Section 4
Protection functions
RER620 145
Technical Manual
Figure 61: 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 line-to-ground capacitances (C
0
) of phases
and leakage resistances (R
0
). This means that the zero sequence current is mainly
capacitive and has a phase shift of –90 degrees compared to the residual voltage (-V
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 ground fault current
is defined in isolated neutral networks.
For definitions of different directional ground-fault characteristics, see
Directional ground-fault principles .
maximum torque line
Min forward angle
Min reverse angle
Min operate
current
Max forward angle
Max reverse angle
zero torque line
Non-operatin
g area
Characteristic angle
= -90 deg
(operating quantity)
- (polarizing quantity)
0
V
0
I