ABB RER620 - Page 151

1MAC309294-MB F 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-operating area
Characteristic angle
= -90 deg
(operating quantity)
- (polarizing quantity)
0
V
0
I