ABB RER620 - Page 152

Section 4 1MAC309294-MB F
Protection functions
146 RER620
Technical Manual
Figure 62: Ground-fault situation in an isolated network
Directional ground-fault protection in a compensated network
In compensated networks, the capacitive fault current and the inductive resonance coil
current compensate each other. The protection cannot be based on the reactive current
measurement, since the current of the compensation coil would disturb the operation of the
relays. In this case, the selectivity is based on the measurement of the active current
component. The magnitude of this component is often small and must be increased by
means of a parallel resistor in the compensation equipment. When measuring the resistive
part of the zero sequence current, the relay characteristic angle (RCA) should be set to 0
degrees and the operation criteria to I
0
cos(φ) or phase angle.The figure below describes
how ground fault current is defined in compensated neutral networks.
A B C
C
0
-
ΣI
01
ΣI
02
-
ΣI
02
I
ef
R
0
0
V
0
V
0
V