ABB RELION REX640 - Page 755

Figure 427: Connection of the current transformers of Type 2. The phase currents
and the neutral current have equal directions at an external earth-fault situation.
L1
L2
L3
1/5A
N
1/5A
N
1/5A
N
IL1IL2IL3
P1 P2
S1
S2
1/5A
N
Io
P2P1
S1
S2
Figure 428: Connection of the current transformers of Type 2. The phase currents
and the neutral current have equal directions at an external earth-fault situation.
Internal and external faults
LREFPNDF does not respond to any faults outside the protected zone. An external
fault is detected by checking the phase angle difference of the neutral current and
the sum of the phase currents. When the difference is less than 90 degrees, the
operation is internally restrained or blocked. Hence the protection is not sensitive to
an external fault. Figure 429 and Figure 430 are valid for CT connection type 1.
1MRS759142 F
Protection functions
REX640
Technical Manual
755