GUID-124047A0-9B33-4D2F-9519-75D98C0A4534 V1 EN
Figure 228: 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.
A
B
I a = 0
I
b = 0
I
b = 0
I
c = 0
I
Io
zone of
protection
C
a
b
c
I
N
I
zs1
I
zs1
I
zs1
Io
I
N
Reference is Neutral
Current
Restrain for
external fault
Operate for
internal fault
For external fault
Uzs
GUID-FAC5E4AD-A4A7-4D39-9EAC-C380EA33CB78 V2 EN
Figure 229: Current flow in all the CTs for an external fault
Section 4 1YHT530004D05 D
Protection functions
428 615 series
Technical Manual
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