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Internal and external faults
87LOZREF 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 274:
Current flow in all the CTs for an external fault
Section 4
1MAC059074-MB A
Protection functions
546
615 series ANSI
Technical Manual
Содержание Relion 615 series
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