A
B
C
a
b
c
I
a = 0
I
I
b = 0
I
c = 0
I
Uzs
Io
zone of
protection
Ifault
I
N
Io
I
N
Reference is Neutral
Current
Restrain for
external fault
Operate for
internal fault
For internal fault
s
o
u
r
c
e
GUID-D5D712D4-2291-4C49-93DE-363F9F10801C V2 EN
Figure 230: Current flow in all the CTs for an internal fault
LREFPNDF does not respond to phase-to-phase faults either, as in this case the
fault current flows between the two line CTs and so the neutral CT does not
experience this fault current.
Blocking based on the second harmonic of the neutral current
The transformer magnetizing inrush currents occur when the transformer is
energized after a period of de-energization. The inrush current can be many times
the rated current, and the halving time can be up to several seconds. For the
differential IED, the inrush current represents the differential current, which causes
the IED to operate almost always when the transformer is connected to the
network. Typically, the inrush current contains a large amount of second harmonics.
The blocking also prevents unwanted operation at the recovery and sympathetic
magnetizing inrushes. At the recovery inrush, the magnetizing current of the
transformer to be protected increases momentarily when the voltage returns to
normal after the clearance of a fault outside the protected area. The sympathetic
inrush is caused by the energization of a transformer running in parallel with the
protected transformer already connected to the network.
Blocking the starting of the restricted earth-fault protection at the magnetizing
inrush is based on the ratio of the second harmonic and the fundamental frequency
amplitudes of the neutral current Io_2H / Io. Typically, the second harmonic
content of the neutral current at the magnetizing inrush is higher than that of the
phase currents.
1YHT530004D05 D Section 4
Protection functions
615 series 429
Technical Manual
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