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 protection relay, the inrush current represents the differential current,
which causes the protection relay 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.
4.3.3.6
Signals
Table 493: LREFPNDF Input signals
Name
Type
Default
Description
I_A
SIGNAL
0
Phase A current
I_B
SIGNAL
0
Phase B current
I_C
SIGNAL
0
Phase C current
Io
SIGNAL
0
Residual current
BLOCK
BOOLEAN
0=False
Block signal for acti-
vating the blocking
mode
Table 494: LREFPNDF Output signals
Name
Type
Description
OPERATE
BOOLEAN
Operate
START
BOOLEAN
Start
BLK2H
BOOLEAN
2nd harmonic block
1MRS757644 H
Protection functions
620 series
Technical Manual
567
